Unexpected ambiguity in a seemingly valid Haskell 2010 program

Roman Cheplyaka roma at ro-che.info
Fri Nov 9 18:09:41 CET 2012

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:

        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.


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