[mercury-users] Higher-order semideterminism in discriminated unions
Fergus Henderson
fjh at cs.mu.OZ.AU
Sun Aug 31 20:20:01 AEST 2003
On 26-Aug-2003, Andr? Platzer <api at gmx.de> wrote:
> :- func cast_det(T::in) = (S::out) is det.
> cast_det(X) = Y :-
> univ_to_type(univ(X), Y)
> ;
> error("casting failed").
This coding style should not be used, because you are assuming
that the compiler does not reorder disjunctions. You should use
an if-then-else rather than a disjunction.
(Also, it's generally much easier to read programs if all disjunctions
and if-then-elses are enclosed in paretheses.)
So it would be better written as
cast_det(X) = Y :-
( univ_to_type(univ(X), Y1) ->
Y = Y1
;
error("casting failed")
).
or
cast_det(X) =
(if univ_to_type(univ(X), Y) then Y
else func_error("casting failed")).
> Erm, yes thanks. Of course you are right, in the simplified example I
> have provided this really solves the problem. But in my real application
> scenario, unfortunately this cannot be applied - as far as I know.
One possibility in your real application scenario would be to have your
data structure store det functions that return a maybe(T) type rather
than being semidet. The functions in most of your application could be
semidet, you'd just need to wrap them up in a lambda expression before
storing them in the data structure.
--
Fergus Henderson <fjh at cs.mu.oz.au> | "I have always known that the pursuit
The University of Melbourne | 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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