[m-users.] Reference manual 8.3.1, higher-order inst

[fabrice nicol]

In the current ROTD Reference manual, chapter 8.3.1 on 'Building 
higher-order insts and modes', the inst/mode of higher-order predicates 
is defined as follows:

    "If you want to define a predicate which returns a higher-order
    predicate term,

      you would use a mode such as ‘free >> pred(…) is …’, or
    ‘out(pred(…) is … )’. For example:

      :- pred foo(pred(int)).
      :- mode foo(free >> pred(out) is det) is det.

      foo(sum([1,2,3]))."

Apparently though, at least from 20.06 on (not checked before), the 
'free >>' part of of higher-order inst/mode declaration is outdated and 
causes compiler errors. The following Mercury chunk code compiles and 
runs OK:

    :- module new.

    :- interface.

    :- import_module io.

    :- pred main(io::di, io::uo) is det.

    :- implementation.

    :- import_module string, int.

    :- pred check_10(pred(string, int), string).

    :- mode check_10(pred(in, out) is det, in) is semidet.

    check_10(Predicate, S) :- call(Predicate, S, L), L < 10.

    main(!IO) :- ( if check_10((pred(S::in, L::out) is det :- length(S,
    L)), "test_str")

                  then write_string("OK!", !IO) elsewrite_string("NO!",
    !IO)).


Replacing 'pred(in, out)' with 'free >> pred(in, out)'  in the above 
mode declaration for 'check_10' causes a compiler (20.06 or DEV) error msg:


    Error: no mode declaration for predicate `check_10'/2.
        (Use `--infer-modes' to enable mode inference.)
      Inferred :- mode check_10(di(/* unique */(pred(in, out) is det)),
    di).
    In the mode declaration of the predicate `check_10': error: a
      higher-order mode should have one of the following forms:
       `pred(<mode1>, ...) is <detism>'
        `any_pred(<mode1>, ...) is <detism>'
        `func(<mode1>, ...) = <return_mode> is <detism>'
        `any_func(<mode1>, ...) = <return_mode> is <detism>'

Also, for independent reasons, the example given in the manual 
(foo(sum[1,2,3])) does not compile. It triggers a "higher-order 
predicate unification error", but this is more understandable and can be 
fixed by changing the example.
Unless I am mistaken or doing something wrong, a reference manual update 
would perhaps be appropriate.

F. Nicol



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