Re: Unexpected ambiguity in a seemingly valid Haskell 2010 program

11 Nov 2012

      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  [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  wrote:
...
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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