Support type guards in Flow

changelog_unreleased/flow/14767.md

@@ -0,0 +1,15 @@
+#### Support type guards (#14767 by @panagosg7)
+
+<!-- prettier-ignore -->
+```jsx
+// Input
+function isString (x: mixed): x is string { return typeof x === "string"; }
+
+// Prettier stable
+// Does not parse
+
+// Prettier main
+function isString(x: mixed): x is string {
+  return typeof x === 'string';
+}
+```

package.json

@@ -62,7 +62,7 @@
     "file-entry-cache": "6.0.1",
     "find-cache-dir": "4.0.0",
     "find-parent-dir": "0.3.1",
-    "flow-parser": "0.204.0",
+    "flow-parser": "0.205.0",
     "get-stdin": "9.0.0",
     "graphql": "16.6.0",
     "hermes-parser": "0.10.1",

src/language-js/needs-parens.js

@@ -585,6 +585,27 @@ function needsParens(path, options) {
         return true;
       }
 
+      /* Matches the following case in Flow:
+         ```
+         const a = (x: any): x is (number => string) => true;
+         ```
+         This case is not necessary in TS since `number => string` is not a valid
+         arrow type there.
+      */
+      if (
+        path.match(
+          undefined,
+          (node, key) =>
+            key === "typeAnnotation" && node.type === "TypePredicate",
+          (node, key) =>
+            key === "typeAnnotation" && node.type === "TypeAnnotation",
+          (node, key) =>
+            key === "returnType" && node.type === "ArrowFunctionExpression"
+        )
+      ) {
+        return true;
+      }
+
       const ancestor =
         parent.type === "NullableTypeAnnotation" ? path.grandparent : parent;
 

src/language-js/print/flow.js

@@ -252,6 +252,15 @@ function printFlow(path, options, print) {
         ")",
       ];
 
+    case "TypePredicate":
+      return [
+        node.asserts ? "asserts " : "",
+        print("parameterName"),
+        node.typeAnnotation
+          ? [" is ", printTypeAnnotationProperty(path, print)]
+          : "",
+      ];
+
     case "TypeParameterDeclaration":
     case "TypeParameterInstantiation":
       return printTypeParameters(path, options, print, "params");

src/language-js/traverse/visitor-keys.evaluate.js

@@ -49,6 +49,7 @@ const additionalVisitorKeys = {
   TupleTypeAnnotation: ["elementTypes"],
   TupleTypeSpreadElement: ["label", "typeAnnotation"],
   TupleTypeLabeledElement: ["label", "elementType", "variance"],
+  TypePredicate: ["parameterName", "typeAnnotation", "asserts"],
   NeverTypeAnnotation: [],
   UndefinedTypeAnnotation: [],
   UnknownTypeAnnotation: [],

tests/format/flow-repo/type-guards/__snapshots__/jsfmt.spec.js.snap

@@ -0,0 +1,117 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`comments-in-type-annotations.js [babel-flow] format 1`] = `
+"Unexpected token, expected "{" (3:51)
+  1 | type F = (x: mixed) /* C1 */ => /* C2 */ x /* C3 */ is /* C4 */ number;
+  2 |
+> 3 | function f(x: any) /* C1 */ : /* C2 */ x /* C3 */ is /* C4 */ number /* C5 */ { return true; }
+    |                                                   ^
+  4 |
+  5 | const arrow = (x: any): /* C1 */ x is (number /* C2 */ => /* C3 */ x /* C4 */ is /* C5 */ string /* C6 */ => /* C7 */ x /* C8 */ is /* C9 */ boolean) => true;
+  6 |"
+`;
+
+exports[`comments-in-type-annotations.js format 1`] = `
+====================================options=====================================
+parsers: ["flow"]
+printWidth: 80
+                                                                                | printWidth
+=====================================input======================================
+type F = (x: mixed) /* C1 */ => /* C2 */ x /* C3 */ is /* C4 */ number;
+
+function f(x: any) /* C1 */ : /* C2 */ x /* C3 */ is /* C4 */ number /* C5 */ { return true; }
+
+const arrow = (x: any): /* C1 */ x is (number /* C2 */ => /* C3 */ x /* C4 */ is /* C5 */ string /* C6 */ => /* C7 */ x /* C8 */ is /* C9 */ boolean) => true;
+
+=====================================output=====================================
+type F = (x: mixed /* C1 */) => /* C2 */ x /* C3 */ is /* C4 */ number;
+
+function f(x: any) /* C1 */ : /* C2 */ x /* C3 */ is /* C4 */ number /* C5 */ {
+  return true;
+}
+
+const arrow = (
+  x: any,
+): /* C1 */ x is ((
+  number /* C2 */,
+) => /* C3 */ x /* C4 */ is /* C5 */ (
+  string /* C6 */,
+) => /* C7 */ x /* C8 */ is /* C9 */ boolean) => true;
+
+================================================================================
+`;
+
+exports[`passing.js [babel-flow] format 1`] = `
+"Unexpected token, expected "{" (5:20)
+  3 | type F = (x: mixed) => x is A;
+  4 |
+> 5 | function f(any): x is A { return true; }
+    |                    ^
+  6 |
+  7 | const arrow0 = (x: any): x is A => true;
+  8 | const arrow1 = (x: any): x is (y: A) => B => true;"
+`;
+
+exports[`passing.js format 1`] = `
+====================================options=====================================
+parsers: ["flow"]
+printWidth: 80
+                                                                                | printWidth
+=====================================input======================================
+// @flow
+
+type F = (x: mixed) => x is A;
+
+function f(any): x is A { return true; }
+
+const arrow0 = (x: any): x is A => true;
+const arrow1 = (x: any): x is (y: A) => B => true;
+const arrow3 = (x: any): x is (y: A) => y is B => true;
+
+const needs_parens_1 = (x: any): x is (A => B) => true;
+const needs_parens_2 = (x: any): x is ((A) => B) => true;
+const needs_parens_3 = (x: any): x is (A => x is B) => true;
+const needs_parens_4 = (x: any): x is (A => x is B => x is C) => true;
+
+const needs_parens_5 = (x: any): x is (y: A) => (B => C) => true;
+const needs_parens_6 = (x: any): x is (y: A) => y is (B => C) => true;
+
+class C {
+  m(): this is D {}
+  f = (): this is D => {}
+}
+
+function asserts_1(x: any): asserts x {}
+function asserts_2(x: any): asserts x is A {}
+
+=====================================output=====================================
+// @flow
+
+type F = (x: mixed) => x is A;
+
+function f(any): x is A {
+  return true;
+}
+
+const arrow0 = (x: any): x is A => true;
+const arrow1 = (x: any): x is ((y: A) => B) => true;
+const arrow3 = (x: any): x is ((y: A) => y is B) => true;
+
+const needs_parens_1 = (x: any): x is ((A) => B) => true;
+const needs_parens_2 = (x: any): x is ((A) => B) => true;
+const needs_parens_3 = (x: any): x is ((A) => x is B) => true;
+const needs_parens_4 = (x: any): x is ((A) => x is (B) => x is C) => true;
+
+const needs_parens_5 = (x: any): x is ((y: A) => (B) => C) => true;
+const needs_parens_6 = (x: any): x is ((y: A) => y is (B) => C) => true;
+
+class C {
+  m(): this is D {}
+  f = (): this is D => {};
+}
+
+function asserts_1(x: any): asserts x {}
+function asserts_2(x: any): asserts x is A {}
+
+================================================================================
+`;

