Expected/known? format_//2 with uninstantiated variable without an error

[haijinSk]

Edit: This was with 5a869e8 version. For 1618739 version see my next comment

Thank you.

---

(From format module/library.)

?- phrase(format_("~s",[_]),Str).
   Str = []
;  Str = [_A]
;  Str = [_A,_B]
;  Str = [_A,_B,_C]
;  ... .

Or even:

?- phrase(format_("~w",[_]),Str).
   Str = "A".

But (as I would expect):

?- phrase(format_("~d",[_]),Str).
   error(instantiation_error,(is)/2).

The first query in Trealla Prolog:

?- phrase(format_("~s",[_]),Str).
   error(instantiation_error,not_sufficiently_instantiated).

[haijinSk]

With the 1618739 version (now, as I would expect):

?- phrase(format_("~s",[_]),Str).
   error(instantiation_error,must_be/2).

But still (expected/known?):

?- phrase(format_("~w",[_]),Str).
   Str = "A".

[triska]

Yes exactly, ~s requires the argument to be a string, and the most recent version of library(format) correctly enforces the type.

~w formats any term similar to how write/1 would write it. We have:

?- write(X).
_469   true.

My recommendation is to use ~q instead of ~w, or a more appropriate specifier such as for example ~d if the argument is known to be an integer.

[UWN]

But why "A" and not a name starting with an underscore as in 7.10.5 a2?

[haijinSk]

Right now, I have understood, my view is, that format_//2, at least in some aspects, is work in progress, in a good way.

Its modes are specified as format_(+FormatString, +Arguments)// (+ = must be instantiated) and I, as a user, have to be cautious: no "run-time help" from the system in this particular case, at this moment.

Anyway, I like format_//2 and company. Thank you.

[triska]

We do get a run-time help in the form of type and instantiation errors. +Arguments does not mean that each argument must be instantiated, but that Arguments itself must be instantiated, for example like [_,_].

[haijinSk]

For me, a naive user, in the context of "what is instantiated", it's easy to be confused with a "duo example" like this:

?- phrase(format_("~s",[_]),Str).
   error(instantiation_error,must_be/2).
?- phrase(format_("~q",[_]),Str).
   Str = "A".

Thank you very much for your answers, for your libraries/modules, for all your work on/for Scryer, and for your
educational content on the modern Prolog...

[haijinSk]

Markus @triska enhanced format library #2869

and now https://github.com/mthom/scryer-prolog/tree/20525e50136b38743cc9d8a7b478069a2bcd1c07

it works as @UWN has suggested #2863 (reply in thread)

Thank you.