问：在SICStus Prolog中，如何摆脱maplist的开销（如SWI的apply_macros）？
A：目标扩展。
首先，我们定义所需的辅助谓词。在下面，我们使用SICStus Prolog 4.5.0。
:- module(maplist_macros, [maplist/2, maplist/3, maplist/4, maplist/5, maplist/6, maplist/7, maplist/8, maplist/9]).
:- meta_predicate maplist(1,?), maplist(2,?,?), maplist(3,?,?,?), maplist(4,?,?,?,?), maplist(5,?,?,?,?,?), maplist(6,?,?,?,?,?,?), maplist(7,?,?,?,?,?,?,?), maplist(8,?,?,?,?,?,?,?,?).
:- use_module(library(lists), [append/2, same_length/2]).
:- use_module(library(ordsets)).
callable_arglists_appended(C0, New, C1) :-
C0 =.. Parts0,
append([Parts0|New], Parts1),
C1 =.. Parts1.
:- dynamic expand_goal_aux/1.
my_expand_goal(Goal, Expanded) :-
asserta((expand_goal_aux(Goal) :- Goal)),
retract((expand_goal_aux(Goal) :- Expanded0)),
strip_module(Expanded0, _, Expanded).
strip_module(MGoal_0, Module, Goal_0) :-
aux_strip_module(MGoal_0, Goal_0, lambda, Module). % lambda?!
aux_strip_module(MG_0,G_0, M0,M) :-
( nonvar(MG_0),
MG_0 = (M1:MG1_0)
-> aux_strip_module(MG1_0,G_0, M1,M)
; MG_0 = G_0,
M0 = M
).
:- dynamic maplist_aux_count/1.
maplist_aux_count(0).
现在进行目标扩展：
% generate specialized `maplist/N'
goal_expansion(Goal0, _Layout0, FromModule, FromModule:Goal, []) :-
Goal0 =.. [maplist, Rel0 | Args],
callable(Rel0),
Args = [_|_],
!,
% get count # of aux preds generated so far and increment it
retract(maplist_aux_count(C)),
C1 is C+1,
asserta(maplist_aux_count(C1)),
% build predicate functor `AuxPred'
number_chars(C, C_chars),
atom_chars(C_atom, C_chars),
atom_concat(maplist_aux_, C_atom, AuxPred), % e.g., maplist_aux_3
% enforce all relevant lists have the same length
lists:maplist(same_length(Args), [Vars_E,Nils]),
lists:maplist(lists:cons, Vars_E, Vars_Es, Vars_E_Es),
lists:maplist(=([]), Nils),
% expand the goal in the right module (`FromModule')
strip_module(Rel0, _, Rel1),
callable_arglists_appended(Rel1, [Vars_E], Rel2),
my_expand_goal(FromModule:Rel2, Rel),
% find out which variables need to be threaded through
term_variables(Rel, Vars_Schema),
list_to_ord_set(Vars_Schema, VSet_Schema),
list_to_ord_set(Vars_E, VSet_E),
ord_subtract(VSet_Schema, VSet_E, VSet_Actual),
% build call of new predicate with proper arguments
callable_arglists_appended(AuxPred, [Args,VSet_Actual], Goal),
% callee clauses (new predicate)
callable_arglists_appended(AuxPred, [Nils, VSet_Actual], Head0), % fact
callable_arglists_appended(AuxPred, [Vars_E_Es,VSet_Actual], Head1), % rule
callable_arglists_appended(AuxPred, [Vars_Es, VSet_Actual], Rec1), %
% dump generated clauses to a file
atom_concat('/tmp/x', C_atom, FileName), % TODO: get actual tmpfilnam
open(FileName, write, S),
portray_clause(S, (Head0 :- true)),
portray_clause(S, (Head1 :- Rel, Rec1)),
close(S),
% compile temporary file in proper module
compile(FromModule:FileName).
到目前为止一切都很好；）这就是问题所在...
如何确保目标扩展变体与替换的谓词完全一样？
（我有一种直觉，不是，但是我不能完全把手指放在上面...）
allequal(Xs) :- maplist(=(_), Xs).
成为
allequal(A) :- maplist_aux_0(A, _).
maplist_aux_0([], _).
maplist_aux_0([A|B], C) :- C=A, maplist_aux_0(B, C).
答案 0 :(得分：3)
最简单的解决方案可能是扩展到do/2，类似于SICStus中的library(lists)用于实现maplist/n。
/* -*- Mode:Prolog; coding:iso-8859-1; indent-tabs-mode:nil; prolog-indent-width:8; prolog-paren-indent:4; tab-width:8; -*- */
/*
Replacement for maplist from library lists, that inlines the calls when possible.
In your code, instead of doing:
:- use_module(library(lists),[maplist/2, maplist/3, ... other non-maplist things ...]).
Do:
:- use_module(library(lists),[... other non-maplist things ...]).
:- use_module(maplist_inliner, [maplist/2,maplist/3]).
*/
:- module(maplist_inliner, [maplist/2,maplist/3]).
% We can not import (and reexport) maplist/2 etc from the module 'lists' (because
% our goal_expansion will only be used from our own predicates, not predicates we reexport).
% Instead we use thin wrappers for those cases where we are unable to inline the calls to maplist.