tests/format/flow-repo/type-guards/comments-in-type-annotations.js

@@ -0,0 +1,5 @@
+type F = (x: mixed) /* C1 */ => /* C2 */ x /* C3 */ is /* C4 */ number;
+
+function f(x: any) /* C1 */ : /* C2 */ x /* C3 */ is /* C4 */ number /* C5 */ { return true; }
+
+const arrow = (x: any): /* C1 */ x is (number /* C2 */ => /* C3 */ x /* C4 */ is /* C5 */ string /* C6 */ => /* C7 */ x /* C8 */ is /* C9 */ boolean) => true;

tests/format/flow-repo/type-guards/jsfmt.spec.js

@@ -0,0 +1,5 @@
+run_spec(import.meta, ["flow"], {
+  errors: {
+    "babel-flow": ["passing.js", "comments-in-type-annotations.js"],
+  },
+});

tests/format/flow-repo/type-guards/passing.js

@@ -0,0 +1,25 @@
+// @flow
+
+type F = (x: mixed) => x is A;
+
+function f(any): x is A { return true; }
+
+const arrow0 = (x: any): x is A => true;
+const arrow1 = (x: any): x is (y: A) => B => true;
+const arrow3 = (x: any): x is (y: A) => y is B => true;
+
+const needs_parens_1 = (x: any): x is (A => B) => true;
+const needs_parens_2 = (x: any): x is ((A) => B) => true;
+const needs_parens_3 = (x: any): x is (A => x is B) => true;
+const needs_parens_4 = (x: any): x is (A => x is B => x is C) => true;
+
+const needs_parens_5 = (x: any): x is (y: A) => (B => C) => true;
+const needs_parens_6 = (x: any): x is (y: A) => y is (B => C) => true;
+
+class C {
+  m(): this is D {}
+  f = (): this is D => {}
+}
+
+function asserts_1(x: any): asserts x {}
+function asserts_2(x: any): asserts x is A {}

tests/unit/__snapshots__/visitor-keys.js.snap

@@ -1010,6 +1010,11 @@ exports[`visitor keys estree 1`] = `
   "TypeParameterInstantiation": [
     "params",
   ],
+  "TypePredicate": [
+    "parameterName",
+    "typeAnnotation",
+    "asserts",
+  ],
   "TypeofTypeAnnotation": [
     "argument",
   ],

yarn.lock

@@ -3989,10 +3989,10 @@ __metadata:
   languageName: node
   linkType: hard
 
-"flow-parser@npm:0.204.0":
-  version: 0.204.0
-  resolution: "flow-parser@npm:0.204.0"
-  checksum: 295a85bbb3e9f6f46766d751ab9af9977b278b3a41648a4f1a5a8e123b5388822fd120bb16b2ee68a43199b309e0472732f1f2ff04cc841d4472aab3a67d8848
+"flow-parser@npm:0.205.0":
+  version: 0.205.0
+  resolution: "flow-parser@npm:0.205.0"
+  checksum: 84e79867128c414a332e506b43d436baca15eea86100fcc9fb0791dabefd02e9ba72feb1706262378950a10fd97340d8f30363b4fe798ae6c86d74bb255ee0b6
   languageName: node
   linkType: hard
 
@@ -6762,7 +6762,7 @@ __metadata:
     file-entry-cache: 6.0.1
     find-cache-dir: 4.0.0
     find-parent-dir: 0.3.1
-    flow-parser: 0.204.0
+    flow-parser: 0.205.0
     get-stdin: 9.0.0
     graphql: 16.6.0
     hermes-parser: 0.10.1