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proposed extenion to patterns
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Date: -- (:)
From: skaller <skaller@u...>
Subject: proposed extenion to patterns
Pattern matches in Ocaml now allow alternatives within 
a branch subject to a restriction that all alternatives
supply the same set of pattern variables (and of the same type).

This is useful but still quite restrictive, for example:

| A (i,j) | B i with j = 1 -> i + j

cannot be coded, because there's no way to introduce
the variable j.

However this isn't really general enough either,
because it can't HIDE variables!

This form looks more general:

| in A (h,k) let i = h and k = k
| in B s let i = s + 1 let k = i

These forms are inverted let/in constructions,
with all the equivalent options (and, rec, and sequence).

The variables h,k in the first alternative are hidden.

The implementation is roughly syntactic sugar:

match x with A (h,k) -> let i = h and k = k in i,k

The 'with' form is a simplification, as is the
usual form. 

Any comments on this idea?


-- 
John Skaller <skaller at users dot sf dot net>
Felix, successor to C++: http://felix.sf.net