Add general ? operator

docs/content/3.manual/manual.yml

@@ -1584,6 +1584,16 @@ sections:
             input: 'true'
             output: ['"some exception"']
 
+      - title: `?` operator
+        body: |
+
+          The `?` operator, used as `EXP?`, is shorthand for `try EXP`.
+
+        examples:
+          - program: '[.[]|(.a)?]'
+            input: '[{}, true, {"a":1}'
+            output: ['[null, 1]']
+
   - title: Advanced features
     body: |
       Variables are an absolute necessity in most programming languages, but

parser.y

@@ -255,6 +255,10 @@ Term "as" '$' IDENT '|' Exp {
   $$ = $2;
 } |
 
+Exp '?' {
+  $$ = gen_try($1, gen_op_simple(BACKTRACK));
+} |
+
 
 Exp '=' Exp {
   $$ = gen_call("_assign", BLOCK(gen_lambda($1), gen_lambda($3)));

tests/all.test

@@ -670,6 +670,14 @@ def inc(x): x |= .+1; inc(.[].a)
 [0,1,2,3]
 ["foo","Cannot index number with string \"a\"",3]
 
+[.[]|(.a, .a)?]
+[null,true,{"a":1}]
+[null,null,1,1]
+
+[[.[]|[.a,.a]]?]
+[null,true,{"a":1}]
+[]
+
 # string operations
 [.[]|startswith("foo")]
 ["fo", "foo", "barfoo", "foobar", "barfoob"]