#4875: ghc misdiagnoses a compile time error concerning parameterized types
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Reporter: Stef Joosten | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 6.12.3 | Keywords:
Testcase: | Blockedby:
Os: Windows | Blocking:
Architecture: Unknown/Multiple | Failure: Other
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Please find the script HaskellBug.hs at the bottom of this message.
Here is how to reproduce the behaviour in two steps.
Step 1: verify that Module HaskellBug produces the following error
message:
[1 of 1] Compiling HaskellBug ( \HaskellBug.hs, interpreted )
\HaskellBug.hs:12:22:
`rel' is not applied to enough type arguments
Expected kind `??', but `rel' has kind `* -> *'
In the type `rel -> concept'
In the class declaration for `Association'
Step 2: remove the definition of class Morphic r c and verify that the
script is now error free.
You have now reproduced the symptoms.
Assessment:
In this situation, ghc has diagnosed an error in the definition of
class Association. This error was resolved by removing another
definition, without altering the definition of class Association.
This proves that the definition of class Association was correct in the
first place. Therefore, ghc has made a mistake. It has diagnosed
the error in the wrong location. I whould have expected an error
message like
\HaskellBug.hs:23:22:
`r' is applied to too many type arguments
Expected <something sensible here>
In the type `r -> c'
In the class declaration for `Morphic'
Epilog:
I have taken the trouble to isolate the problem in a small script,
because this error has caused serious havoc in a large Haskell
project. Students, who were still learning about parameterized
types, have systematically been 'fixing' the wrong code, because
ghc has never pointed in the direction of the real error. In the
end, they were unable to resolve their mistake.
Besides haarvesting a frustrating experience with parameterized
types, wasting two entire days before giving up, they have turned a
large script upside down and introduced many more mistakes. That is
why I think it is important to get this fixed.
Here is the script:
{{{
{-# OPTIONS -XMultiParamTypeClasses -XFunctionalDependencies
-XFlexibleInstances #-}
module HaskellBug where
data Relation c -- The basic Relation
= Rel { relnm :: String -- The name of the relation
, relsrc :: c -- Source concept
, reltrg :: c -- ^Target concept
}
deriving Eq
class (Eq concept)=> Association rel concept | rel -> concept where
source, target :: rel -> concept -- e.g. Declaration Concept ->
Concept
sign :: rel -> (concept,concept)
sign x = (source x,target x)
homogeneous :: rel -> Bool
homogeneous s = source s == target s
instance (Eq c)=>Association (Relation c) c where
source = relsrc
target = reltrg
class (Eq c, Association r c) => Morphic r c where
multiplicities :: r c -> [c]
multiplicities _ = []
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4875>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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