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

[Bernie Pope]

On 26/09/2006, at 12:19 AM, Christian Sievers wrote:

> Bernie Pope answered:
>
>>> 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.
>
> Indeed, I meant to allude to this translation.
>
>> A simple pattern binding is one where the lhs is a variable only.
>
> That's consistent with the second reason for rule one of the MR.
>
> However, the mentioned section 4.4.3.2 defines it differently:
>
>    A simple pattern binding has form p = e.
>
> And if there is any doubt about what p stands for, it goes on:
>
>    The pattern p ...
>
> Contrasting to that:
>
>    The general form of a pattern binding is p match, where a match  
> is the same
>    structure as for function bindings above; in other words, a  
> pattern binding
>    is:
>
>    p 	| g1 	= e1
> 	| g2 	= e2
> 	...
> 	| gm 	= em
> 	where { decls }
>
>
> So according to this definition, a pattern binding is simple iff
> there are no guards (unless they are in the expression).
> Also the translation to a "simple pattern binding" only gets rid of  
> guards.
>
> So there seems to be an error in the report, which can be fixed by  
> either
> redefining "simple pattern binding", or using a differnet  
> description in the
> MR.

Aha,

Christian I see what you mean. It seems I did not read section  
4.4.3.2 carefully. In fact
I think I was interpreting that section in light of the MR.

So I am now as puzzled as you are.

Anyway, thanks for persisting on this point.

Cheers,
Bernie.