Re: Unexpected ambiguity in a seemingly valid Haskell 2010 program

Apparently not — the code comilers with any of -XNoMonoLocalBinds and -XMonoLocalBinds, but not with -XNoMonomorphismRestriction. * wagnerdm@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 :

...

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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