#5892: Compiler crash and other oddities when using RecordWildCards with fields
not imported
--------------------------------+-------------------------------------------
Reporter: parcs | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 7.4.1 | Keywords:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: Compile-time crash | Testcase:
Blockedby: | Blocking:
Related: |
--------------------------------+-------------------------------------------
GHC exhibits an irrefutable pattern failure when attempting to compile or
interpret a module that uses the RecordWildCards extension to bind a
record's field names when its field accessors are not in scope.
Error:
{{{
*** Exception: compiler/rename/RnPat.lhs:535:39-79: Irrefutable pattern
failed for pattern gres@(gre : _)
}}}
Examples:
{{{
{-# LANGUAGE RecordWildCards #-}
import Data.Tree (Tree(Node))
-- Note that Node's record accessors 'subForest' and 'rootLabel' are not
imported.
Node{..} = Node () []
}}}
{{{
{-# LANGUAGE RecordWildCards #-}
import Data.Tree (Tree(Node))
foo = let rootLabel = (); subForest = [] in Node{..}
}}}
With GHC HEAD + a trivial patch for RnPat.lhs the programs compiled but
other oddities appeared. With the patch, the first program does not have
the names 'subForest' and 'rootLabel' in scope, and the second program
causes GHCi to complain that the names 'rootLabel' and 'subForest' are
unused, and that the fields 'Data.Tree.rootLabel' and
'Data.Tree.subForest' are not initialized in the constructor 'Node'.
I'm not sure what the correct behavior should be in these edge cases but
thankfully I found a workaround in my program. (I was using
RecordWildCards to extract the fields of Text.Parsec.Token.TokenParser on
to the top level. If I don't import its record accessors I get the
described error, and if I import the accessors unqualified I get ambiguous
name errors, so instead I just import them qualified.)
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5892>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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