first commit, moving out of hcel.

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diff --git a/COPYING.agpl3 b/COPYING.agpl3
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--- /dev/null
+++ b/COPYING.agpl3
@@ -0,0 +1,661 @@
+                    GNU AFFERO GENERAL PUBLIC LICENSE
+                       Version 3, 19 November 2007
+
+ Copyright (C) 2007 Free Software Foundation, Inc. <https://fsf.org/>
+ Everyone is permitted to copy and distribute verbatim copies
+ of this license document, but changing it is not allowed.
+
+                            Preamble
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+IS WITH YOU.  SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF
+ALL NECESSARY SERVICING, REPAIR OR CORRECTION.
+
+  16. Limitation of Liability.
+
+  IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
+WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MODIFIES AND/OR CONVEYS
+THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY
+GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE
+USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF
+DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD
+PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS),
+EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF
+SUCH DAMAGES.
+
+  17. Interpretation of Sections 15 and 16.
+
+  If the disclaimer of warranty and limitation of liability provided
+above cannot be given local legal effect according to their terms,
+reviewing courts shall apply local law that most closely approximates
+an absolute waiver of all civil liability in connection with the
+Program, unless a warranty or assumption of liability accompanies a
+copy of the Program in return for a fee.
+
+                     END OF TERMS AND CONDITIONS
+
+            How to Apply These Terms to Your New Programs
+
+  If you develop a new program, and you want it to be of the greatest
+possible use to the public, the best way to achieve this is to make it
+free software which everyone can redistribute and change under these terms.
+
+  To do so, attach the following notices to the program.  It is safest
+to attach them to the start of each source file to most effectively
+state the exclusion of warranty; and each file should have at least
+the "copyright" line and a pointer to where the full notice is found.
+
+    <one line to give the program's name and a brief idea of what it does.>
+    Copyright (C) <year>  <name of author>
+
+    This program is free software: you can redistribute it and/or modify
+    it under the terms of the GNU Affero General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU Affero General Public License for more details.
+
+    You should have received a copy of the GNU Affero General Public License
+    along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+Also add information on how to contact you by electronic and paper mail.
+
+  If your software can interact with users remotely through a computer
+network, you should also make sure that it provides a way for users to
+get its source.  For example, if your program is a web application, its
+interface could display a "Source" link that leads users to an archive
+of the code.  There are many ways you could offer source, and different
+solutions will be better for different programs; see section 13 for the
+specific requirements.
+
+  You should also get your employer (if you work as a programmer) or school,
+if any, to sign a "copyright disclaimer" for the program, if necessary.
+For more information on this, and how to apply and follow the GNU AGPL, see
+<https://www.gnu.org/licenses/>.
diff --git a/README.org b/README.org
new file mode 100644
index 0000000..290455e
--- /dev/null
+++ b/README.org
@@ -0,0 +1,42 @@
+#+title: hc.el
+
+hc.el is the emacs client for hcel.
+
+* Install
+
+Clone this repo, and add load path (assuming you clone to ~~/.emacs.d~):
+
+#+begin_src sh
+cd ~/.emacs.d
+git clone https://g.ypei.me/hc.el.git
+#+end_src
+
+#+begin_src elisp
+(add-to-list 'load-path "~/.emacs.d/hc.el")
+#+end_src
+
+You'll also need an hcel server.  To host one yourself, clone the [[https://g.ypei.me/hcel.git][repo]]
+and follow the instructions there.
+
+Once you have a server set up (say at ~localhost:8080~) and serving
+indexed packages, do the following and you are all set.
+
+#+begin_src elisp
+(require 'hc)
+(setq hcel-endpoint "localhost:8080")
+#+end_src
+
+* Use
+
+The main entry points are two commands: ~hcel~, which opens up an
+hcel-outline mode buffer, which you can use to browse all packages,
+modules and identifiers in outline mode, and open any module source or
+jump to the definition of any identifier you like; ~hcel-package~
+prompts you for a package id, followed by a module path, to open the
+module source.  If you want to search for an identifier, the command
+~hcel-global-ids~ is for you.
+
+* Contact and Copyright
+
+~hc.el~ is covered by [[https://www.gnu.org/licenses/agpl-3.0.en.html][GNU AGPLv3+]].  You may find the license text in a
+file named COPYING.agpl3 in the project tree.
diff --git a/hc.el b/hc.el
new file mode 100644
index 0000000..f6239f6
--- /dev/null
+++ b/hc.el
@@ -0,0 +1,58 @@
+;;; hc.el --- Haskell codebase explorer -*- lexical-binding: t; -*-
+
+;; Author: Yuchen Pei <id@ypei.org>
+;; Maintainer: Yuchen Pei <id@ypei.org>
+;; Created: 2022
+;; Version: 0
+;; Keywords: haskell
+;; Package-Requires: ((emacs "28") (haskell-mode))
+;; Package-Type: multi
+;; Homepage: https://g.ypei.me/hcel.git
+
+;; Copyright (C) 2022 Yuchen Pei.
+;; 
+;; This file is part of hcel.
+;; 
+;; hcel is free software: you can redistribute it and/or modify it under
+;; the terms of the GNU Affero General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+;; 
+;; hcel is distributed in the hope that it will be useful, but WITHOUT
+;; ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+;; or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Affero General
+;; Public License for more details.
+;; 
+;; You should have received a copy of the GNU Affero General Public
+;; License along with hcel.  If not, see <https://www.gnu.org/licenses/>.
+
+(require 'hcel-source)
+(require 'hcel-outline)
+(require 'hcel-results)
+(require 'hcel-utils)
+
+(defun hcel-package (package-id)
+  "Select a package, followed by selecting a module to display the source."
+  (interactive
+   (list
+    (completing-read "Select package: "
+                     (mapcar 'hcel-format-package-id
+                             (hcel-api-packages)))))
+  (call-interactively (hcel-module-selector (hcel-parse-package-id package-id))))
+
+(defun hcel-module ()
+  "Select a module to display source."
+  (interactive)
+  (call-interactively
+   (hcel-module-selector hcel-package-id)))
+
+(defun hcel-module-selector (package-id)
+  (lambda (module-path)
+    (interactive
+     (list
+      (completing-read "Select module: "
+                       (hcel-list-modules package-id))))
+    (switch-to-buffer
+     (hcel-load-module-source package-id module-path))))
+
+(provide 'hc)
diff --git a/hcel-client.el b/hcel-client.el
new file mode 100644
index 0000000..1aa0d0f
--- /dev/null
+++ b/hcel-client.el
@@ -0,0 +1,161 @@
+;;; hcel-client.el --- talks to a haskell-code-server. -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2022 Yuchen Pei.
+;; 
+;; This file is part of hcel.
+;; 
+;; hcel is free software: you can redistribute it and/or modify it under
+;; the terms of the GNU Affero General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+;; 
+;; hcel is distributed in the hope that it will be useful, but WITHOUT
+;; ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+;; or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Affero General
+;; Public License for more details.
+;; 
+;; You should have received a copy of the GNU Affero General Public
+;; License along with hcel.  If not, see <https://www.gnu.org/licenses/>.
+
+(defcustom hcel-endpoint "localhost:8080"
+  "hcel endpoint"
+  :group 'hcel)
+(defcustom hcel-indexed-dir "/.haskell-code-explorer"
+  "hcel indexed dir"
+  :group 'hcel)
+
+(defvar hcel-client-buffer-name "*hcel-client*")
+
+(defun hcel-api-packages ()
+  (let ((packages
+         (hcel-url-fetch-json (concat hcel-endpoint "/api/packages"))))
+    (mapcan
+     (lambda (package)
+       (mapcar
+        (lambda (version) (list (cons 'name (alist-get 'name package))
+                                (cons 'version version)))
+        (alist-get 'versions package)))
+     packages)))
+
+(defun hcel-api-package-info (package-id)
+  (mapcar
+   (lambda (tuple)
+     (prin1-to-string (car tuple) t))
+   (alist-get
+    'modules
+    (hcel-url-fetch-json (concat
+                     hcel-endpoint "/files/" (hcel-format-package-id package-id "-")
+                     hcel-indexed-dir "/packageInfo.json")))))
+
+(defun hcel-api-definition-site
+    (package-id component-id module-name entity name)
+  (hcel-url-fetch-json
+   (concat hcel-endpoint "/api/definitionSite/"
+           (hcel-format-package-id package-id "-")
+           "/" component-id "/" module-name "/" entity "/" name)))
+
+(defun hcel-definition-site-location-info (approx-location-info)
+  "Call definitionSite with info from an approximate location."
