Implement add/1

docs/content/manual/dev/manual.yml

@@ -1300,7 +1300,7 @@ sections:
             input: '[1,[[],{"a":2}]]'
             output: ['[[0],[1,1,"a"]]']
 
-      - title: "`add`"
+      - title: "`add`, `add(generator)`"
         body: |
 
           The filter `add` takes as input an array, and produces as
@@ -1311,6 +1311,9 @@ sections:
 
           If the input is an empty array, `add` returns `null`.
 
+          `add(generator)` operates on the given generator rather than
+          the input.
+
         examples:
           - program: add
             input: '["a","b","c"]'
@@ -1321,6 +1324,9 @@ sections:
           - program: add
             input: '[]'
             output: ["null"]
+          - program: add(.[].a)
+            input: '[{"a":3}, {"a":5}, {"b":6}]'
+            output: ['8']
 
       - title: "`any`, `any(condition)`, `any(generator; condition)`"
         body: |

src/builtin.jq

@@ -8,7 +8,8 @@ def unique: group_by(.) | map(.[0]);
 def unique_by(f): group_by(f) | map(.[0]);
 def max_by(f): _max_by_impl(map([f]));
 def min_by(f): _min_by_impl(map([f]));
-def add: reduce .[] as $x (null; . + $x);
+def add(f): reduce f as $x (null; . + $x);
+def add: add(.[]);
 def del(f): delpaths([path(f)]);
 def abs: if . < 0 then - . else . end;
 def _assign(paths; $value): reduce path(paths) as $p (.; setpath($p; $value));

tests/jq.test

@@ -642,6 +642,19 @@ map_values(.+1)
 [0,1,2]
 [1,2,3]
 
+[add(null), add(range(range(10))), add(empty), add(10,range(10))]
+null
+[null,120,null,55]
+
+# Real-world use case for add(empty)
+.sum = add(.arr[])
+{"arr":[]}
+{"arr":[],"sum":null}
+
+add({(.[]):1}) | keys
+["a","a","b","a","d","b","d","a","d"]
+["a","b","d"]
+
 #
 # User-defined functions
 # Oh god.