% However, these will never be used, because we always fallback to expanding to a plain lists:maplist/n call.
:- use_module(library(lists), []).
:- meta_predicate maplist(1, +).
:- meta_predicate maplist(2, +, +).
% TODO: Add more arities
% A thin wrapper around lists:maplist/2. See module documentation for rationale.
maplist(G_1, L1) :-
lists:maplist(G_1, L1).
% A thin wrapper around lists:maplist/3. See module documentation for rationale.
maplist(G_2, L1, L2) :-
lists:maplist(G_2, L1, L2).
get_module(X, ModuleContext, G, M) :-
var(X),
!,
G = X,
M = ModuleContext.
get_module(M1:X, _ModuleContext, G, M) :-
!,
get_module(X, M1, G, M).
get_module(X, ModuleContext, G, M) :-
!,
G = X,
M = ModuleContext.
:- if(fail).
goal_expansion(G, Layout0, ModuleContext, Expansion, Layout1) :-
writeq(goal_expansion(G,Layout0,ModuleContext,Expansion,Layout1)),
nl,
fail.
:- endif.
goal_expansion(maplist(G, L1), _Layout0, ModuleContext, Expansion, Layout1) :-
callable(G),
get_module(G, ModuleContext, G_1, M),
callable(G_1),
atom(M),
!,
Layout1 = [], % No source info
inline_maplist_2(G_1, M, L1, Expansion).
goal_expansion(maplist(G, L1), _Layout0, ModuleContext, Expansion, Layout1) :-
!,
Layout1 = [], % No source info
Expansion = lists:maplist(ModuleContext:G,L1).
goal_expansion(maplist(G, L1, L2), _Layout0, ModuleContext, Expansion, Layout1) :-
callable(G),
get_module(G, ModuleContext, G_2, M),
callable(G_2),
atom(M),
!,
Layout1 = [], % No source info
inline_maplist_3(G_2, M, L1, L2, Expansion).
goal_expansion(maplist(G, L1, L2), _Layout0, ModuleContext, Expansion, Layout1) :-
!,
Layout1 = [], % No source info
Expansion = lists:maplist(ModuleContext:G,L1,L2).
inline_maplist_2(G_1, M, L1, Expansion) :-
G_1 =.. [F|ClosureArgs],
append([F|ClosureArgs], [X], G_ClosureArgs_X),
BodyGoal =.. G_ClosureArgs_X,
Expansion =
(foreach(X,L1),
param(G_1)
do
M:BodyGoal
).
inline_maplist_3(G_2, M, L1, L2, Expansion) :-
G_2 =.. [F|ClosureArgs],
append([F|ClosureArgs], [X1,X2], G_ClosureArgs_X1_X2),
BodyGoal =.. G_ClosureArgs_X1_X2,
Expansion =
(foreach(X1,L1),
foreach(X2,L2),
param(G_2)
do
M:BodyGoal
).
示例
/* -*- Mode:Prolog; coding:iso-8859-1; indent-tabs-mode:nil; prolog-indent-width:8; prolog-paren-indent:4; tab-width:8; -*- */
:- use_module(maplist_inline, [maplist/2,maplist/3]).
p_1(X1) :-
writeq(call(p_1(X1))).
p_2(X1, X2) :-
writeq(call(p_2(X1,X2))).
p_3(X1, X2, X3) :-
writeq(call(p_3(X1,X2,X3))).
test(L1) :-
ClosureArg1 = 'a',
maplist(p_2(ClosureArg1), L1).
test(L1, L2) :-
maplist(p_2, L1, L2).
test_1(L1, Arg) :-
maplist(p_2(Arg), L1).
test_1(L1, L2, Arg) :-
maplist(p_3(Arg), L1, L2).
test_noinline(L1, L2, Arg) :-
G_2 = p_3(Arg), % Inliner will not see this
maplist(G_2, L1, L2).
使用consult/1和listing/1显示发生的情况：
bash$ /usr/local/sicstus4.5.0/bin/sicstus
SICStus 4.5.0 (x86_64-darwin-17.7.0): Thu Jan 17 17:17:35 CET 2019
Licensed to SICS
| ?- consult(test).
% ...
| ?- listing.
maplist_inliner:maplist(A, B) :-
lists:maplist(A, B).
maplist_inliner:maplist(A, B, C) :-
lists:maplist(A, B, C).
p_1(A) :-
writeq(call(p_1(A))).
p_2(A, B) :-
writeq(call(p_2(A,B))).
p_3(A, B, C) :-
writeq(call(p_3(A,B,C))).
test(A) :-
B=a,
( foreach(C, A),
fromto(B, D, D, _)
do p_2(D, C)
).
test(A, B) :-
( foreach(C, A),
foreach(D, B)
do p_2(C, D)
).
test_1(A, B) :-
( foreach(C, A),
fromto(B, D, D, _)
do p_2(D, C)
).
test_1(A, B, C) :-
( foreach(D, A),
foreach(E, B),
fromto(C, F, F, _)
do p_3(F, D, E)
).
test_noinline(A, B, C) :-
D=p_3(C),
lists:maplist(user:user:D, A, B).
当心。几分钟后，我还没有对此进行测试。