[commit: ghc] master: Add comment to Parser.y re extra API Annotation (638fde5)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/638fde50365f29bc82fdf7058ab966a33d19e5ca/ghc

>---------------------------------------------------------------

commit 638fde50365f29bc82fdf7058ab966a33d19e5ca
Author: Alan Zimmerman <alan.zimm at gmail.com>
Date:   Sun Nov 22 21:18:24 2015 +0200

    Add comment to Parser.y re extra API Annotation
    
    Document extra AnnRarrow annotation on a HsFunTy


>---------------------------------------------------------------

638fde50365f29bc82fdf7058ab966a33d19e5ca
 compiler/parser/Parser.y | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/compiler/parser/Parser.y b/compiler/parser/Parser.y
index 7b40574..e761828 100644
--- a/compiler/parser/Parser.y
+++ b/compiler/parser/Parser.y
@@ -1607,12 +1607,28 @@ context :: { LHsContext RdrName }
                                                    else return ()
                                                 ; ams ctx anns
                                                 } }
+{- Note [GADT decl discards annotations]
+~~~~~~~~~~~~~~~~~~~~~
+The type production for
+
+    btype `->` btype
+
+adds the AnnRarrow annotation twice, in different places.
+
+This is because if the type is processed as usual, it belongs on the annotations
+for the type as a whole.
+
+But if the type is passed to mkGadtDecl, it discards the top level SrcSpan, and
+the top-level annotation will be disconnected. Hence for this specific case it
+is connected to the first type too.
+-}
+
 -- See Note [Parsing ~]
 type :: { LHsType RdrName }
         : btype                         { splitTilde $1 }
         | btype qtyconop type           { sLL $1 $> $ mkHsOpTy $1 $2 $3 }
         | btype tyvarop  type           { sLL $1 $> $ mkHsOpTy $1 $2 $3 }
-        | btype '->'     ctype          {% ams $1 [mu AnnRarrow $2]
+        | btype '->'     ctype          {% ams $1 [mu AnnRarrow $2] -- See note [GADT decl discards annotations]
                                         >> ams (sLL $1 $> $ HsFunTy (splitTilde $1) $3)
                                                [mu AnnRarrow $2] }
         | btype SIMPLEQUOTE qconop type  {% ams (sLL $1 $> $ mkHsOpTy $1 $3 $4)