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Predicate write_length/3
Availability:built-in
[semidet]write_length(+Term, -Length, +Options)
True when Length is the number of characters emitted for write_term(Term, Options). In addition to valid options for write_term/2, it processes the option:
max_length(+MaxLength)
If provided, fail if Length would be larger than MaxLength. The implementation ensures that the runtime is limited when computing the length of a huge term with a bounded maximum.
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LogicalCaptain said (2021-02-17T09:34:21):0 upvotes 0 0 downvotes
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Nonground arguments will give arbitrary length:

?- X=foo(bar(baz(_))),write_length(X,L,[]).
X = foo(bar(baz(_2500))),
L = 20.

?- X=foo(bar(baz(_))),write_length(X,L,[]).
X = foo(bar(baz(_12000))),
L = 21.

Note that this does NOT "stringify" codelists or charlists before giving the length, unlike what atom_length/2 or string_length/2 do

?- X=tee,write_length(X,L,[]).
X = tee,
L = 3.

?- X=`tee`,write_length(X,L,[]).
X = [116,101,101],
L = 13.

?- atom_chars(tee,X),write_length(X,L,[]).
X = [t,e,e],
L = 7.

?- X is pi,write_length(X,L,[]).
X = 3.141592653589793,
L = 17.

Whereas:

?- X=tee,atom_length(X,L).
X = tee,
L = 3.

?- X=`tee`,atom_length(X,L).
X = [116,101,101],
L = 3.

?- atom_chars(tee,X),atom_length(X,L).
X = [t,e,e],
L = 3.

?- X is pi,atom_length(X,L).
X = 3.141592653589793,
L = 17.