+  (when (string= (hcel-location-tag approx-location-info) "ExactLocation")
+    (error "An ExactLocation supplied."))
+  (when-let* ((package-id (alist-get 'packageId approx-location-info))
+              (component-id (alist-get 'componentId approx-location-info))
+              (module-name (alist-get 'moduleName approx-location-info))
+              (entity (alist-get 'entity approx-location-info))
+              (name (alist-get 'name approx-location-info)))
+    (hcel-api-definition-site package-id component-id module-name entity name)))
+
+(defun hcel-api-module-info (package-id module-path)
+  (hcel-url-fetch-json
+   (concat
+    hcel-endpoint "/files/" (hcel-format-package-id package-id "-")
+    hcel-indexed-dir
+    "/" (replace-regexp-in-string "/" "%252F" module-path) ".json.gz")
+   t))
+
+(defun hcel-api-expressions
+    (package-id module-path line-beg col-beg line-end col-end)
+  (hcel-url-fetch-json
+   (concat
+    hcel-endpoint "/api/expressions/" (hcel-format-package-id package-id "-")
+    "/" (replace-regexp-in-string "/" "%2F" module-path)
+    "/" (number-to-string (1+ line-beg))
+    "/" (number-to-string (1+ col-beg))
+    "/" (number-to-string (1+ line-end))
+    "/" (number-to-string (1+ col-end)))))
+
+(defun hcel-api-hoogle-docs (package-id module-name entity name)
+  (hcel-url-fetch-json
+   (concat hcel-endpoint "/api/hoogleDocs/"
+           (hcel-format-package-id package-id "-") "/"
+           module-name "/" entity "/" name)))
+
+(defun hcel-format-pagination-query (page per-page)
+  (when (or page per-page)
+    (concat "?"
+            (string-join 
+             (list
+              (when page (concat "page=" page))
+              (when per-page (concat "per_page=" per-page)))
+             (when (and page per-page) "&")))))
+
+(defun hcel-api-references (package-id name &optional page per-page)
+  (hcel-url-fetch-json
+   (concat hcel-endpoint "/api/references/"
+           (hcel-format-package-id package-id "-") "/"
+           name
+           (hcel-format-pagination-query page per-page))))
+
+(defun hcel-api-identifiers (scope query package-id &optional page per-page
+                                   with-header)
+  (hcel-url-fetch-json
+   (concat hcel-endpoint
+           (if (eq scope 'global)
+               "/api/globalIdentifiers/"
+             (concat "/api/identifiers/"
+                     (hcel-format-package-id package-id "-")
+                     "/"))
+           query
+           (hcel-format-pagination-query page per-page))
+   nil with-header))
+
+(defun hcel-api-global-references (name)
+  (hcel-url-fetch-json (concat hcel-endpoint "/api/globalReferences/" name)))
+
+(defun hcel-parse-http-header (text)
+  (let ((status) (fields))
+    (with-temp-buffer
+      (insert text)
+      (goto-char (point-min))
+      (re-search-forward "^HTTP.*\\([0-9]\\{3\\}\\).*$")
+      (setq status (match-string 1))
+      (while (re-search-forward "^\\(.*?\\): \\(.*\\)$" nil t)
+        (push (cons (intern (match-string 1)) (match-string 2)) fields)))
+    (list (cons 'status status) (cons 'fields fields))))
+
+(defun hcel-url-fetch-json (url &optional decompression with-header)
+  (with-current-buffer (get-buffer-create hcel-client-buffer-name)
+    (goto-char (point-max))
+    (insert "[" (current-time-string) "] Request: " url "\n"))
+  (with-current-buffer (url-retrieve-synchronously url t)
+    (let ((header) (status) (fields) (json))
+      (delete-http-header)
+      (setq header (hcel-parse-http-header (car kill-ring))
+            status (alist-get 'status header)
+            fields (alist-get 'fields header))
+      (with-current-buffer hcel-client-buffer-name
+        (insert "[" (current-time-string) "] Response: " status "\n"))
+      (when decompression
+        (call-process-region (point) (point-max) "gunzip" t t t)
+        (goto-char (point-min)))
+      (call-interactively 'delete-trailing-whitespace)
+      (if (string= status "200")
+          (unless (= (point) (point-max))
+            (if with-header
+                (list
+                 (cons 'header fields)
+                 (cons 'json (json-read)))
+              (json-read)))
+        (error "HTTP error: %s" (buffer-substring (point) (point-max)))))))
+
+(provide 'hcel-client)
diff --git a/hcel-haddorg.el b/hcel-haddorg.el
new file mode 100644
index 0000000..ad797e0
--- /dev/null
+++ b/hcel-haddorg.el
@@ -0,0 +1,46 @@
+;; -*- lexical-binding: t; -*-
+;; Copyright (C) 2022 Yuchen Pei.
+;; 
+;; This file is part of hcel.
+;; 
+;; hcel is free software: you can redistribute it and/or modify it under
+;; the terms of the GNU Affero General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+;; 
+;; hcel is distributed in the hope that it will be useful, but WITHOUT
+;; ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+;; or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Affero General
+;; Public License for more details.
+;; 
+;; You should have received a copy of the GNU Affero General Public
+;; License along with hcel.  If not, see <https://www.gnu.org/licenses/>.
+
+(require 'hcel-source)
+(require 'hcel-client)
+(require 'hcel-utils)
+
+(defun hcel-haddorg-to-hcel-definition ()
+  (interactive)
+  (save-excursion
+    (org-back-to-heading t)
+    (let* ((id (org-entry-get (point) "CUSTOM_ID"))
+           (splitted (split-string id "/"))
+           (module-name (car splitted))
+           (entity (if (equal "v" (cadr splitted)) "Val" "Typ"))
+           (name (caddr splitted))
+           (package) (unparsed) (package-id))
+      (goto-char (point-min))
+      (setq package (org-entry-get (point) "ITEM"))
+      (setq package-id
+            (hcel-parse-package-id
+             (progn
+               (string-match "^\\(.*?\\)\\(:.*\\)?$" package)
+               (match-string 1 package))
+             "-"))
+      (hcel-load-module-location-info
+       (alist-get 'location
+                  (hcel-api-definition-site
+                   package-id "lib" module-name entity name))))))
+
+(provide 'hcel-haddorg)
diff --git a/hcel-outline.el b/hcel-outline.el
new file mode 100644
index 0000000..a46db14
--- /dev/null
+++ b/hcel-outline.el
@@ -0,0 +1,215 @@
+;;; hcel-outline.el --- shows package-module-identifier hierarchy in an outline mode -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2022 Yuchen Pei.
+;; 
+;; This file is part of hcel.
+;; 
+;; hcel is free software: you can redistribute it and/or modify it under
+;; the terms of the GNU Affero General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+;; 
+;; hcel is distributed in the hope that it will be useful, but WITHOUT
+;; ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+;; or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Affero General
+;; Public License for more details.
+;; 
+;; You should have received a copy of the GNU Affero General Public
+;; License along with hcel.  If not, see <https://www.gnu.org/licenses/>.
+
+(defvar hcel-outline-buffer-name "*hcel-outline*")
+(defvar hcel-outline-indentation 2)
+
+(defvar hcel-outline-mode-map
+  (let ((kmap (make-sparse-keymap)))
+    (define-key kmap "n" 'outline-next-visible-heading)
+    (define-key kmap "p" 'outline-previous-visible-heading)
+    (define-key kmap "f" 'outline-forward-same-level)
+    (define-key kmap "F" 'hcel-outline-follow-mode)
+    (define-key kmap "b" 'outline-backward-same-level)
+    (define-key kmap "u" 'outline-up-heading)
+    (define-key kmap "\t" 'hcel-outline-toggle-children)
+    (define-key kmap "\r" 'hcel-outline-open-thing-at-point)
+    (define-key kmap "o" 'hcel-outline-open-thing-at-point-other-window)
+    (define-key kmap "q" 'quit-window)
+    kmap))
+(define-derived-mode hcel-outline-mode outline-mode "hcel-outline"
+  "Major mode for browsing Haskell codebases"
+  (setq-local package-filter nil
+              module-filter nil
+              outline-regexp "\\( *\\)."
