Unexpected ambiguity in a seemingly valid Haskell 2010 program

[Roman Cheplyaka]

Right. What I meant is that with -XMonomorphismRestriction, it compiles
with with both -XMonoLocalBinds and -XNoMonoLocalBinds.

That means that MonoLocalBinds can not be solely responsible for this
behaviour.

Anyway, I just noticed that a very similar example (using Read) is
described in the Haskell report's section on the monomorphism
restriction.

Roman

* Erik Hesselink <hesselink at gmail.com> [2012-11-11 16:43:20+0100]
> That's strange. Here, it only fails with both NoMonomorphismRestriction and
> NoMonoLocalBinds (which makes sense). I've tested on 7.4.1 and 7.6.1.
> 
> Erik
> 
> 
> On Sun, Nov 11, 2012 at 3:54 PM, Roman Cheplyaka <roma at ro-che.info> wrote:
> 
> > 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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