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

[Bernie Pope]

On 23/09/2006, at 4:33 AM, Christian Sievers wrote:

> Hello,
>
> I don't take my advice to go to haskell-cafe  :-)

I will take your advice :)

>
> The discussion continued outside the mailing list, and now I have
> two questions myself:
>
> 1. Why do the rules of the monomorphism restriction explicitly mention
>    *simple* pattern bindings?
>    Where is the difference, especially as there is a translation to
>    simple pattern bindings?
>    Why should
>
>    p | "a"=="b"  = 2
>      | otherwise = 3
>
>    be treated different than
>
>    p = if "a"=="b" then 2 else 3


They are the same (both are simple pattern bindings). The report says  
in section 4.4.3.2 that the first can be translated into the second.

A simple pattern binding is one where the lhs is a variable only.

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.

Cheers,
Bernie.