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

[Fergus Henderson]

On 09-Aug-2004, Peter Schachte <schachte at cs.mu.OZ.AU> wrote:
> > 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?

 ( AList = [_, _, _] ->
	Alist = [V1, V2, V3]
 ;
	throw("Something is wrong")
 )
 
> 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.

The reason that I did not want something along those lines is that
the semantics becomes mode-dependent.

-- 
Fergus Henderson                    |  "I have always known that the pursuit
                                    |  of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh>  |     -- the last words of T. S. Garp.
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