+              outline-level (lambda () (1+ (/ (length (match-string 1))
+                                              hcel-outline-indentation)))
+              buffer-read-only t))
+(add-hook 'hcel-outline-mode-hook 'hcel-minor-mode)
+
+(defun hcel ()
+  (interactive)
+  (let ((buffer (get-buffer hcel-outline-buffer-name)))
+    (if buffer (switch-to-buffer buffer)
+      (switch-to-buffer (get-buffer-create hcel-outline-buffer-name))
+      (save-excursion
+        (mapc
+         (lambda (package-id)
+           (insert
+            (concat (propertize
+                     (hcel-format-package-id package-id)
+                     'thing 'package
+                     'package-id package-id
+                     'children-loaded nil)
+                    "\n")))
+         (hcel-api-packages)))
+      (hcel-outline-mode))))
+
+(define-key hcel-mode-map "o" 'hcel)
+
+;; TODO: maybe remove
+(defun hcel-outline-update-opened (package-id module-path)
+  "Update the outline tree depending on openness of packages and modules.
+
+If a package is opened outside of the outline mode (mainly source mode), then
+update in the outline mode too."
+  (with-current-buffer hcel-outline-buffer-name
+    (save-excursion
+      (hcel-outline-goto-package package-id)
+      (hcel-outline-load-modules-at-point)
+      (hcel-outline-goto-module module-path)
+      (hcel-outline-load-identifiers-at-point))))
+
+(defun hcel-outline-goto-package (package-id)
+  (goto-char (point-min))
+  (re-search-forward
+   (format "^%s$" (hcel-format-package-id package-id)))
+  (beginning-of-line))
+
+(defun hcel-outline-goto-module (module-path)
+  "Goto module, assuming point is on the package."
+  (re-search-forward
+   (format "^%s%s$" (make-string hcel-outline-indentation 32) module-path))
+  (beginning-of-line))
+
+(defun hcel-outline-goto-identifier (position)
+  "Goto identifier declared at POSITION, assuming point is on the module."
+  (text-property-search-forward 'position position))
+
+(defun hcel-outline-load-modules-at-point ()
+  (interactive)
+  (unless (eq (get-text-property (point) 'thing) 'package)
+    (error "Point is not at a package!"))
+  (unless (get-text-property (point) 'children-loaded)
+    (save-excursion
+      (let ((inhibit-read-only t)
+            (package-id (get-text-property (point) 'package-id)))
+        (put-text-property
+         (progn (beginning-of-line) (point))
+         (progn (end-of-line) (point))
+         'children-loaded t)
+        (beginning-of-line 2)
+        (mapc
+         (lambda (module)
+           (insert
+            (propertize (concat
+                         (make-string hcel-outline-indentation 32) module "\n")
+                        'thing 'module
+                        'package-id package-id
+                        'module-path module
+                        'folded t)))
+         (hcel-api-package-info package-id))))))
+
+(defun hcel-outline-toggle-children ()
+  (interactive)
+  (let ((thing (get-text-property (point) 'thing))
+        (children-loaded (get-text-property (point) 'children-loaded)))
+    (cond (children-loaded (outline-toggle-children))
+          ((eq thing 'package) (hcel-outline-load-modules-at-point))
+          ((eq thing 'module) (hcel-outline-load-identifiers-at-point))
+          (t nil))))
+
+(defun hcel-outline-load-identifiers-at-point ()
+  (interactive)
+  (unless (eq (get-text-property (point) 'thing) 'module)
+    (error "Point is not at a module!"))
+  (unless (get-text-property (point) 'children-loaded)
+    (save-excursion
+      (let* ((inhibit-read-only t)
+             (package-id (get-text-property (point) 'package-id))
+             (module-path (get-text-property (point) 'module-path))
+             (imenu-index))
+        (put-text-property
+         (progn (beginning-of-line) (point))
+         (progn (end-of-line) (point))
+         'children-loaded t)
+        (when (or (get-buffer (hcel-buffer-name package-id module-path))
+                  (y-or-n-p "Open module source?"))
+          (with-current-buffer
+              (hcel-load-module-source package-id module-path)
+            (setq imenu-index (save-excursion (hcel-imenu-create-index))))
+          (beginning-of-line 2)
+          (mapc
+           (lambda (pair)
+             (insert
+              (propertize
+               (concat (make-string (* 2 hcel-outline-indentation) 32)
+                       (car pair)
+                       "\n")
+               'thing 'identifier
+               'package-id package-id
+               'module-path module-path
+               'position (cdr pair))))
+           imenu-index))))))
+
+(defun hcel-outline-open-module-source-at-point (&optional other-window)
+  (interactive)
+  (let ((props (text-properties-at (point))))
+    (unless (eq (plist-get props 'thing) 'module)
+      (error "Point is not at a module!"))
+    (let ((buffer
+           (hcel-load-module-source
+            (plist-get props 'package-id)
+            (plist-get props 'module-path))))
+      (hcel-outline-load-identifiers-at-point)
+      (if other-window
+          (switch-to-buffer-other-window buffer)
+        (switch-to-buffer buffer)))))
+
+(defun hcel-outline-open-identifier-at-point (&optional other-window)
+  (interactive)
+  (let ((props (text-properties-at (point))))
+    (unless (eq (plist-get props 'thing) 'identifier)
+      (error "Point is not at an identifier!"))
+    (let ((buffer
+           (hcel-load-module-source
+            (plist-get props 'package-id)
+            (plist-get props 'module-path))))
+      (if other-window
+          (switch-to-buffer-other-window buffer)
+        (switch-to-buffer buffer))
+      (goto-char (plist-get props 'position)))))
+
+(defun hcel-outline-open-thing-at-point (&optional other-window)
+  (interactive)
+  (cond ((eq (get-text-property (point) 'thing) 'module)
+         (hcel-outline-open-module-source-at-point other-window))
+        ((eq (get-text-property (point) 'thing) 'identifier)
+         (hcel-outline-open-identifier-at-point other-window))
+        (t nil)))
+
+(defun hcel-outline-open-thing-at-point-other-window ()
+  (interactive)
+  (let ((current-window (car (window-list))))
+    (hcel-outline-open-thing-at-point t)
+    (select-window current-window)))
+
+(define-minor-mode hcel-outline-follow-mode
+  "Display modules and identifiers as point moves."
+  :lighter " hcel-outline-follow"
+  :after-hook
+  (if hcel-outline-follow-mode
+      (if (not (eq major-mode 'hcel-outline-mode))
+          (error "Not in hcel-outline mode!")
+        (add-hook 'post-command-hook
+                  'hcel-outline-open-thing-at-point-other-window nil t))
+    (remove-hook 'post-command-hook
+                 'hcel-outline-open-thing-at-point-other-window t)))
+
+(provide 'hcel-outline)
diff --git a/hcel-results.el b/hcel-results.el
new file mode 100644
index 0000000..5f27b6a
--- /dev/null
+++ b/hcel-results.el
@@ -0,0 +1,393 @@
+;;; hc-results.el --- Shows query results in a compilation mode -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2022 Yuchen Pei.
+;; 
+;; This file is part of hcel.
+;; 
+;; hcel is free software: you can redistribute it and/or modify it under
+;; the terms of the GNU Affero General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+;; 
+;; hcel is distributed in the hope that it will be useful, but WITHOUT
+;; ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+;; or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Affero General
+;; Public License for more details.
+;; 
+;; You should have received a copy of the GNU Affero General Public
+;; License along with hcel.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; hcel-results provides functions for hcel modes derived from
+;; compilation-mode.
+
+;;; Code:
+
+(defun hcel-results-next-error-no-open (n)
+  (interactive "p")
+  (hcel-results-next-error-internal n))
+
+(defun hcel-results-previous-error-no-open (n)
+  (interactive "p")
+  (hcel-results-next-error-no-open (- n)))
+
+(defun hcel-results-next-error-internal (n &optional reset)
+  (interactive "p")
+  (if reset
+      (progn
+        (goto-char (point-min))
+        (hcel-results-next-error-internal 1 nil))
+    (if (> n 0)
+        (dotimes (unused n)
+          (condition-case nil
+              (progn
+                (goto-char (next-single-property-change (point) 'match-line))
+                (unless (get-text-property (point) 'match-line)
+                  (goto-char
+                   (next-single-property-change (point) 'match-line))))
+            (error (hcel-results-next-page))))
+      (dotimes (unused (- n))
+        (condition-case nil
+            (progn
+              (goto-char (previous-single-property-change (point) 'match-line))
+              (unless (get-text-property (point) 'match-line)
+                (goto-char
+                 (previous-single-property-change (point) 'match-line))))
+          (error (hcel-results-previous-page)))))))
+
+(defun hcel-results-next-error (n &optional reset)
+  (interactive "p")
+  (hcel-results-next-error-internal n reset)
+  (hcel-results-open))
+
+(defun hcel-results-open ()
+  (interactive)
+  ;; TODO: arrow not working
+  (compilation-set-overlay-arrow (selected-window))
+  (hcel-load-module-location-info (get-text-property (point) 'location-info)))
+
+(defun hcel-results-next-page ()
+  (interactive)
+  (unless (memq major-mode '(hcel-refs-mode hcel-ids-mode))
+    (error "Not in hcel-refs or hcel-ids mode: %S" major-mode))
+  (when (= hcel-results-page-number hcel-results-max-page-number)
+    (error "Already on the last page."))
