Remove the insane "fold" operation, replace with saner "reduce".

builtin.c

@@ -554,10 +554,10 @@ static const char* const jq_builtins[] = {
   "def unique: group_by(.) | map(.[0]);",
   "def max_by(f): _max_by_impl(map([f]));",
   "def min_by(f): _min_by_impl(map([f]));",
-  "def add: fold null as $sum (.[] | $sum + .);",
+  "def add: reduce .[] as $x (null; . + $x);",
   "def del(f): delpaths([path(f)]);",
-  "def _assign(paths; value): value as $v | fold . as $obj (path(paths) as $p | $obj | setpath($p; $v));",
-  "def _modify(paths; update): fold . as $obj (path(paths) as $p | $obj | setpath($p; getpath($p) | update));",
+  "def _assign(paths; value): value as $v | reduce path(paths) as $p (.; setpath($p; $v));",
+  "def _modify(paths; update): reduce path(paths) as $p (.; setpath($p; getpath($p) | update));",
   "def recurse(f): ., (f | select(. != null) | recurse(f));",
   "def to_entries: [keys[] as $k | {key: $k, value: .[$k]}];",
   "def from_entries: map({(.key): .value}) | add;",

compile.c

@@ -320,15 +320,24 @@ block gen_collect(block expr) {
                gen_op_var_bound(LOADV, array_var));
 }
 
-block gen_fold(const char* varname, block init, block fold) {
-  block loop = BLOCK(fold, gen_op_var_unbound(STOREV, varname), gen_op_simple(BACKTRACK));
+block gen_reduce(const char* varname, block source, block init, block body) {
+  block res_var = block_bind(gen_op_var_unbound(STOREV, "reduce"),
+                             gen_noop(), OP_HAS_VARIABLE);
+
+  block loop = BLOCK(gen_op_simple(DUP),
+                     source,
+                     block_bind(gen_op_var_unbound(STOREV, varname),
+                                BLOCK(gen_op_var_bound(LOADVN, res_var),
+                                      body,
+                                      gen_op_var_bound(STOREV, res_var)),
+                                OP_HAS_VARIABLE),
+                     gen_op_simple(BACKTRACK));
   return BLOCK(gen_op_simple(DUP),
                init,
-               block_bind(gen_op_var_unbound(STOREV, varname),
-                          BLOCK(gen_op_target(FORK, loop),
-                                loop,
-                                gen_op_var_unbound(LOADV, varname)),
-                          OP_HAS_VARIABLE));
+               res_var,
+               gen_op_target(FORK, loop),
+               loop,
+               gen_op_var_bound(LOADVN, res_var));
 }
 
 block gen_definedor(block a, block b) {

compile.h

@@ -34,7 +34,7 @@ block gen_call(const char* name, block body);
 block gen_subexp(block a);
 block gen_both(block a, block b);
 block gen_collect(block expr);
-block gen_fold(const char* varname, block init, block body);
+block gen_reduce(const char* varname, block source, block init, block body);
 block gen_definedor(block a, block b);
 block gen_condbranch(block iftrue, block iffalse);
 block gen_and(block a, block b);

docs/content/3.manual/manual.yml

@@ -1016,7 +1016,7 @@ sections:
       (many jq functions such as `map` and `find` are in fact written
       in jq).
 
-      Finally, jq has a `fold` operation, which is very powerful but a
+      Finally, jq has a `reduce` operation, which is very powerful but a
       bit tricky. Again, it's mostly used internally, to define some
       useful bits of jq's standard library.
 
@@ -1126,36 +1126,26 @@ sections:
             input: '[[1,2],[10,20]]'
             output: ['[[1,2,1,2], [10,20,1,2]]']
 
-      - title: Fold
+      - title: Reduce
         body: |
         
-          The `fold` syntax in jq allows you to combine all of the
+          The `reduce` syntax in jq allows you to combine all of the
           results of an expression by accumulating them into a single
           answer. As an example, we'll pass `[3,2,1]` to this expression:
 
-              fold 0 as $sum (.[] | $sum + .)
+              reduce .[] as $item (0; . + $item)
           
-          The variable `$sum` is first given the value `0`. The body
-          of the fold (i.e. `.[] | $sum + .`) is evaluated. `.[]`
-          produces three results, `3`, `2`, and `1`. For the first
-          one, `$sum + .` gives `3`.
+          For each result that `.[]` produces, `. + $item` is run to
+          accumulate a running total, starting from 0. In this
+          example, `.[]` produces the results 3, 2, and 1, so the
+          effect is similar to running something like this:
 
-          Having produced this answer, jq backtracks to find the next
-          result as per usual. However, this time, `$sum` is set to
-          the previous value of the body, so `$sum + .` gives
-          `5`. After the final backtracking, `$sum + .` gives
-          `6`. This final value is used as the value of the entire
-          `fold` expression, so the above filter returns `6`.
-
-          More formally, in order to evaluate `fold INIT as $VAR
-          (BODY)`, jq first sets `$VAR` to the value of `INIT`. It
-          then runs through `BODY`. Each time `BODY` produces a value,
-          `$VAR` is set to that value and jq backtracks to find the
-          next one. When `BODY` stops producing values, the final
-          value of `$VAR` is the result of the entire expression.
+              0 | (3 as $item | . + $item) | 
+                  (2 as $item | . + $item) |
+                  (1 as $item | . + $item)
 
         examples:
-          - program: 'fold 0 as $sum (.[] | $sum + .)'
+          - program: 'reduce .[] as $item (0; . + $item)'
             input: '[10,2,5,3]'
             output: ['20']
       

lexer.l

@@ -49,7 +49,7 @@ struct lexer_param;
 "and" { return AND; }
 "or" { return OR; }
 "end" { return END; }
-"fold" { return FOLD; }
+"reduce" { return REDUCE; }
 "//" { return DEFINEDOR; }
 "|=" { return SETPIPE; }
 "+=" { return SETPLUS; }

parser.y

@@ -58,7 +58,7 @@ struct lexer_param;
 %token THEN "then"
 %token ELSE "else"
 %token ELSE_IF "elif"
-%token FOLD "fold"
+%token REDUCE "reduce"
 %token END "end"
 %token AND "and"
 %token OR "or"
@@ -226,8 +226,8 @@ Term "as" '$' IDENT '|' Exp {
   jv_free($4);
 } |
 
-"fold" Term "as" '$' IDENT '(' Exp ')' {
-  $$ = gen_fold(jv_string_value($5), $2, $7);
+"reduce" Term "as" '$' IDENT '(' Exp ';' Exp ')' {
+  $$ = gen_reduce(jv_string_value($5), $2, $7, $9);
   jv_free($5);
 } |
 

tests/all.test

@@ -379,7 +379,7 @@ def fac: if . == 1 then 1 else . * (. - 1 | fac) end; [.[] | fac]
 # []
 # 1001
 
-fold 0 as $s (.[] | $s + .)
+reduce .[] as $x (0; . + $x)
 [1,2,4]
 7