#4338: weird discrepancies between TFs and FDs in GHC7
--------------------------+-------------------------------------------------
Reporter: illissius | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler (Type checker)
Version: 6.13 | Keywords:
Testcase: | Blockedby:
Os: Linux | Blocking:
Architecture: x86 | Failure: Compile-time crash
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I'm trying to do some seemingly equivalent code with GHC7 as of 09/19,
using !TypeFamilies on the one hand and !FunctionalDependencies on the
other, and my experience is that the TFs version results in some really
weird-ass error messages from the compiler -- and a hang in one case --
whereas the FDs version works just fine. I'm not sure about the errors,
though they certainly seem bizarre, but I'm pretty sure the compiler
hanging is a bug. (And I assume a hang is morally equivalent to a crash,
so I'm marking this as such.)
Here's the version with TFs:
{{{
{-# LANGUAGE MultiParamTypeClasses, TypeFamilies, FlexibleContexts #-}
class (There a ~ b, BackAgain b ~ a) => Foo a b where
type There a
type BackAgain b
there :: a -> b
back :: b -> a
tickle :: b -> b
instance Foo Char Int where
type There Char = Int
type BackAgain Int = Char
there = fromEnum
back = toEnum
tickle = (+1)
test :: (Foo a b) => a -> a
test = back . tickle . there
main :: IO ()
main = print $ test 'F'
}}}
and the one with FDs:
{{{
{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies #-}
class Bar a b | a -> b, b -> a where
there :: a -> b
back :: b -> a
tickle :: b -> b
instance Bar Char Int where
there = fromEnum
back = toEnum
tickle = (+1)
test :: (Bar a b) => a -> a
test = back . tickle . there
main :: IO ()
main = print $ test 'F'
}}}
Are these as functionally-equivalent as they seem, or are there some
subtle differences I'm missing? (Is it possible there's some kind of
configuration problem on my end?)
In any case, the result is that the TFs version gives me different errors
depending on which type signatures I supply or omit, whereas the version
with FDs compiles and works correctly in all cases.
The TFs version, if I supply both type signatures (as listed):
{{{
$ ghc Foo.hs
[1 of 1] Compiling Main ( Foo.hs, Foo.o )
Foo.hs:18:15:
Could not deduce (Foo (BackAgain (There a)) (There a))
from the context (Foo a b)
arising from a use of `tickle'
Possible fix:
add (Foo (BackAgain (There a)) (There a)) to the context of
the type signature for `test'
or add an instance declaration for
(Foo (BackAgain (There a)) (There a))
In the first argument of `(.)', namely `tickle'
In the second argument of `(.)', namely `tickle . there'
In the expression: back . tickle . there
Foo.hs:21:16:
Overlapping instances for Foo Char Int
arising from a use of `test'
Matching instances:
instance Foo Char Int -- Defined at Foo.hs:10:10-21
(The choice depends on the instantiation of `'
To pick the first instance above, use -XIncoherentInstances
when compiling the other instance declarations)
In the second argument of `($)', namely `test 'F''
In the expression: print $ test 'F'
In an equation for `main': main = print $ test 'F'
}}}
If I leave off the type signature for main, but not test:
{{{
$ ghc Foo.hs
[1 of 1] Compiling Main ( Foo.hs, Foo.o )
Foo.hs:18:15:
Could not deduce (Foo (BackAgain (There a)) (There a))
from the context (Foo a b)
arising from a use of `tickle'
Possible fix:
add (Foo (BackAgain (There a)) (There a)) to the context of
the type signature for `test'
or add an instance declaration for
(Foo (BackAgain (There a)) (There a))
In the first argument of `(.)', namely `tickle'
In the second argument of `(.)', namely `tickle . there'
In the expression: back . tickle . there
}}}
If I leave off the signature for test, regardless of whether I supply one
for main:
{{{
$ ghc Foo.hs
[1 of 1] Compiling Main ( Foo.hs, Foo.o )
^C
-- a seemingly infinite loop
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4338>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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