Simon Peyton-Jones wrote:
| What have you got in mind? ANY tupling of bindings may change the SCC
| structure, and hence the results of type inference--I'm taking that as
| read. But that still leaves the question of whether the dynamic
| semantics of the program is changed. Let's assume for the time being
| that all bindings carry a type signature--then the SCC structure is
| irrelevant, isn't it? Or am I missing something here? I'm under the
| impression that the *dynamic* semantics of
|
| p1 = e1
| p2 = e2
|
| *would* be the same as (p1,p2) = (e1,e2) under my strict matching
| proposal. I don't see how the SCC structure can affect that.
Well I put the example that you sent me on the Wiki, right at the
bottom. Did I get it wrong?
let { (y:ys) = xs; (z:zs) = ys } in body
means
case xs of (y:ys) -> case ys of (z:zs) -> body
whereas
let (y:ys, z:zs) = (xs,ys) in body
means
case (fix (\~(y:ys, z:zs). (xs,ys))) of (y:ys, z:zs) -> body
which isn't the same.
Simon
Oh yes, you're right of course.
In the denotational semantics I wrote last night, multiple bindings are
combined using (+), which *is* the same as tupling them. But remember
the thing I left unproven, because it was late at night?
E[[let defs1 in let defs2 in exp]]env = E[[let defs1; defs2 in exp]]env
It's not true, as this example shows. That'll teach me! In
let y:ys = xs; z:zs = ys in body
then the result is _|_, because matching the entire *group* against (xs, _|_)
fails, but once the example is split into two nested lets then everything
works. Yuck.
John
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