builtin: add pick(stream)

pick(stream) works on both arrays and objects

Restrictions on the dot-paths are specified in manual.yml

See #2578

docs/content/manual/manual.yml

@@ -925,6 +925,24 @@ sections:
             input: '{"a": 1, "b": 2, "c": 3}'
             output: ['{"a": 2, "b": 3, "c": 4}']
 
+      - title: "`pick(stream)`"
+        body: |
+        
+          Emit the projection of the input object or array defined by the specified stream of
+          dot-path specifications, such that if p is one of these specifications,
+          and `(. | p)` evaluates to $x, then `pick(stream) | p` will also be $x.
+          For arrays, negative indices and .[m:n] specifications should not be used.
+
+        examples:
+          - program: 'pick(.a, .b.c, .x)'
+            input: '{"a": 1, "b": {"c": 2, "d": 3}, "e": 4}'
+            output: ['{"a":1,"b":{"c":2},"x":null}']
+
+          - program: '[1,2,3,4] | pick(.[2], .[0], .[0])'
+            input: '[1,2,3,4]'
+            output: ['[1,null,3]']
+
+
       - title: "`path(path_expression)`"
         body: |
 

src/builtin.jq

@@ -260,6 +260,12 @@ def walk(f):
   else f
   end;
 
+# stream should be a stream of dot-paths
+def pick(stream):
+  . as $in
+  | reduce path(stream) as $a (null;
+      setpath($a; $in|getpath($a)) );
+
 # SQL-ish operators here:
 def INDEX(stream; idx_expr):
   reduce stream as $row ({}; .[$row|idx_expr|tostring] = $row);

tests/man.test

@@ -219,6 +219,14 @@ map_values(.+1)
 {"a": 1, "b": 2, "c": 3}
 {"a": 2, "b": 3, "c": 4}
 
+pick(.a, .b.c, .x)
+{"a": 1, "b": {"c": 2, "d": 3}, "e": 4}
+{"a":1,"b":{"c":2},"x":null}
+
+[1,2,3,4] | pick(.[2], .[0], .[0])
+[1,2,3,4]
+[1,null,3]
+
 path(.a[0].b)
 null
 ["a",0,"b"]