Fix: Generate pattern signatures for constructors exported as patterns (#663)

* Fix pretty-printing of pattern signatures

Pattern synonyms can have up to two contexts, both having a
different semantic meaning: The first holds the constraints
required to perform the matching, the second contains the
constraints provided by a successful pattern match. When the
first context is empty but the second is not it is necessary
to render the first, empty context.

* Generate pattern synonym signatures for ctors exported as patterns

This fixes #653.

* Simplify extractPatternSyn

It is not necessary to generate the simplest type signature since
it will be simplified when pretty-printed.

* Add changelog entries for PR #663

* Fix extractPatternSyn error message

2 files changed, 33 insertions, 1 deletions

diff --git a/html-test/src/ConstructorPatternExport.hs b/html-test/src/ConstructorPatternExport.hs
new file mode 100644
index 00000000..7897b4bc
--- /dev/null
+++ b/html-test/src/ConstructorPatternExport.hs
@@ -0,0 +1,26 @@
+{-# LANGUAGE PatternSynonyms #-}
+{-# LANGUAGE ExistentialQuantification #-}
+{-# LANGUAGE GADTs #-}
+{-# LANGUAGE KindSignatures #-}
+
+module ConstructorPatternExport (
+    pattern FooCons
+  , pattern MyRecCons
+  , pattern (:+)
+  , pattern BlubCons
+  , pattern MyGADTCons
+  ) where
+
+data Foo a = FooCons String a
+
+data MyRec = MyRecCons { one :: Bool, two :: Int }
+
+data MyInfix a = String :+ a
+
+data Blub = forall b. Show b => BlubCons b
+
+data MyGADT :: * -> * where
+  MyGADTCons :: forall a. Eq a => a -> Int -> MyGADT (Maybe String)
+
+pattern MyGADTCons' :: () => forall a. Eq a => a -> Int -> MyGADT (Maybe String)
+pattern MyGADTCons' x y = MyGADTCons x y
\ No newline at end of file
diff --git a/html-test/src/PatternSyns.hs b/html-test/src/PatternSyns.hs
index a8de113c..bf0f7848 100644
--- a/html-test/src/PatternSyns.hs
+++ b/html-test/src/PatternSyns.hs
@@ -1,4 +1,4 @@
-{-# LANGUAGE PatternSynonyms, PolyKinds, TypeOperators #-}
+{-# LANGUAGE ExistentialQuantification, PatternSynonyms, PolyKinds, TypeOperators #-}
 
 -- | Testing some pattern synonyms
 module PatternSyns where
@@ -15,6 +15,12 @@ pattern Bar x = FooCtor (Foo x)
 -- | Pattern synonym for (':<->')
 pattern x :<-> y = (Foo x, Bar y)
 
+-- | BlubType is existentially quantified
+data BlubType = forall x. Show x => BlubCtor x
+
+-- | Pattern synonym for 'Blub' x
+pattern Blub x = BlubCtor x
+
 -- | Doc for ('><')
 data (a :: *) >< b = Empty