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

Bernie Pope bjpop at csse.unimelb.edu.au
Sat Sep 23 08:28:15 EDT 2006

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 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.


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