Unexpected ambiguity in a seemingly valid Haskell 2010 program
roma at ro-che.info
Fri Nov 9 18:09:41 CET 2012
For this module
module Test where
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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