Unexpected ambiguity in a seemingly valid Haskell 2010 program

[Roman Cheplyaka]

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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