[mercury-users] Testing with dependent lists?

[Ralph Becket]

doug.auclair at logicaltypes.com, Sunday, 11 February 2007:
> 
> So, is there a way in Mercury to declare to the compiler the inst of
> one object based on the inst of another in a clause head?

Not to do what you want here.  The subtyping aspect of insts is
occasionally useful, but it isn't a magic bullet.  Sometimes the
easiest (and best) thing to do is just add a couple of extra clauses or
use an if-then-else.

-- Ralph
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