Adds "toboolean" builtin function for converting strings to booleans

docs/content/manual/dev/manual.yml

@@ -1476,6 +1476,18 @@ sections:
             input: '[1, "1"]'
             output: ['1', '1']
 
+      - title: "`toboolean`"
+        body: |
+
+          The `toboolean` function parses its input as a boolean. It
+          will convert correctly-formatted strings to their boolean
+          equivalent, leave booleans alone, and give an error on all other input.
+
+        examples:
+          - program: '.[] | toboolean'
+            input: '["true", "false", true, false]'
+            output: ['true', 'false', 'true', 'false']
+
       - title: "`tostring`"
         body: |
 

src/builtin.c

@@ -486,6 +486,23 @@ static jv f_tonumber(jq_state *jq, jv input) {
   return type_error(input, "cannot be parsed as a number");
 }
 
+static jv f_toboolean(jq_state *jq, jv input) {
+  if (jv_get_kind(input) == JV_KIND_TRUE || jv_get_kind(input) == JV_KIND_FALSE) {
+    return input;
+  }
+  if (jv_get_kind(input) == JV_KIND_STRING) {
+    const char *s = jv_string_value(input);
+    if (strcmp(s, "true") == 0) {
+      jv_free(input);
+      return jv_true();
+    } else if (strcmp(s, "false") == 0) {
+      jv_free(input);
+      return jv_false();
+    }
+  }
+  return type_error(input, "cannot be parsed as a boolean");
+}
+
 static jv f_length(jq_state *jq, jv input) {
   if (jv_get_kind(input) == JV_KIND_ARRAY) {
     return jv_number(jv_array_length(input));
@@ -1850,6 +1867,7 @@ BINOPS
   CFUNC(f_dump, "tojson", 1),
   CFUNC(f_json_parse, "fromjson", 1),
   CFUNC(f_tonumber, "tonumber", 1),
+  CFUNC(f_toboolean, "toboolean", 1),
   CFUNC(f_tostring, "tostring", 1),
   CFUNC(f_keys, "keys", 1),
   CFUNC(f_keys_unsorted, "keys_unsorted", 1),

tests/jq.test

@@ -638,6 +638,21 @@ null
 4
 15
 
+map(toboolean)
+["false","true",false,true]
+[false,true,false,true]
+
+.[] | try toboolean catch .
+[null,0,"tru","truee","fals","falsee",[],{}]
+"null (null) cannot be parsed as a boolean"
+"number (0) cannot be parsed as a boolean"
+"string (\"tru\") cannot be parsed as a boolean"
+"string (\"truee\") cannot be parsed as a boolean"
+"string (\"fals\") cannot be parsed as a boolean"
+"string (\"falsee\") cannot be parsed as a boolean"
+"array ([]) cannot be parsed as a boolean"
+"object ({}) cannot be parsed as a boolean"
+
 [{"a":42},.object,10,.num,false,true,null,"b",[1,4]] | .[] as $x | [$x == .[]]
 {"object": {"a":42}, "num":10.0}
 [true,  true,  false, false, false, false, false, false, false]