Unexpected ambiguity in a seemingly valid Haskell 2010 program
Roman Cheplyaka
roma at ro-che.info
Sun Nov 11 15:54:42 CET 2012
Apparently not — the code comilers with any of -XNoMonoLocalBinds and
-XMonoLocalBinds, but not with -XNoMonomorphismRestriction.
* wagnerdm at seas.upenn.edu <wagnerdm at seas.upenn.edu> [2012-11-09 14:07:59-0500]
> It's possible that the below blog post is related.
> ~d
>
> http://hackage.haskell.org/trac/ghc/blog/LetGeneralisationInGhc7
>
> Quoting Roman Cheplyaka <roma at ro-che.info>:
>
> >For this module
> >
> > module Test where
> >
> > import System.Random
> >
> > data RPS = Rock | Paper | Scissors deriving (Show, Enum)
> >
> > instance Random RPS where
> > random g =
> > let (x, g') = randomR (0, 2) g
> > in (toEnum x, g')
> > randomR = undefined
> >
> >ghc (7.4.1 and 7.6.1) reports an error:
> >
> > rand.hs:9:9:
> > No instance for (Random t0) arising from the ambiguity check for g'
> > The type variable `t0' is ambiguous
> > Possible fix: add a type signature that fixes these type variable(s)
> > Note: there are several potential instances:
> > instance Random RPS -- Defined at rand.hs:7:10
> > instance Random Bool -- Defined in `System.Random'
> > instance Random Foreign.C.Types.CChar -- Defined in `System.Random'
> > ...plus 34 others
> > When checking that g' has the inferred type `g'
> > Probable cause: the inferred type is ambiguous
> > In the expression: let (x, g') = randomR (0, 2) g in (toEnum x, g')
> > In an equation for `random':
> > random g = let (x, g') = randomR ... g in (toEnum x, g')
> > Failed, modules loaded: none.
> >
> >There should be no ambiguity since 'toEnum' determines the type of x
> >(Int), and that in turn fixes types of 0 and 2. Interestingly,
> >annotating 0 or 2 with the type makes the problem go away.
> >
> >jhc 0.8.0 compiles this module fine.
> >
> >Roman
> >
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> >
> >
>
>
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