[Haskell-cafe] Re: [Haskell] MR details (was: Implicit type of numeric constants)

[Bernie Pope]

On 24/09/2006, at 1:46 AM, Michael Shulman wrote:

> On 9/23/06, Bernie Pope <bjpop at csse.unimelb.edu.au> wrote:
>> If a pattern binding is not simple, it must have a data constructor
>> on the lhs, therefore it cannot be overloaded. So the (dreaded) MR  
>> only
>> applies to simple pattern bindings.
>
> I thought it was simple pattern bindings that could be *exempted* from
> the MR by supplying an explicit type signature, while non-simple
> pattern bindings are always subject to it.
>

Actually, I realised after I posted my message that what I wrote was  
dubious. I'm sorry about that, please ignore it.

This section from the Report seems to clear things up:

In "Motivation", section 4.5.5:

"Rule 1 prevents ambiguity. For example, consider the declaration group

   [(n,s)] = reads t

Recall that reads is a standard function whose type is given by the  
signature

   reads :: (Read a) => String -> [(a,String)]

Without Rule 1, n would be assigned the type forall a. Read a =>a and  
s the type forall a. Read a =>String. The latter is an invalid type,  
because it is inherently ambiguous. It is not possible to determine  
at what overloading to use s, nor can this be solved by adding a type  
signature for s. Hence, when non-simple pattern bindings are used  
(Section 4.4.3.2), the types inferred are always monomorphic in their  
constrained type variables, irrespective of whether a type signature  
is provided. In this case, both n and s are monomorphic in a."

Sorry for  the confusion,
Bernie.