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

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

On 9/23/06, Bernie Pope 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.