Simon Peyton-Jones wrote:
| The report doesn't actually mention this translation although it is
| widely used to implement pattern bindings, and in some compilers (not
| GHC) the translation is done before type checking.
|
| What's interesting to me is that perhaps this gives us a way to
| understand what the static semantics of pattern bindings should be,
| absent MPB. e.g.
Yes, that's a fine point. If this became the formal definition of the *static*
semantics of pattern bindings, that would be a significant improvement, because
it'd give us a precise way to answer the various questions I asked. (We might
or might not like the answers, but at least we could answer them.)
Ok. So I counter-propose that we deal with pattern bindings like this:
The static semantics of a pattern binding are given by the following
translation. A binding 'p = e' has the same meaning as the set of
bindings
z = e
x1 = case z of { p -> x1 }
...
xn = case z of { p -> xn }
where z is fresh, and x1..xn are the variables of the pattern p.
For bang patterns, I think it suffices to say that a bang at the top
level of p is carried to the binding for z, and then separately define
what banged variable bindings mean (i.e. add appropriate seqs).
Does anyone see any problems with this?
Oh, and I also propose to use the terminology "variable binding" instead
of "simple pattern binding", which is currently used inconsistently in
the report (see section 4.4.3.2).
Cheers,
Simon
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