+  (setq hcel-results-page-number (1+ hcel-results-page-number))
+  (cond ((eq major-mode 'hcel-refs-mode) (hcel-refs-update-references))
+        ((eq major-mode 'hcel-ids-mode) (hcel-ids-update))
+        (t (error "wrong major mode: %S" major-mode)))
+  (hcel-results-next-error-internal 1))
+
+(defun hcel-results-previous-page ()
+  (interactive)
+  (unless (memq major-mode '(hcel-refs-mode hcel-ids-mode))
+    (error "Not in hcel-refs or hcel-ids mode: %S" major-mode))
+  (when (= hcel-results-page-number 1)
+    (error "Already on the first page."))
+  (setq hcel-results-page-number (1- hcel-results-page-number))
+  (cond ((eq major-mode 'hcel-refs-mode) (hcel-refs-update-references))
+        ((eq major-mode 'hcel-ids-mode) (hcel-ids-update))
+        (t (error "wrong major mode: %S" major-mode)))
+  (goto-char (point-max))
+  (hcel-results-next-error-internal -1))
+
+;; hcel-refs-mode
+(defcustom hcel-refs-per-page 50
+  "hcel refs number of results per page."
+  :group 'hcel-refs)
+
+(define-compilation-mode hcel-refs-mode "hcel-refs"
+  "Major mode for showing references"
+  (setq-local next-error-function 'hcel-results-next-error
+              hcel-refs-id nil
+              hcel-refs-package-id nil
+              hcel-results-page-number nil
+              hcel-results-max-page-number nil))
+
+(define-key hcel-refs-mode-map (kbd "M-n")
+  'hcel-results-next-error-no-open)
+
+(define-key hcel-refs-mode-map (kbd "M-p")
+  'hcel-results-previous-error-no-open)
+
+(defun hcel-refs-update-references ()
+  "Find references and update the current hcel-refs-mode buffer."
+  (unless (eq major-mode 'hcel-refs-mode)
+    (error "Not in hcel-refs mode!"))
+  (let ((inhibit-read-only t)
+        (modules-refs
+         (hcel-api-references
+          hcel-refs-package-id hcel-refs-id
+          (number-to-string hcel-results-page-number)
+          (number-to-string hcel-refs-per-page))))
+    (erase-buffer)
+    (insert (format "References of %s, Page %d of %d.\n"
+                    (hcel-refs-format-id hcel-refs-id)
+                    hcel-results-page-number hcel-results-max-page-number))
+    (mapc
+     (lambda (module-refs)
+       (let ((module-path (alist-get 'name module-refs))
+             (refs (alist-get 'references module-refs)))
+         (insert "References in "
+                 (hcel-format-package-id hcel-refs-package-id "-")
+                 " " module-path ": \n")
+         (mapc
+          (lambda (ref)
+            (insert
+             "--\n"
+             (propertize
+              (hcel-unquote-html
+               (alist-get 'sourceCodeHtml ref))
+              'location-info (hcel-id-src-span-to-location-info
+                              hcel-refs-package-id module-path
+                              (alist-get 'idSrcSpan ref)))))
+          refs)))
+     modules-refs)
+    (goto-char (point-min))
+    (while (re-search-forward "<b>\\(.*?\\)</b>" nil t)
+      (replace-match
+       (propertize (match-string 1) 'font-lock-face 'match))
+      (save-excursion
+        (add-text-properties (progn (beginning-of-line) (point))
+                             (progn (end-of-line) (point))
+                             (list 'match-line t)))))
+  (goto-char (point-min)))
+
+(defun hcel-refs-reload ()
+  (interactive)
+  (hcel-refs-update-references))
+(define-key hcel-refs-mode-map "g" 'hcel-refs-reload)
+
+(define-key hcel-refs-mode-map "f" 'hcel-results-next-page)
+
+(define-key hcel-refs-mode-map "b" 'hcel-results-previous-page)
+
+(defun hcel-refs-buffer-name (id)
+  (format "*hcel-refs %s*" (hcel-refs-format-id id)))
+
+(defun hcel-refs-format-id (id)
+  (let* ((tuple (split-string id "|")))
+    (format "%s (%s %s)"
+            (cadddr tuple) (car tuple) (cadr tuple))))
+
+(defun hcel-refs-update-references-package ()
+  "Find references and update the current hcel-refs buffer.
+
+Start by choosing a package."
+  (interactive)
+  (unless (eq major-mode 'hcel-refs-mode)
+    (error "Not in hcel-refs mode!"))
+  (let* ((global-refs (hcel-api-global-references hcel-refs-id))
+         (name (cadddr (split-string hcel-refs-id "|")))
+         (package-id-and-count
+          (split-string
+           (completing-read
+            (format "References of %s from: " name)
+            (mapcar (lambda (pkg-count)
+                      (format "%s (%s)"
+                              (alist-get 'packageId pkg-count)
+                              (alist-get 'count pkg-count)))
+                    global-refs))))
+         (package-id (car package-id-and-count))
+         (count (string-to-number (substring (cadr package-id-and-count) 1 -1)))
+         (max-page-number (1+ (/ count hcel-refs-per-page))))
+    (setq hcel-refs-package-id (hcel-parse-package-id package-id "-")
+          hcel-results-page-number 1
+          hcel-results-max-page-number max-page-number)
+    (hcel-refs-update-references)))
+(define-key hcel-refs-mode-map "P" 'hcel-refs-update-references-package)
+
+(defun hcel-find-references-at-point ()
+  "Find references of the identifier at point."
+  (interactive)
+  (hcel-find-references-internal hcel-package-id hcel-module-path
+                                 (get-text-property (point) 'identifier)))
+(define-key hcel-mode-map (kbd "M-?") 'hcel-find-references-at-point)
+
+(defun hcel-minor-find-references-at-point ()
+  (interactive)
+  (let ((props (text-properties-at (point))))
+    (cond ((or (eq major-mode 'hcel-outline-mode)
+               (eq (current-buffer) eldoc--doc-buffer))
+           (hcel-find-references-internal
+            (plist-get props 'package-id)
+            (plist-get props 'module-path)
+            (plist-get props 'internal-id)))
+          ((eq major-mode 'hcel-ids-mode)
+           (hcel-find-references-internal
+            (alist-get 'packageId (plist-get props 'location-info))
+            (alist-get 'modulePath (plist-get props 'location-info))
+            (plist-get props 'internal-id)))
+          (t (error "%S not supported and not in eldoc doc buffer." major-mode)))))
+
+(defun hcel-find-references-internal (package-id module-path identifier)
+  (when (and package-id module-path identifier)
+    (let ((hcel-buffer (hcel-buffer-name package-id module-path)))
+      (when (or (get-buffer hcel-buffer)
+                (and (y-or-n-p "Open module source?")
+                     (hcel-load-module-source
+                      package-id module-path))))
+      (with-current-buffer hcel-buffer
+        (when-let* ((id (alist-get
+                         'externalId
+                         (alist-get (intern identifier) hcel-identifiers)))
+                    (buffer-name (hcel-refs-format-id id)))
+          (with-current-buffer (get-buffer-create buffer-name)
+            (hcel-refs-mode)
+            (setq hcel-refs-id id)
+            (hcel-refs-update-references-package))
+          (switch-to-buffer-other-window buffer-name))))))
+;; hcel-ids-mode
+(defcustom hcel-ids-per-page 20
+  "hcel-ids mode number of results per page."
+  :group 'hcel-ids)
+(defcustom hcel-ids-live-per-page 10
+  "hcel-ids live search results per page."
