builtin.jq: naive abs/0

manual.yml explains that the def is naive, and mentions fabs, etc.

docs/content/manual/manual.yml

@@ -854,6 +854,21 @@ sections:
             input: '[1,0,-1]'
             output: ['1', '-1']
 
+      - title: "`abs`"
+        body: |
+
+          The builtin function `abs` is defined naively as: `if . < 0 then - . else . end`.
+
+          For numeric input, this is the absolute value.  See the
+          section on the identity filter for the implications of this
+          definition for numeric input.
+
+          To compute the absolute value of a number as a floating point number, you may wish use `fabs`.
+
+        examples:
+          - program: 'map(abs)'
+            input: '[-10, -1.1, -1e-1, 1000000000000000002, -1000000000000000002]'
+            output: ['[10,1.1,1e-1,10000000000000002,1e+18]']
 
       - title: "`length`"
         body: |

src/builtin.jq

@@ -10,6 +10,7 @@ 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 del(f): delpaths([path(f)]);
+def abs: if . < 0 then - . else . end;
 def _assign(paths; $value): reduce path(paths) as $p (.; setpath($p; $value));
 def _modify(paths; update):
     reduce path(paths) as $p ([., []];

tests/jq.test

@@ -1808,6 +1808,18 @@ false
 1
 1
 
+# abs, fabs, length
+abs
+"abc"
+"abc"
+
+map(abs)
+[-10, -1.1, -1e-1, 1000000000000000002, -1000000000000000002]
+[10,1.1,0.1,1000000000000000002,1e+18]
+
+map(fabs == length) | unique[0]
+[-10, -1.1, -1e-1, 1000000000000000002, -1000000000000000002]
+true
 
 # Using a keyword as variable/label name