Add filter command as an alias for map(select())

I (and I've heard others) use this often enough that it makes sense to have more user-friendly syntax.

docs/content/manual/dev/manual.yml

@@ -1207,6 +1207,21 @@ sections:
             output: ['{"id": "second", "val": 2}']
 
 
+      - title: "`filter(boolean_expression)`"
+        body: |
+
+          The function `filter(f)` takes an array and returns an array
+          with only the elements for which `f` returns true for that element.
+
+          The behavior is identical to map(select(`f`)) but it's more user
+          friendly.
+
+        examples:
+          - program: 'filter(. >= 2)'
+            input: '[1,5,3,0,7]'
+            output: ['[5,3,7]']
+
+
       - title: "`arrays`, `objects`, `iterables`, `booleans`, `numbers`, `normals`, `finites`, `strings`, `nulls`, `values`, `scalars`"
         body: |
 

src/builtin.jq

@@ -2,6 +2,7 @@ def halt_error: halt_error(5);
 def error(msg): msg|error;
 def map(f): [.[] | f];
 def select(f): if f then . else empty end;
+def filter(f): map(select(f));
 def sort_by(f): _sort_by_impl(map([f]));
 def group_by(f): _group_by_impl(map([f]));
 def unique: group_by(.) | map(.[0]);