Fix Trac #2490: sections should be parenthesised

[ghc-hetmet.git] / compiler / parser / Parser.y.pp

diff --git a/compiler/parser/Parser.y.pp b/compiler/parser/Parser.y.pp
index 86ce98c..67b2dca 100644 (file)
--- a/compiler/parser/Parser.y.pp
+++ b/compiler/parser/Parser.y.pp
@@ -1429,16 +1429,27 @@ cvtopdecls0 :: { [LHsDecl RdrName] }
        : {- empty -}           { [] }
        | cvtopdecls            { $1 }
 
--- tuple expressions: things that can appear unparenthesized as long as they're
+-- "texp" is short for tuple expressions: 
+-- things that can appear unparenthesized as long as they're
 -- inside parens or delimitted by commas
 texp :: { LHsExpr RdrName }
        : exp                           { $1 }
-       -- Technically, this should only be used for bang patterns,
-       -- but we can be a little more liberal here and avoid parens
-       -- inside tuples
-       | infixexp qop  { LL $ SectionL $1 $2 }
+
+       -- Note [Parsing sections]
+       -- ~~~~~~~~~~~~~~~~~~~~~~~
+       -- We include left and right sections here, which isn't
+       -- technically right according to Haskell 98.  For example
+       --      (3 +, True) isn't legal
+       -- However, we want to parse bang patterns like
+       --      (!x, !y)
+       -- and it's convenient to do so here as a section
+        -- Then when converting expr to pattern we unravel it again
+       -- Meanwhile, the renamer checks that real sections appear
+       -- inside parens.
+        | infixexp qop         { LL $ SectionL $1 $2 }
        | qopm infixexp       { LL $ SectionR $1 $2 }
-       -- view patterns get parenthesized above
+
+       -- View patterns get parenthesized above
        | exp '->' exp   { LL $ EViewPat $1 $3 }
 
 texps :: { [LHsExpr RdrName] }