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.
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