[m-users.] Use of length() as function giving unexpected (to me) error

[Julien Fischer]

On Sat, 30 Oct 2021, Zoltan Somogyi wrote:

> On Fri, 29 Oct 2021 21:57:35 +0100, "Sean Charles (emacstheviking)" <objitsu at gmail.com> wrote:
>
>> Given that Args and PStrings are lists of different things, I just sought to compare their length:
>> 
>> args_to_pstrings(Args, [], _, PStrings, !T),
>> ( if length(Args) = length(PStrings) then …
>> 
>> translate.m:174: In clause for predicate `translate_call'/4:
>> translate.m:174:   error: ambiguous overloading causes type ambiguity.
>> translate.m:174:   Possible type assignments include:
>> translate.m:174:   V_27: `int' or `(pred int)'
>> translate.m:174:   You will need to add an explicit type qualification to
>> translate.m:174:   resolve the type ambiguity. The way to add an explicit type
>> translate.m:174:   qualification is to use "with_type". For details see the
>> translate.m:174:   "Explicit type qualification" sub-section of the
>> translate.m:174:   "Data-terms" section of the "Syntax" chapter of the Mercury
>> translate.m:174:   language reference manual.
>> ** Error making `Mercury/opts/translate.opt’.
>> 
>> changing to the predicate form removes the error,
>> 
>> list.length(Args, L1),
>> list.length(PStrings, L2),
>> ( if L1 = L2 then …
>> 
>> Compiles fine. I know that from the error message it seems that I have somehow produced a curried call ?! I am also using the `int` module as well. Just seeking some explanation!
>
> The explanation is in the error message: the compiler does not know
> whether each occurrence of length refers to function list.length/1,
> in which case e.g. length(Args) is an int, or to predicate list.length/2,
> in which case length(Args) is a curried call whose type is "pred int".
>
> The error message also tells you how to fix the problem: add an explicit
> type qualification:
>
> ( if length(Args) : int = length(PStrings) then ...

An more readable alternative in this case would be to use:

     ( if same_length(Args, PStrings) then

Julien.