rishi/scryer-prolog
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ENHANCED: partial_string/3 no longer creates atoms

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As a consequence, resulting strings are now quickly reclaimed on
backtracking.

This addresses #2912.

Test case:

    :- use_module(library(iso_ext)).
    :- use_module(library(lists)).

    ab(a).
    ab(b).

Sample query:

    ?- length(Ls, 1_000_000),
       maplist(ab, Ls),
       partial_string(Ls, Es0, []),
       Es0 == Ls.
       Ls = "aaaaaaaaaaaaaaaaaaa ...", Es0 = "aaaaaaaaaaaaaaaaaaa ..."
    ;  Ls = "aaaaaaaaaaaaaaaaaaa ...", Es0 = "aaaaaaaaaaaaaaaaaaa ..."
    ;  Ls = "aaaaaaaaaaaaaaaaaaa ...", Es0 = "aaaaaaaaaaaaaaaaaaa ..."
    ;  Ls = "aaaaaaaaaaaaaaaaaaa ...", Es0 = "aaaaaaaaaaaaaaaaaaa ..."
    ;  Ls = "aaaaaaaaaaaaaaaaaaa ...", Es0 = "aaaaaaaaaaaaaaaaaaa ..."
    ;  ... .

running in constant memory.
This commit is contained in:
Markus Triska
2025-04-25 22:16:44 +02:00
committed by Mark Thom
parent e9bb41f0d6
commit d51cbd67e0
2 changed files with 26 additions and 25 deletions
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13
src/lib/iso_ext.pl
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@@ -22,6 +22,7 @@ but they're not part of the ISO Prolog standard at the moment.
copy_term/3]).
:- use_module(library(error), [can_be/2,
must_be/2,
domain_error/3,
instantiation_error/1,
type_error/3]).
@@ -275,17 +276,15 @@ call_with_inference_limit(_, _, R, Bb, B) :-
; nonvar(R)
).
%% partial_string(String, L, L0)
%% partial_string(String, Ls0, Ls)
%
% Explicitly construct a partial string "manually". It can be used as an optimized append/3.
% It's not recommended to use this predicate in application code.
partial_string(String, L, L0) :-
partial_string(String, Ls0, Ls) :-
must_be(chars, String),
( String == [] ->
L = L0
; catch(atom_chars(Atom, String),
error(E, _),
throw(error(E, partial_string/3))),
'$create_partial_string'(Atom, L, L0)
Ls0 = Ls
; '$create_partial_string'(String, Ls0, Ls)
).
%% partial_string(+String)