#3176: erroneous defaulting? behaviour for existentials
-----------------------------+----------------------------------------------
Reporter: claus | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 6.11 | Severity: normal
Keywords: | Testcase:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
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consider
{{{
{-# LANGUAGE NoExtendedDefaultRules #-}
{-# OPTIONS_GHC -fwarn-type-defaults #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ExistentialQuantification #-}
data EShow = forall a. Show a => EShow a
smallPrint t = concatMap (\f-> case f t of EShow a -> show a) [EShow .
foo, EShow . bar, EShow . baz]
data ES = forall a. Show a => ES {unES:: a}
smallPrintES t = concatMap (\f-> show $ unES $ f t) [ES . foo, ES . bar,
ES . baz]
data Test = Test { foo :: Int, bar :: Char, baz :: Bool }
main = print $ smallPrintES $ Test 1 'x' False
}}}
for which
{{{
$ /cygdrive/c/ghc/ghc-6.11.20090320/bin/ghc -e "main" exists.hs
"()()()"
}}}
while
{{{
$ /cygdrive/c/ghc/ghc-6.8.3/bin/ghc -e "main" exists.hs
exists.hs:12:40:
Cannot use record selector `unES' as a function due to escaped type
variables
Probably fix: use pattern-matching syntax instead
In the first argument of `($)', namely `unES'
In the second argument of `($)', namely `unES $ f t'
In the expression: show $ unES $ f t
}}}
The expected results were error message or this
{{{
$ /cygdrive/c/ghc/ghc-6.11.20090320/bin/ghc -e "smallPrint $ Test 1 'x'
False" exists.hs
"1'x'False"
}}}
There seem to be two issues:
- shouldn't ghc head report the use of `unES` as an error?
- how does ghc head arrive at that result, with these flags, without any
warnings?
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3176>
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