+  :group 'hcel-ids)
+(define-compilation-mode hcel-ids-mode "hcel-ids"
+  "Major mode for showing identifiers"
+  (setq-local next-error-function 'hcel-results-next-error
+              hcel-ids-scope nil
+              hcel-ids-query nil
+              hcel-ids-package-id nil
+              hcel-results-page-number nil
+              hcel-results-max-page-number nil))
+(add-hook 'hcel-ids-mode-hook 'hcel-minor-mode)
+
+(defun hcel-ids-update ()
+  (unless (eq major-mode 'hcel-ids-mode)
+    (error "Not in hcel-ids mode!"))
+  (when (and (eq hcel-ids-scope 'package) (not hcel-ids-package-id))
+    (error "No package-id supplied for identifiers call!"))
+  (let* ((inhibit-read-only t)
+         (results
+          (hcel-api-identifiers
+           hcel-ids-scope hcel-ids-query hcel-ids-package-id
+           (number-to-string hcel-results-page-number)
+           (number-to-string hcel-ids-per-page)
+           t)))
+    (erase-buffer)
+    (setq hcel-results-max-page-number
+          (1+ (/ (string-to-number
+                  (alist-get 'X-Total-Count
+                             (alist-get 'header results)))
+                 hcel-ids-per-page)))
+    (insert (format "Results of %s, Page %d of %d.\n"
+                    hcel-ids-query hcel-results-page-number
+                    hcel-results-max-page-number))
+    (mapc
+     (lambda (result)
+       (let* ((location-info (alist-get 'locationInfo result))
+              (doc (hcel-render-html
+                    (or (alist-get 'doc result)
+                        (alist-get 'documentation
+                                   (ignore-errors
+                                     (hcel-definition-site-location-info
+                                      location-info)))))))
+         (insert "--\n")
+         (insert (propertize
+                  (format "%s :: %s\n"
+                          (alist-get 'demangledOccName result)
+                          (hcel-render-id-type (alist-get 'idType result)))
+                  'location-info location-info
+                  'match-line t))
+         (insert (format "Defined in %s %s\n"
+                         (hcel-format-package-id
+                          (alist-get 'packageId location-info) "-")
+                         (alist-get 'modulePath location-info)))
+         (when doc (insert doc))))
+     (alist-get 'json results))
+    (goto-char (point-min))))
+
+(defun hcel-ids-reload ()
+  (interactive)
+  (hcel-ids-update))
+(define-key hcel-ids-mode-map "g" 'hcel-ids-reload)
+
+(define-key hcel-ids-mode-map (kbd "M-n")
+  'hcel-results-next-error-no-open)
+(define-key hcel-ids-mode-map (kbd "M-p")
+  'hcel-results-previous-error-no-open)
+(define-key hcel-ids-mode-map "f" 'hcel-results-next-page)
+(define-key hcel-ids-mode-map "b" 'hcel-results-previous-page)
+
+(defun hcel-ids-update-query (query)
+  "Search for identities matching query."
+  (interactive (list (progn
+                       (unless (eq major-mode 'hcel-ids-mode)
+                         (error "Not in hcel-ids mode!"))
+                       (read-string "Query: " hcel-ids-query))))
+  (setq hcel-ids-query query
+        hcel-results-page-number 1)
+  (hcel-ids-update))
+(define-key hcel-ids-mode-map "s" 'hcel-ids-update-query)
+
+(defun hcel-ids-buffer-name (scope query)
+  (format "*hcel-ids-%S %s*" scope query))
+
+;; Caching results to prevent to many hits
+(defvar hcel-ids--minibuffer-saved-query nil)
+(defvar hcel-ids--minibuffer-saved-results nil)
+
+(defun hcel-ids-minibuffer-collection (scope query &optional package-id)
+  (when (and (eq scope 'package) (not package-id))
+    (error "No package-id supplied for identifiers call!"))
+  (unless (length= query 0)
+    (if (string= hcel-ids--minibuffer-saved-query query)
+        hcel-ids--minibuffer-saved-results
+      (setq hcel-ids--minibuffer-saved-query query
+            hcel-ids--minibuffer-saved-results
+            (mapcar
+             (lambda (result)
+               (propertize
+                (alist-get 'demangledOccName result)
+                'location-info (alist-get 'locationInfo result)))
+             (hcel-api-identifiers
+              scope query package-id nil
+              (number-to-string hcel-ids-live-per-page))))
+      hcel-ids--minibuffer-saved-results)))
+
+(defun hcel-global-ids-minibuffer-collection (query unused unused)
+  (hcel-ids-minibuffer-collection 'global query))
+
+(defun hcel-package-ids-minibuffer-collection (package-id)
+  (lambda (query unused unused)
+    (hcel-ids-minibuffer-collection 'package query package-id)))
+
+(defun hcel-ids (scope query &optional package-id)
+  (if (length= hcel-ids--minibuffer-saved-results 1)
+      (hcel-load-module-location-info
+       (with-temp-buffer
+         (insert (car hcel-ids--minibuffer-saved-results))
+         (get-text-property (point-min) 'location-info)))
+    (let ((buffer-name (hcel-ids-buffer-name scope query)))
+      (with-current-buffer (get-buffer-create buffer-name)
+        (hcel-ids-mode)
+        (setq hcel-ids-scope scope
+              hcel-ids-package-id package-id)
+        (hcel-ids-update-query query))
+      (switch-to-buffer buffer-name))))
+
+(defun hcel-global-ids (query)
+  (interactive (list
+                (let ((minibuffer-allow-text-properties t))
+                  (completing-read "Search for identifier globally: "
+                                   'hcel-global-ids-minibuffer-collection))))
+  (hcel-ids 'global query))
+(define-key hcel-mode-map "I" 'hcel-global-ids)
+
+(defun hcel-package-ids (query)
+  (interactive (list
+                (let ((minibuffer-allow-text-properties t)
+                      (package-id hcel-package-id))
+                  (unless (eq major-mode 'hcel-mode)
+                    (error "Not in hcel-mode!"))
+                  (completing-read
+                   (format "Search for identifier in %s: "
+                           (hcel-format-package-id package-id "-"))
+                   (hcel-package-ids-minibuffer-collection package-id)))))
+  (hcel-ids 'package query hcel-package-id))
+(define-key hcel-mode-map "i" 'hcel-package-ids)
+
+(provide 'hcel-results)
diff --git a/hcel-source.el b/hcel-source.el
new file mode 100644
index 0000000..688fff1
--- /dev/null
+++ b/hcel-source.el
@@ -0,0 +1,523 @@
+;;; hcel-source.el --- displays Haskell module source. -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2022 Yuchen Pei.
+;; 
+;; This file is part of hcel.
+;; 
+;; hcel is free software: you can redistribute it and/or modify it under
+;; the terms of the GNU Affero General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+;; 
+;; hcel is distributed in the hope that it will be useful, but WITHOUT
+;; ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+;; or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Affero General
+;; Public License for more details.
+;; 
+;; You should have received a copy of the GNU Affero General Public
+;; License along with hcel.  If not, see <https://www.gnu.org/licenses/>.
+
+(require 'hcel-client)
+(define-derived-mode hcel-mode special-mode "hcel"
+  "Major mode for exploring Haskell codebases"
+  (setq-local eldoc-documentation-strategy 'eldoc-documentation-compose-eagerly
+              eldoc-documentation-functions
+              '(hcel-eldoc-id-type hcel-eldoc-expression-type hcel-eldoc-docs)
+              imenu-create-index-function 'hcel-imenu-create-index
+              imenu-space-replacement " "
+              hcel-identifiers nil
+              hcel-declarations nil
+              hcel-occurrences nil
+              hcel-package-id nil
+              hcel-module-path nil
+              hcel-highlight-id nil))
+(defun hcel-buffer-name (package-id module-path)
+  (concat "*hcel " (hcel-format-package-id package-id "-")
+          "/" module-path "*"))
+
+(defun hcel-load-module-source (package-id module-path &optional force)
+  "Loads module source in a buffer and returns the buffer.
+
+When FORCE is non-nil, kill existing source buffer if any."
+  (let ((buffer-name (hcel-buffer-name package-id module-path)))
+    (when (or force (not (get-buffer buffer-name)))
+      (let ((json (hcel-api-module-info package-id module-path)))
+        (ignore-errors (kill-buffer buffer-name))
+        (with-current-buffer (get-buffer-create buffer-name)
+          ;; (hcel-write-source-to-buffer (alist-get 'tokenizedLines json))
+          (hcel-write-html-source-to-buffer (hcel-source-html json))
+          (fontify-with-haskell-mode)
+          ;; it is important the setq of local vars are after the (hcel-mode)
+          ;; otherwise they may be rewritten
+          (hcel-mode)
+          (setq hcel-declarations (alist-get 'declarations json)
+                hcel-identifiers (alist-get 'identifiers json)
+                hcel-occurrences (alist-get 'occurrences json)
+                hcel-package-id package-id
+                hcel-module-path module-path)
+          (goto-char (point-min)))))
+    (get-buffer buffer-name)))
+
+(defun hcel-reload-module-source ()
+  "Reloads current module source."
