[mercury-users] Documentation error? Higher-order-inst example needs extra parentheses.

[Julian Fondren]

>From the Mercury Language Reference Manual, section 8.3,
Higher Order Modes:

     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])).

However, the trailing analogous code example emits the following
inst errors unless I put parentheses around the higher-order inst,
to say (pred(out is det)).

  inst_fail.m:017: In mode declaration for predicate `inst_fail.foo'/1:
  inst_fail.m:017:   error: undefined inst `in'/0.
  inst_fail.m:017: In mode declaration for predicate `inst_fail.foo'/1:
  inst_fail.m:017:   error: undefined inst `func'/1.
  inst_fail.m:017: In mode declaration for predicate `inst_fail.foo'/1:
  inst_fail.m:017:   error: undefined inst `>>'/2.
  inst_fail.m:017: In mode declaration for predicate `inst_fail.foo'/1:
  inst_fail.m:017:   error: undefined inst `out'/0.
  inst_fail.m:017: In mode declaration for predicate `inst_fail.foo'/1:
  inst_fail.m:017:   error: undefined inst `='/2.
  inst_fail.m:017: In mode declaration for predicate `inst_fail.foo'/1:
  inst_fail.m:017:   error: undefined inst `det'/0.
  inst_fail.m:017: In mode declaration for predicate `inst_fail.foo'/1:
  inst_fail.m:017:   error: undefined mode `is'/2.

:- module inst_fail.
:- interface.
:- import_module io.
:- pred main(io::di, io::uo) is det.
:- implementation.
:- import_module int, list, string.

main(!IO) :-
        foo(F),
        io.format("sum([1,2,3]) = %d\n", [i(F([1,2,3]))], !IO).

:- func sum(list(int)) = int.
sum([]) = 0.
sum([H|T]) = H + sum(T).

:- pred foo(func(list(int)) = int).
:- mode foo(free >> func(in) = out is det) is det.
foo(sum).
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