From 32d5953f2985f8e39454d9ad6891a48c47839656 Mon Sep 17 00:00:00 2001
From: Simon Forman <sforman@hushmail.com>
Date: Sun, 21 Jul 2019 08:28:56 -0700
Subject: [PATCH] Implement map combinator.

---
 thun/defs.txt |  5 ++++-
 thun/thun.pl  | 28 ++++++++++++++++++++++++++++
 2 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/thun/defs.txt b/thun/defs.txt
index e883053..487dfe5 100644
--- a/thun/defs.txt
+++ b/thun/defs.txt
@@ -1,9 +1,11 @@
 -- == 1 -
-++ == 1 +
 ? == dup bool
+++ == 1 +
 anamorphism == [pop []] swap [dip swons] genrec
 app1 == grba infrst
 app2 == [grba swap grba swap] dip [infrst] cons ii
+app3 == 3 appN
+appN == [grabN] cons dip map disenstacken
 at == drop first
 average == [sum 1.0 *] [size] cleave /
 b == [i] dip i
@@ -23,6 +25,7 @@ flatten == [] swap [concat] step
 fork == [i] app2
 fourth == rest third
 gcd == true [tuck mod dup 0 >] loop pop
+grabN == [] swap [cons] times
 grba == [stack popd] dip
 hypot == [sqr] ii + sqrt
 ifte == [nullary] dipd swap branch
diff --git a/thun/thun.pl b/thun/thun.pl
index 3b803ce..5717c31 100644
--- a/thun/thun.pl
+++ b/thun/thun.pl
@@ -242,6 +242,34 @@ combo(genrec, [R1, R0, Then, If|S],
     Quoted = [If, Then, R0, R1, genrec],
     append(R0, [Quoted|R1], Else).
 
+/*
+This is a crude but servicable implementation of map combinator.
+Obviously it would be nice to take advantage of the implied parallelism.
+Instead the quoted program, stack, and each term in the arg list are
+transformed to a simple Joy expression that runs the program on a prepared
+stack.  These expressions are collected in a list and the whole thing is
+evaluated with infra on an empty list, so the result is the mapped list.
+
+The chief advantage of doing it this way (as opposed to using Prolog's map)
+is that the whole state remains in the continuation expression.
+*/
+
+combo(map, [_,   []|S],         [[]|S], E,        E ) :- !.
+combo(map, [P, List|S], [Mapped, []|S], E, [infra|E]) :-
+    prepare_mapping(P, S, List, Mapped).
+
+% Set up a program for each term in ListIn
+%
+%     [term S] [P] infrst
+%
+% prepare_mapping(P, S, ListIn, ListOut).
+
+prepare_mapping(P, S, In, Out) :- prepare_mapping(P, S, In, [], Out).
+
+prepare_mapping(    _, _,     [],                   Out,  Out) :- !.
+prepare_mapping(    P, S, [T|In],                   Acc,  Out) :-
+    prepare_mapping(P, S,    In,  [[T|S], P, infrst|Acc], Out).
+
 
 /*
 Compiler