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

[Ralph Becket]

Julian Fondren, Saturday, 10 February 2007:
> >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.

use `out(pred(out) is det)' rather than (free >> ...).  It's more
readable.

>  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).

Place the `func(in) = out is det' in parentheses and your errors will go
away.  The problem is that Mercury currently uses a precedence grammar -
it's a hangover from its Prolog heritage.
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