[mercury-users] Binding variables in an if then else?

[Peter Schachte]

> Suppose we have a variable that we want to decompose or unify, say 
> AList = [1,2,3]
> 
> I want to assign the values in AList to V1, V2, and V3, respectively.
> 
> ( AList = [A1, A2, A3] ->
> 	V1 = A1,
> 	V2 = A2,
> 	V3 = A3
> ;
> 	throw("Something is wrong")
> )
> 
> Is it possible to write this more compactly?

I think the nicest thing for cases like this would be to write
something like:

	expect(Alist = [A1, A2, A3])

(maybe even make expect a prefix operator.)

The semantics of expect(X) would be the same as that of X, except that
failure of X would be transformed into an exception much like your
example above, so if the determinism of X were semidet, expect would
be det, and similarly for nondet => multi.

I'm sure something like this would be in the library by now, except
that the mode system won't let you code it.  But it could be built
into the compiler.

-- 
Peter Schachte              A rock pile ceases to be a rock pile the moment a
schachte at cs.mu.OZ.AU        single man contemplates it, bearing within him the
www.cs.mu.oz.au/~schachte/  image of a cathedral.
Phone: +61 3 8344 1338          -- Antoine de Saint-Exupery 
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