Add $ElementType<A, B> type

Reviewed By: samwgoldman

Differential Revision: D5222718

fbshipit-source-id: de926de63a6c4f42b47929bfbbc3b20f5a094777

src/typing/debug_js.ml

@@ -994,6 +994,9 @@ and json_of_destructor_impl json_cx = Hh_json.(function
   | PropertyType x -> JSON_Object [
       "propName", JSON_String x;
     ]
+  | ElementType t -> JSON_Object [
+      "elementType", _json_of_t json_cx t
+    ]
   | Bind t -> JSON_Object [
       "thisType", _json_of_t json_cx t
     ]
@@ -1416,6 +1419,7 @@ and dump_t_ (depth, tvars) cx t =
     let string_of_destructor = function
     | NonMaybeType -> "non-maybe type"
     | PropertyType x -> spf "property type `%s`" x
+    | ElementType _ -> "element type"
     | Bind _ -> "bind"
     | SpreadType _ -> "spread"
     in
@@ -1983,6 +1987,7 @@ let string_of_selector = function
 let string_of_destructor = function
   | NonMaybeType -> "NonMaybeType"
   | PropertyType x -> spf "PropertyType %s" x
+  | ElementType _ -> "ElementType"
   | Bind _ -> "Bind"
   | SpreadType _ -> "Spread"
 

src/typing/flow_js.ml

@@ -6455,6 +6455,7 @@ and eval_destructor cx ~trace reason curr_t s i =
             let maybe_r = replace_reason (fun desc -> RMaybe desc) reason in
             UseT (UnknownUse, DefT (maybe_r, MaybeT tvar))
         | PropertyType x -> GetPropT(reason, Named (reason, x), tvar)
+        | ElementType t -> GetElemT(reason, t, tvar)
         | Bind t -> BindT(reason, mk_methodcalltype t [] tvar, true)
         | SpreadType (make_exact, todo_rev) ->
             let open ObjectSpread in
@@ -6489,6 +6490,9 @@ and subst_destructor cx force map s = match s with
   | NonMaybeType
   | PropertyType _
     -> s
+  | ElementType t ->
+    let t_ = subst cx ~force map t in
+    if t_ == t then s else ElementType t_
   | Bind t ->
     let t_ = subst cx ~force map t in
     if t_ == t then s else Bind t_

src/typing/gc_js.ml

@@ -362,6 +362,7 @@ and gc_selector cx state = function
 and gc_destructor cx state = function
   | NonMaybeType -> ()
   | PropertyType _ -> ()
+  | ElementType t -> gc cx state t
   | Bind t -> gc cx state t
   | SpreadType (_, ts) -> List.iter (gc cx state) ts
 

src/typing/type.ml

@@ -803,6 +803,7 @@ module rec TypeTerm : sig
   and destructor =
   | NonMaybeType
   | PropertyType of string
+  | ElementType of t
   | Bind of t
   | SpreadType of bool * t list
 

src/typing/type_annotation.ml

@@ -239,6 +239,17 @@ let rec convert cx tparams_map = Ast.Type.(function
         error_type cx loc (FlowError.EPropertyTypeAnnot loc)
     )
 
+  (* $ElementType<T, string> acts as the type of the string elements in object
+     type T *)
+  | "$ElementType" ->
+    check_type_param_arity cx loc typeParameters 2 (fun () ->
+      let ts = convert_type_params () in
+      let t = List.nth ts 0 in
+      let e = List.nth ts 1 in
+      EvalT (t, TypeDestructorT
+        (mk_reason (RCustom "element type") loc, ElementType e), mk_id())
+    )
+
   (* $NonMaybeType<T> acts as the type T without null and void *)
   | "$NonMaybeType" ->
     check_type_param_arity cx loc typeParameters 1 (fun () ->

src/typing/type_visitor.ml

@@ -209,6 +209,7 @@ class ['a] t = object(self)
   method private destructor cx acc = function
   | NonMaybeType -> acc
   | PropertyType _ -> acc
+  | ElementType t -> self#type_ cx acc t
   | Bind t -> self#type_ cx acc t
   | SpreadType (_, ts) -> self#list (self#type_ cx) acc ts
 

tests/type-destructors/element_type.js

@@ -0,0 +1,23 @@
+type BadArity = $ElementType<number, number, number>;
+
+type Arr = Array<number>;
+type Arr_Elem = $ElementType<Arr, number>;
+
+(42: Arr_Elem); // OK: `Arr_Elem` is `number`
+('hello world': Arr_Elem);
+
+function foo(a: Arr): $ElementType<Arr, number> {
+  if (false) return a[0];
+  else return 0;
+}
+
+type Obj = { [key: string]: number };
+type Obj_Elem = $ElementType<Obj, string>;
+
+(42: Obj_Elem); // OK: `Obj_Elem` is `number`
+('hello world': Obj_Elem);
+
+function bar(o: Obj): $ElementType<Obj, string> {
+  if (false) return o['buz'];
+  else return 0;
+}

tests/type-destructors/type-destructors.exp

@@ -1,3 +1,19 @@
+element_type.js:1
+  1: type BadArity = $ElementType<number, number, number>;
+                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Incorrect number of type parameters (expected 2)
+
+element_type.js:7
+  7: ('hello world': Arr_Elem);
+      ^^^^^^^^^^^^^ string. This type is incompatible with
+  7: ('hello world': Arr_Elem);
+                     ^^^^^^^^ number
+
+element_type.js:18
+ 18: ('hello world': Obj_Elem);
+      ^^^^^^^^^^^^^ string. This type is incompatible with
+ 18: ('hello world': Obj_Elem);
+                     ^^^^^^^^ number
+
 non_maybe_type.js:5
   5:   else return 0; // this should be an error
                    ^ number. This type is incompatible with
@@ -95,4 +111,4 @@ union.js:4
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ string
 
 
-Found 7 errors
+Found 10 errors