+  (interactive)
+  (if (equal major-mode 'hcel-mode)
+      (switch-to-buffer
+       (hcel-load-module-source hcel-package-id hcel-module-path t))
+    (error "Not in hcel-mode!")))
+(define-key hcel-mode-map "g" 'hcel-reload-module-source)
+
+(defun hcel-load-module-location-info (location-info &optional no-jump)
+  "Load a module from exact location info.
+
+Example of LOCATION-INFO:
+
+    \"locationInfo\": {
+      \"endColumn\": 14,
+      \"endLine\": 317,
+      \"moduleName\": \"Main\",
+      \"modulePath\": \"app/Server.hs\",
+      \"packageId\": {
+        \"name\": \"hcel\",
+        \"version\": \"0.1.0.0\"
+      },
+      \"startColumn\": 5,
+      \"startLine\": 317,
+      \"tag\": \"ExactLocation\"
+    },
+
+If NO-JUMP is non-nil, just open the source and does not jump to the location with pulsing.
+"
+  (unless (string= (hcel-location-tag location-info) "ExactLocation")
+    (error "Location tag is not ExactLocation."))
+  (when-let ((package-id (alist-get 'packageId location-info))
+             (module-path (alist-get 'modulePath location-info))
+             (line-beg (alist-get 'startLine location-info))
+             (col-beg (alist-get 'startColumn location-info))
+             (line-end (alist-get 'endLine location-info))
+             (col-end (alist-get 'endColumn location-info))
+             (buffer (hcel-load-module-source package-id module-path)))
+    (unless no-jump
+      (switch-to-buffer-other-window buffer)
+      (goto-line-column line-beg (1- col-beg))
+      (pulse-momentary-highlight-region
+       (point) (save-excursion
+                 (goto-line-column line-end (1- col-end))
+                 (point))
+       'next-error))
+    buffer))
+
+(defun hcel-switch-buffer ()
+  (interactive)
+  (switch-to-buffer
+   (read-buffer
+    "Switch to buffer: " nil t
+		(lambda (buffer)
+		  (equal 
+		   (buffer-local-value
+        'major-mode
+        (get-buffer (if (stringp buffer) buffer (car buffer))))
+		   'hcel-mode)))))
+(define-key hcel-mode-map "b" 'hcel-switch-buffer)
+
+(defun hcel-lookup-occurrence-at-point ()
+  (when-let ((occurrence (get-text-property (point) 'occurrence)))
+    (alist-get (intern occurrence) hcel-occurrences)))
+
+(defun hcel-get-location-info (id occ)
+  (or (when id (alist-get 'locationInfo id))
+      ;; happens for import modules
+      (when occ (alist-get 'contents (alist-get 'sort occ)))))
+
+(defun hcel-occ-symbol-at-point ()
+  (when-let* ((occ (get-text-property (point) 'occurrence))
+              (splitted (split-string occ "-"))
+              (line (string-to-number (car splitted)))
+              (col-beg (string-to-number (cadr splitted)))
+              (col-end (string-to-number (caddr splitted))))
+    (buffer-substring-line-column line (1- col-beg) line (1- col-end))))
+
+(defun hcel-type-at-point ()
+  (interactive)
+  (hcel-render-type-internal hcel-package-id hcel-module-path
+                             (get-text-property (point) 'identifier)))
+
+(defun hcel-render-type-internal (package-id module-path identifier)
+  (when (and package-id module-path identifier)
+    (let ((hcel-buffer (hcel-buffer-name package-id module-path)))
+      (when (get-buffer hcel-buffer)
+        (with-current-buffer hcel-buffer
+          (when-let* ((id (alist-get (intern identifier)  hcel-identifiers))
+                      (id-type
+                       (or (alist-get 'idType id)
+                           (alist-get 'idOccType
+                                      (hcel-lookup-occurrence-at-point)))))
+            (concat
+             (hcel-render-id-type id-type)
+             (when-let* ((external-id (alist-get 'externalId id))
+                         (splitted (split-string external-id "|"))
+                         (package-id (car splitted))
+                         (module-name (cadr splitted)))
+               (concat "\nDefined in: " package-id " " module-name "")))))))))
+
+(defun hcel-hoogle-docs-location-info (location-info)
+  (when-let* ((package-id (alist-get 'packageId location-info))
+              (module-name (alist-get 'moduleName location-info))
+              (entity (alist-get 'entity location-info))
+              (name (alist-get 'name location-info)))
+    (hcel-api-hoogle-docs package-id module-name entity name)))
+
+(defun hcel-id-docs-at-point ()
+  (hcel-id-docs-internal hcel-package-id hcel-module-path
+                         (get-text-property (point) 'identifier)))
+
+(defun hcel-id-docs-internal (package-id module-path identifier)
+  (when (and package-id module-path identifier)
+    (let ((hcel-buffer (hcel-buffer-name package-id module-path)))
+      (when (get-buffer hcel-buffer)
+        (with-current-buffer hcel-buffer
+          (when-let*
+              ((id (alist-get (intern identifier) hcel-identifiers))
+               (location-info (hcel-get-location-info id nil))
+               (docs
+                (or
+                 ;; same module
+                 (alist-get 'doc id)
+                 ;; other module
+                 (alist-get
+                  'documentation
+                  (ignore-errors
+                    (hcel-definition-site-location-info location-info)))
+                 ;; hoogle
+                 (when-let ((hoogle-docs
+                             (ignore-errors
+                               (hcel-hoogle-docs-location-info location-info))))
+                   (when (length> hoogle-docs 0) (concat "Hoogle: " hoogle-docs))))))
+            (hcel-render-html docs)))))))
+
+;; TODO: multiple expressions
+(defun hcel-expressions-type (beg end)
+  (interactive "r")
+  (when mark-active
+    (save-excursion
+      (let ((line-beg) (col-beg) (line-end) (col-end))
+        (goto-char beg)
+        (setq line-beg (current-line)
+              col-beg (current-column))
+        (goto-char end)
+        (setq line-end (current-line)
+              col-end (current-column))
+        (when-let ((expr
+                    (ignore-errors
+                      (hcel-api-expressions hcel-package-id
+                                            hcel-module-path line-beg col-beg
+                                            line-end col-end))))
+          (unless (length= expr 0)
+            (hcel-expression-and-type (elt expr (1- (length expr))))))))))
+
+(defun hcel-outline-package-module ()
+  (interactive)
+  (let ((package-id hcel-package-id)
+        (module-path hcel-module-path))
+    (hcel)
+    (hcel-outline-goto-package package-id)
+    (hcel-outline-load-modules-at-point)
+    (hcel-outline-goto-module module-path)
+    (hcel-outline-load-identifiers-at-point)))
+(define-key hcel-mode-map "O" 'hcel-outline-package-module)
+
+;; eldoc
+(defun hcel-eldoc-id-type (cb)
+  (when-let ((symbol (hcel-occ-symbol-at-point))
+             (doc (hcel-type-at-point))
+             (docstring
+              (propertize
+               doc
+               'package-id hcel-package-id
+               'module-path hcel-module-path)))
+    (funcall cb docstring
+             :thing symbol
+             :face 'font-lock-variable-name-face)
+    (with-current-buffer eldoc--doc-buffer
+      (hcel-minor-mode))))
+
+(defun hcel-minor-eldoc-id-type (cb)
+  (when-let* ((props (text-properties-at (point)))
+              (identifier (plist-get props 'internal-id))
+              (symbol (save-excursion
+                        (buffer-substring
+                         (progn
+                           (text-property-search-backward
+                            'internal-id identifier 'string=)
+                           (point))
+                         (progn
+                           (text-property-search-forward
+                            'internal-id identifier 'string=)
+                           (point)))))
+              (docstring
+               (cond ((eq major-mode 'hcel-outline-mode)
+                      (hcel-render-type-internal
+                       (plist-get props 'package-id)
+                       (plist-get props 'module-path)
+                       identifier))
+                     ((eq major-mode 'hcel-ids-mode)
+                      (hcel-render-type-internal
+                       (alist-get 'packageId (plist-get props 'location-info))
+                       (alist-get 'modulePath (plist-get props 'location-info))
+                       identifier))
+                     (t nil))))
+    (funcall cb docstring
+             :thing symbol
+             :face 'font-lock-variable-name-face)
+    (with-current-buffer eldoc--doc-buffer
+      (hcel-minor-mode))))
+
+(defun hcel-eldoc-docs (cb)
+  (when-let ((docstring (hcel-id-docs-at-point)))
+    (setq this-command nil)
+    (funcall cb docstring)
+    (with-current-buffer eldoc--doc-buffer
+      (hcel-minor-mode))))
+
+(defun hcel-minor-eldoc-docs (cb)
+  (when-let* ((props (text-properties-at (point)))
+              (docstring
+               (cond ((eq major-mode 'hcel-outline-mode)
+                      (hcel-id-docs-internal
+                       (plist-get props 'package-id)
+                       (plist-get props 'module-path)
+                       (plist-get props 'internal-id)))
+                     ((eq major-mode 'hcel-ids-mode)
+                      (hcel-id-docs-internal
+                       (alist-get 'packageId (plist-get props 'location-info))
+                       (alist-get 'modulePath (plist-get props 'location-info))
+                       (plist-get props 'internal-id)))
+                     (t nil))))
+    (setq this-command nil)
+    (funcall cb docstring)
+    (with-current-buffer eldoc--doc-buffer
+      (hcel-minor-mode))))
+
+(defun hcel-eldoc-expression-type (cb)
+  (when mark-active
+    (when-let
+        ((expr-and-type
+          (hcel-expressions-type (region-beginning) (region-end))))
+      (setq this-command nil)
+      (funcall cb (cdr expr-and-type)
+               :thing (car expr-and-type)
+               :face 'font-lock-variable-name-face)
+      (with-current-buffer eldoc--doc-buffer
+        (hcel-minor-mode)))))
+
+;; highlight
+(defface hcel-highlight-id '((t (:inherit underline)))
+  "Face for highlighting hcel identifier at point.")
