Re: Unexpected ambiguity in a seemingly valid Haskell 2010 program

11 Nov 2012


      That makes sense: MonomorphismRestriction makes bindings without parameters
monomorphic, and MonoLocalBinds makes local bindings monomorphic. So either
one will make this binding monomorphic. Only when both are off does it
become polymorphic and does the error occur.

Erik


On Sun, Nov 11, 2012 at 5:37 PM, Roman Cheplyaka  wrote:
...
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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