Unexpected ambiguity in a seemingly valid Haskell 2010 program

[Roman Cheplyaka]

Oops, you are right — I had -XNoMonomorphismRestriction in .ghci.

I'm not sure whether this fact makes the situation more or less strange :)

Roman

* Nicolas Frisby <nicolas.frisby at gmail.com> [2012-11-09 11:35:58-0600]
> My GHC 7.6.1 (on a Mac) compiles this code without any warnings or errors.
> 
> Do you have some other compilation flags in effect?
> 
> 
> On Fri, Nov 9, 2012 at 11:09 AM, Roman Cheplyaka <roma at ro-che.info> wrote:
> 
> > 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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> >