[m-users.] Using type constructors in predicate heads

[Sean Charles (emacstheviking)]

Hello,

Is it possible / allowed to write a predicate declaration to explicitly mention a single type from a set of types? I seem to have a pattern cropping up again and again lately, here is my current situation. I have a node type:

:- type snode
    --->    sexp(location, list(snode))
    ;       list(location, list(snode))
    ;       map(location, list(snode))
    ;       tk(location,string)
    ;       kw(location, string)
    ;       s1(location,string)
    ;       s2(location,string)
    .

In my code, I have performed some pre-checks on a term 

    ( if Term = sexp(_, [tk(_, F)|_]) then

such that when I call the predicate to handle it, I —know— that it can only be the one containing an s-expression, but it seems that I am forced to define the predicate I call as the containing `snode` type, I guess what I am saying is is it possible to change this:

:- pred do_gencall(int, renderer, snode, tcon, tcon).
:- mode do_gencall(in, renderer, in, in, out) is det.
do_gencall(L, R, Term, !T) :-

to this in some way?

:- pred do_gencall(int, renderer, snode:sexp, tcon, tcon).
:- mode do_gencall(in, renderer, in, in, out) is det.
do_gencall(L, R, Term, !T) :-



Thanks,
Sean.

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