+
+(defun hcel-highlight-update (unused unused unused)
+  ;; if mark is active, change of face will deactivate the mark in transient
+  ;; mark mode
+  (unless mark-active
+    (let ((id (get-text-property (point) 'identifier))
+          (inhibit-read-only t))
+      (when (not (string= hcel-highlight-id id))
+        (hcel-highlight-stop hcel-highlight-id)
+        (hcel-highlight-start id)
+        (setq hcel-highlight-id id)))))
+
+(defun hcel-highlight-stop (id)
+  (when id
+    (save-excursion
+      (goto-char (point-min))
+      (let ((match))
+        (while (setq match
+                     (text-property-search-forward 'identifier id 'string=))
+          (font-lock--remove-face-from-text-property
+           (prop-match-beginning match)
+           (prop-match-end match) 'face 'hcel-highlight-id))))))
+
+(defun hcel-highlight-start (id)
+  (when id
+    (save-excursion
+      (goto-char (point-min))
+      (let ((match))
+        (while (setq match
+                     (text-property-search-forward 'identifier id 'string=))
+          (add-face-text-property
+           (prop-match-beginning match)
+           (prop-match-end match) 'hcel-highlight-id))))))
+
+(add-hook 'hcel-mode-hook 'cursor-sensor-mode)
+
+;; utilities
+(defun hcel-write-source-line-to-buffer (line)
+  (mapc
+   (lambda (token)
+     (let* ((idInfo (alist-get 'idInfo token))
+            (id (alist-get 'identifier idInfo))
+            (occ (alist-get 'occurrence idInfo))
+            (content (alist-get 'content token)))
+       (insert
+        (propertize content
+                    'identifier (unless (string= id "") id)
+                    'occurrence (unless (string= occ "") occ)
+                    'cursor-sensor-functions
+                    (when id (list 'hcel-highlight-update))))))
+   line))
+
+(defun hcel-write-source-to-buffer (lines)
+  (mapc
+   (lambda (line)
+     (hcel-write-source-line-to-buffer (alist-get 'lineContents line))
+     (insert "\n"))
+   lines))
+
+(defun hcel-write-html-source-line-to-buffer (line)
+  (mapc
+   (lambda (span)
+     (let* ((id (dom-attr span 'data-identifier))
+            (occ (dom-attr span 'data-occurrence))
+            (content (dom-text span)))
+       (insert
+        (propertize content
+                    'identifier (unless (string= id "") id)
+                    'occurrence (unless (string= occ "") occ)
+                    'cursor-sensor-functions
+                    (when id (list 'hcel-highlight-update))))))
+   (dom-by-tag line 'span))
+  (insert "\n"))
+
+(defun hcel-write-html-source-to-buffer (lines)
+  (mapc
+   'hcel-write-html-source-line-to-buffer
+   lines))
+
+(defun hcel-source-html (json)
+  (with-temp-buffer
+    (insert (alist-get 'sourceCodeHtml json))
+    (dom-by-class
+     (libxml-parse-html-region (point-min) (point-max))
+     "line-content")))
+
+;; imenu
+(defun hcel-imenu-create-index ()
+  (unless (eq major-mode 'hcel-mode)
+    (error "Not in hcel-mode!"))
+  (mapcar
+   (lambda (decl)
+     (cons
+      (hcel-render-components
+       (alist-get 'components
+                  (alist-get 'declType decl))
+       (alist-get 'name decl))
+      (progn (goto-line (alist-get 'lineNumber decl)) (point))))
+   hcel-declarations))
+(define-key hcel-mode-map "j" 'imenu)
+
+;; xref
+(add-hook 'hcel-mode-hook
+          (lambda ()
+            (add-hook 'xref-backend-functions
+                      #'hcel--xref-backend nil t)))
+(defun hcel--xref-backend () 'hcel-xref)
+(cl-defmethod xref-backend-definitions ((_backend (eql hcel-xref)) _identifiers)
+  (hcel-find-definition))
+
+(defun hcel-find-definition ()
+  (hcel-find-definition-internal
+   hcel-package-id hcel-module-path
+   (get-text-property (point) 'identifier)
+   (get-text-property (point) 'occurrence)))
+
+(add-hook 'hcel-minor-mode-hook
+          (lambda ()
+            (add-hook 'xref-backend-functions
+                      #'hcel-minor--xref-backend nil t)))
+(defun hcel-minor--xref-backend () 'hcel-minor-xref)
+(cl-defmethod xref-backend-definitions ((_backend (eql hcel-minor-xref)) _identifiers)
+  (hcel-minor-find-definition-at-point))
+(defun hcel-minor-find-definition-at-point ()
+  (interactive)
+  (let ((props (text-properties-at (point))))
+    (cond ((or (eq major-mode 'hcel-outline-mode)
+               (eq (current-buffer) eldoc--doc-buffer))
+           (hcel-find-definition-internal
+            (plist-get props 'package-id)
+            (plist-get props 'module-path)
+            (plist-get props 'internal-id)))
+          ((eq major-mode 'hcel-ids-mode)
+           (hcel-find-definition-internal
+            (alist-get 'packageId (plist-get props 'location-info))
+            (alist-get 'modulePath (plist-get props 'location-info))
+            (plist-get props 'internal-id)))
+          (t (error "%S not supported and not in eldoc doc buffer." major-mode)))))
+
+(defun hcel-find-definition-internal (package-id module-path identifier
+                                                 &optional occurrence)
+  (when (and package-id module-path (or identifier occurrence))
+    (let ((hcel-buffer (hcel-buffer-name package-id module-path)))
+      (when (or (get-buffer hcel-buffer)
+                (and (y-or-n-p "Open module source?")
+                     (hcel-load-module-source
+                      package-id module-path))))
+      (with-current-buffer hcel-buffer
+        (let ((location-info
+               (hcel-get-location-info
+                (when identifier
+                  (alist-get (intern identifier) hcel-identifiers))
+                (when occurrence
+                  (alist-get (intern occurrence) hcel-occurrences)))))
+          (when (string= (hcel-location-tag location-info) "ApproximateLocation")
+            (setq location-info (hcel-approx-to-exact-location location-info)))
+          (let ((module-path (alist-get 'modulePath location-info))
+                (line-beg (alist-get 'startLine location-info))
+                (col-beg (alist-get 'startColumn location-info))
+                (line-end (alist-get 'endLine location-info))
+                (col-end (alist-get 'endColumn location-info)))
+            (cond ((string= (hcel-location-tag location-info) "ExactLocation")
+                   (let ((pos) (len)
+                         (buffer (hcel-load-module-location-info location-info t)))
+                     (with-current-buffer buffer
+                       (save-excursion
+                         (goto-line-column line-beg col-beg)
+                         (setq pos (1- (point)))
+                         (goto-line-column line-end col-end)
+                         (setq len (- (point) pos 1))))
+                     (list (xref-make-match
+                            "hcel match"
+                            (xref-make-buffer-location buffer pos)
+                            len))))
+                  (t
+                   (error "unimplemented: %s" (hcel-location-tag location-info))))))))))
+
+;; hcel-minor mode
+(defvar hcel-minor-major-modes
+  '(hcel-outline-mode hcel-ids-mode)
+  "Major modes where hcel-minor mode can live in.")
