Explore surface syntax for optional attributes.

Author: cristianoc

src/res_core.ml

@@ -161,6 +161,8 @@ let jsxAttr = (Location.mknoloc "JSX", Parsetree.PStr [])
 let uncurryAttr = (Location.mknoloc "bs", Parsetree.PStr [])
 let ternaryAttr = (Location.mknoloc "ns.ternary", Parsetree.PStr [])
 let ifLetAttr = (Location.mknoloc "ns.iflet", Parsetree.PStr [])
+let optionalAttr = (Location.mknoloc "optional", Parsetree.PStr [])
+
 let suppressFragileMatchWarningAttr =
   ( Location.mknoloc "warning",
     Parsetree.PStr
@@ -2923,6 +2925,14 @@ and parseRecordExprRowWithStringKey p =
     | _ -> Some (field, Ast_helper.Exp.ident ~loc:field.loc field))
   | _ -> None
 
+and parseOptionalExpression p =
+  match p.Parser.token with
+  | Token.Question ->
+    Parser.next p;
+    let e = parseExpr p in
+    {e with pexp_attributes = optionalAttr :: e.pexp_attributes}
+  | _ -> parseExpr p
+
 and parseRecordExprRow p =
   let attrs = parseAttributes p in
   let () =
@@ -2939,7 +2949,7 @@ and parseRecordExprRow p =
     match p.Parser.token with
     | Colon ->
       Parser.next p;
-      let fieldExpr = parseExpr p in
+      let fieldExpr = parseOptionalExpression p in
       Some (field, fieldExpr)
     | _ ->
       let value = Ast_helper.Exp.ident ~loc:field.loc ~attrs field in

tests/parsing/grammar/expressions/record.res

@@ -24,7 +24,7 @@ let r = {...expr, pexp_attributes: [],} // trailing comma
 let r = {...make() : myRecord, foo: bar}
 let r = {...(make() : myRecord), foo: bar} // parens optional
 
-let r = {x: @optional None, y: @optional None, z: @optional (None:tt)}
+let r = {x: @optional None, y: ?None, z: @optional (None:tt)}
 
 let z = name => { name : @optional name, x: 3}