[Haskell-beginners] Ambiguous MonadIO and Monad

[Daniel Fischer]

On Wednesday 07 March 2012, 00:42:05, Ken KAWAMOTO wrote:
> Hi,
> 
> I'm trying to understand how Network.OAuth.Consumer works, and
> run sample code found here with Twitter API. (the full code is
> attached at the bottom)
> http://hackage.haskell.org/packages/archive/hoauth/0.3.3/doc/html/Networ
> k-OAuth-Consumer.html
> 
> When I load the test code with GHCi 7.0.2, it raises error:
> 
> --- Error message begin
> TestOAuth.hs:21:103:
>     Ambiguous type variable `m0' in the constraints:
>       (MonadIO m0) arising from a use of `serviceRequest'
>                    at TestOAuth.hs:21:103-116
>       (Monad m0) arising from a use of `>>=' at TestOAuth.hs:21:99-101
>     Possible cause: the monomorphism restriction applied to the
> following: response :: m0 Response (bound at TestOAuth.hs:17:1)
>     Probable fix: give these definition(s) an explicit type signature
>                   or use -XNoMonomorphismRestriction
>     In the second argument of `(>>=)', namely

> 
> I seem using NoMonomorphismRestriction is workaround, so I want to add
> a proper type signature, but I cannot figure out what it is.
> 
> What confuses me is why GHC didn't just take MonadIO as MonadIO is a
> Monad according to the definition.

From the definition of response, the two constraints

MonadIO m0
Monad m0

are inferred. Since response is bound by a simple pattern binding (no 
function arguments) and without a type signature, by the monomorphism 
restriction, it must get a monomorphic type, so the compiler tries to find 
a specific MonadIO (and Monad, but that would follow) to use. But there is 
no defaulting for those constraints (defaulting takes place for constraints 
where at least one of the involved classes is a numeric class [Num, 
Integral, Fractional, ...], and all involved classes are defined in the 
standard libraries), so that fails (no numeric class, and MonadIO is 
defined outside the standard libraries).

To fix it, you just have to define response with a type signature,

response :: MonadIO m => m Response

You should give all your top-level definitions type signatures anyway. It's 
good for documentation, and it gives you much better error messages if you 
make a mistake.