Fix explicit forall in instances (closes #218)

TESTS.md

@@ -49,6 +49,21 @@ Type declaration
 type EventSource a = (AddHandler a, a -> IO ())
 ```
 
+Instance declaration without decls
+
+``` haskell
+instance C a
+```
+
+Instance declaration with decls
+
+``` haskell
+instance C a where
+    foobar = do
+        x y
+        k p
+```
+
 # Expressions
 
 Lazy patterns in a lambda
@@ -364,7 +379,7 @@ main = putStrLn "Hello, World!"
 
 ## Regression tests
 
-cocreature removed from declaration issue
+cocreature removed from declaration issue #186
 
 ``` haskell
 -- https://github.com/chrisdone/hindent/issues/186
@@ -376,6 +391,16 @@ trans One e n =
          })
 ```
 
+sheyll explicit forall in instances #218
+
+``` haskell
+-- https://github.com/chrisdone/hindent/issues/218
+instance forall x. C
+
+instance forall x. Show x =>
+         C x
+```
+
 # Behaviour checks
 
 Unicode

src/HIndent/Pretty.hs

@@ -771,7 +771,7 @@ decl (InlineSig _ inline _ name) = do
   pretty name
 
   write " #-}"
-decl x = pretty' x
+decl x' = pretty' x'
 
 instance Pretty Deriving where
   prettyInternal (Deriving _ heads) =
@@ -973,7 +973,9 @@ instance Pretty InstRule where
   prettyInternal (IRule _ mvarbinds mctx ihead) =
     do case mvarbinds of
          Nothing -> return ()
-         Just xs -> spaced (map pretty xs)
+         Just xs -> do write "forall "
+                       spaced (map pretty xs)
+                       write ". "
        withCtx mctx (pretty ihead)
 
 instance Pretty InstHead where