[m-users.] Issue with assoc_list and other container types

[Volker Wysk]

Hi!!

The search predicate in the assoc_list module is like this:

:- pred search(assoc_list(K, V)::in, K::in, V::out) is semidet.

This leads to a problem. The mode of the output variable is "out", this
means that its instantiation state is "ground", after the predicate has been
called. I have a "call_action" predicate like this:

:- inst assoc_list(K, V) == list(pair(K, V)).

:- func actions =
    ( assoc_list(string, pred(string, string))
      :: out(assoc_list(ground, pred(in, out) is det))
    ).

:- pred call_action(string::in, string::in, string::out) is det.
    
call_action(Action, Str, Str1) :-
    (
        if   assoc_list.search(actions, Action, Pred)
        then Pred(Str, Str1)
        else throw("Bug: unknown action " ++ Action)
    ).

The variable "Pred" gets the instantiatedness "ground". This means that
"Pred(Str, Str1)" leads to a compile time error, because the
instantiatedness of Pred needs to be "pred(in, out) is det" for the call.
And such a higher-order inst is not ground.

This means that functions/predicates can't be usefully stored in association
lists, because you can't call them (what else would you want to do with
them?).

I've reimplemented the assoc_list.search/3 predicate and changed the mode:

:- pred assoc_list_search(
    assoc_list(string, pred(string, string)),
    string,
    pred(string, string)).

:- mode assoc_list_search(
    in(assoc_list(Key, Val)),
    in,
    out(Val))
is semidet.

assoc_list_search(L, K, V) :-
    ( L = [K - V]
    ; L = [_, K1 - V1 | Rest],
      assoc_list_search([K1 - V1 | Rest], K, V)
    ).

This predicate works for call_action/3. You get the higher-order-inst of the
predicate, which has been found, in the search result. 

I guess, the assoc_list.search/3 and other predicates/functions should be
modified, and it be done this way.

It should be the same problem with other container types (I haven't
checked).


Cheers,
Volker
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