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 <r...@ro-che.info> wrote: > Apparently not — the code comilers with any of -XNoMonoLocalBinds and > -XMonoLocalBinds, but not with -XNoMonomorphismRestriction. > > * wagne...@seas.upenn.edu <wagne...@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 <r...@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 > > > > > >_______________________________________________ > > >Glasgow-haskell-users mailing list > > >Glasgow-haskell-users@haskell.org > > >http://www.haskell.org/mailman/listinfo/glasgow-haskell-users > > > > > > > > > > > > _______________________________________________ > > Glasgow-haskell-users mailing list > > Glasgow-haskell-users@haskell.org > > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users > > _______________________________________________ > Glasgow-haskell-users mailing list > Glasgow-haskell-users@haskell.org > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users >
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