[mercury-users] higher order modes

[Fergus Henderson]

On 12-Sep-2000, Michael Day <mcda at students.cs.mu.oz.au> wrote:
> 
> Incidentally, do true and fail exist in any real form anywhere?

Yes, the standard library module std_util defines procedures
`semidet_succeed' and `semidet_fail'.

> As I can't seem to return them in lambda expressions.

Syntactically, `true' and `fail' are builtin goals, not predicates,
so you can't use them directly in expressions as higher-order terms.
Thus `list__filter(true, L0, L)' won't do what you want.
You can however use them in lambda expressions,
e.g. `list__filter((pred is semidet :- true), L0, L)'.
Alternatively you can use semidet_succeed and semidet_fail,
e.g. `list__filter(semidet_succeed, L0, L)'.

If `true' and `fail' were treated as builtin predicates rather than
as goals, so that they could be used directly in expressions as
higher-order terms, that probably wouldn't help, since they wouldn't
have the right determinism.  The determinism of `true' would be `det',
and the determinism of `fail' would be `failure', but no doubt the
higher-order procedure that you're trying to pass them to expects
something of determinism `semidet'.

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
WWW: <http://www.cs.mu.oz.au/~fjh>  |  of excellence is a lethal habit"
PGP: finger fjh at 128.250.37.3        |     -- the last words of T. S. Garp.
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