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.