From 19c77b1b65d580cd978b5c1c572445f24efe171a Mon Sep 17 00:00:00 2001
From: Daniel Gröber <dxld@darkboxed.org>
Date: Sun, 31 Mar 2019 00:17:36 +0100
Subject: Promote 'Ex' to exported API

We will need it for the project discovery module later.
---
 lib/Distribution/Helper.hs           |  1 +
 src/CabalHelper/Compiletime/Types.hs | 30 +++++++++++++++++++++++++++++-
 tests/GhcSession.hs                  |  2 --
 3 files changed, 30 insertions(+), 3 deletions(-)

diff --git a/lib/Distribution/Helper.hs b/lib/Distribution/Helper.hs
index f38d6d5..bb1f67f 100644
--- a/lib/Distribution/Helper.hs
+++ b/lib/Distribution/Helper.hs
@@ -68,6 +68,7 @@ module Distribution.Helper (
   , SProjType(..)
   , demoteSProjType
   , ProjLoc(..)
+  , Ex(..)
 
   , Programs(..)
   , defaultPrograms
diff --git a/src/CabalHelper/Compiletime/Types.hs b/src/CabalHelper/Compiletime/Types.hs
index 6b7d74a..9911aec 100644
--- a/src/CabalHelper/Compiletime/Types.hs
+++ b/src/CabalHelper/Compiletime/Types.hs
@@ -15,7 +15,7 @@
 -- along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 {-# LANGUAGE DeriveGeneric, DeriveDataTypeable, DefaultSignatures,
-  StandaloneDeriving, GADTs, DataKinds, KindSignatures, RankNTypes #-}
+  StandaloneDeriving, GADTs, DataKinds, KindSignatures, RankNTypes, PolyKinds #-}
 
 {-|
 Module      : CabalHelper.Compiletime.Types
@@ -105,6 +105,34 @@ data DistDir (pt :: ProjType) where
 
 deriving instance Show (DistDir pt)
 
+-- | General purpose existential wrapper. Useful for hiding a phantom type
+-- argument.
+--
+-- Say you have:
+--
+-- @
+-- {-# LANGUAGE DataKinds, GADTS #-}
+-- data K = A | B | ...
+-- data Q k where
+--   QA :: ... -> Q 'A
+--   QB :: ... -> Q 'B
+-- @
+--
+-- and you want a list of @Q@. You can use @Ex@ to hide the phantom type
+-- argument and recover it later by matching on the GADT constructors:
+--
+-- @
+-- qa :: Q A
+-- qa = QA
+--
+-- qb :: Q B
+-- qb = QB
+--
+-- mylist :: [Ex Q]
+-- mylist = [Ex qa, Ex qb]
+-- @
+data Ex a = forall x. Ex (a x)
+
 -- | Environment for running a 'Query'. The constructor is not exposed in the
 -- API to allow extending it with more fields without breaking user code.
 --
diff --git a/tests/GhcSession.hs b/tests/GhcSession.hs
index 46ccab9..2fda204 100644
--- a/tests/GhcSession.hs
+++ b/tests/GhcSession.hs
@@ -159,8 +159,6 @@ testLocPath (TN test_name) = (projdir, ".", cabal_file)
 testLocPath (TF topdir projdir cabal_file) =
     (topdir, projdir, cabal_file)
 
-data Ex a = forall x. Ex (a x)
-
 -- | Check version bounds of tests against available versions, if successful run
 -- the test.
 checkAndRunTestConfig
-- 
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