+
+(defvar hcel-minor-mode-map
+  (let ((kmap (make-sparse-keymap)))
+    (define-key kmap (kbd "M-?") 'hcel-minor-find-references-at-point)
+    kmap))
+
+(define-minor-mode hcel-minor-mode
+  "A minor mode for exploring haskell codebases."
+  :lighter " hcel-minor"
+  :after-hook
+  (if hcel-minor-mode
+      (if (and (not (memq major-mode hcel-minor-major-modes))
+               (not (eq (current-buffer) eldoc--doc-buffer)))
+          (progn
+            (hcel-minor-mode 0)
+            (error "Not in one of the supported modes (%s) or the eldoc buffer."
+                   (string-join (mapcar 'prin1-to-string hcel-minor-major-modes)
+                                ", ")))
+        (add-hook
+         'eldoc-documentation-functions #'hcel-minor-eldoc-docs nil t)
+        (add-hook
+         'eldoc-documentation-functions #'hcel-minor-eldoc-id-type nil t)
+        (setq-local eldoc-documentation-strategy 'eldoc-documentation-compose))
+    (remove-hook 'eldoc-documentation-functions #'hcel-minor-eldoc-id-type t)
+    (remove-hook 'eldoc-documentation-functions #'hcel-minor-eldoc-docs t)))
+
+(provide 'hcel-source)
diff --git a/hcel-utils.el b/hcel-utils.el
new file mode 100644
index 0000000..18e1062
--- /dev/null
+++ b/hcel-utils.el
@@ -0,0 +1,169 @@
+;;; hc-utils.el --- Commonly used utilities -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2022 Yuchen Pei.
+;; 
+;; This file is part of hcel.
+;; 
+;; hcel is free software: you can redistribute it and/or modify it under
+;; the terms of the GNU Affero General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+;; 
+;; hcel is distributed in the hope that it will be useful, but WITHOUT
+;; ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+;; or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Affero General
+;; Public License for more details.
+;; 
+;; You should have received a copy of the GNU Affero General Public
+;; License along with hcel.  If not, see <https://www.gnu.org/licenses/>.
+
+;; data conversions
+(defun hcel-parse-package-id (package-id &optional divider)
+  (unless (stringp divider) (setq divider " "))
+  (string-match (format "^\\(.*\\)%s\\(.*\\)$" divider) package-id)
+  (list (cons 'name (match-string 1 package-id))
+        (cons 'version (match-string 2 package-id))))
+
+(defun hcel-location-tag (location-info)
+  "Gets the tag of LOCATION-INFO."
+  (alist-get 'tag location-info))
+
+(defun hcel-approx-to-exact-location (approx-location-info)
+  "Fetch exact location given approximate location.
+
+Example of approximate location:
+
+      \"locationInfo\": {
+        \"componentId\": \"exe-haskell-code-server\",
+        \"entity\": \"Typ\",
+        \"haddockAnchorId\": \"PackageInfo\",
+        \"moduleName\": \"HaskellCodeExplorer.Types\",
+        \"name\": \"PackageInfo\",
+        \"packageId\": {
+          \"name\": \"haskell-code-explorer\",
+          \"version\": \"0.1.0.0\"
+        },
+        \"tag\": \"ApproximateLocation\"
+      }"
+  (alist-get 'location
+             (hcel-definition-site-location-info approx-location-info)))
+
+(defun hcel-id-src-span-to-location-info (package-id module-path id-src-span)
+  "Converts an idSrcSpan to an exact location.
+
+Example of an idSrcSpan:
+
+        \"idSrcSpan\": {
+          \"endColumn\": 43,
+          \"line\": 228,
+          \"modulePath\": \"src/HaskellCodeExplorer/Types.hs\",
+          \"startColumn\": 26
+        }
+"
+  (list
+   (cons 'packageId package-id)
+   (cons 'modulePath module-path)
+   (cons 'startLine (alist-get 'line id-src-span))
+   (cons 'startColumn (alist-get 'startColumn id-src-span))
+   (cons 'endLine (alist-get 'line id-src-span))
+   (cons 'endColumn (alist-get 'endColumn id-src-span))
+   (cons 'tag "ExactLocation")))
+
+(defun hcel-expression (expr)
+  (when-let* ((span (alist-get 'srcSpan expr))
+              (line-beg (alist-get 'line (alist-get 'start span)))
+              (col-beg (alist-get 'column (alist-get 'start span)))
+              (line-end (alist-get 'line (alist-get 'end span)))
+              (col-end (alist-get 'column (alist-get 'end span))))
+    (buffer-substring-line-column line-beg (1- col-beg) line-end (1- col-end))))
+
+;; buffers and strings manipulation
+(defun goto-line-column (line column)
+  (goto-line line)
+  (move-to-column column))
+
+(defun hcel-unquote-html (html)
+  (replace-regexp-in-string
+   "&amp;" "&"
+   (replace-regexp-in-string
+    "&#39;" "'"
+    (replace-regexp-in-string
+     "&quot;" "\""
+     (replace-regexp-in-string
+      "&gt;" ">"
+      (replace-regexp-in-string
+       "&lt;" "<" html))))))
+
+(defun buffer-substring-line-column (line-beg col-beg line-end col-end)
+  (save-excursion
+    (buffer-substring
+     (progn (goto-line-column line-beg col-beg) (point))
+     (progn (goto-line-column line-end col-end) (point)))))
+
+(defun fontify-with-haskell-mode ()
+  "Fontify using haskell-mode"
+  (require 'haskell)
+  (let ((text (buffer-string)))
+    (with-temp-buffer
+      (haskell-mode)
+      (insert text)
+      (if (fboundp 'font-lock-ensure)
+          (font-lock-ensure)
+        (with-no-warnings (font-lock-fontify-buffer)))
+      (setq text (buffer-substring (point-min) (point-max))))
+    (erase-buffer)
+    (insert text)))
+
+(defun remove-html-markup (html)
+  (replace-regexp-in-string
+   "<.*?>" "" 
+   (replace-regexp-in-string "</p>" "\n\n" html)))
+
+(defun fill-string (text)
+  (with-temp-buffer
+    (insert text)
+    (fill-region (point-min) (point-max))
+    (buffer-substring (point-min) (point-max))))
+
+;; string formatting
+(defun hcel-format-package-id (package &optional divider)
+  (unless (stringp divider) (setq divider " "))
+  (concat (alist-get 'name package) divider (alist-get 'version package)))
+
+(defun hcel-render-components (components &optional name)
+  (when (or components name)
+    (concat (when name (replace-regexp-in-string "\n" " " name))
+            (when components
+              (concat (when name " :: ")
+                      (replace-regexp-in-string
+                       "\n" " " (mapconcat
+                                 (lambda (component)
+                                   (propertize
+                                    (or (alist-get 'name component)
+                                        (alist-get 'contents component))
+                                    'internal-id (alist-get 'internalId component)))
+                                 components
+                                 "")))))))
+
+(defun hcel-render-id-type (id-type)
+  (concat
+   (hcel-render-components (alist-get 'components id-type))
+   (when-let ((expanded (hcel-render-components
+                         (alist-get 'componentsExpanded id-type))))
+     (concat "\nExpands to: " expanded))))
+
+(defun hcel-expression-and-type (expr)
+  "Returns a pair of expression text and its type text."
+  (when-let ((expression (hcel-expression expr))
+             (type (hcel-render-id-type
+                    (alist-get 'exprType (alist-get 'info expr)))))
+    (cons expression type)))
+
+(defun hcel-render-html (html)
+  (when html
+    (with-temp-buffer
+      (insert html)
+      (shr-render-region (point-min) (point-max))
+      (buffer-string))))
+
+(provide 'hcel-utils)