feat: Implement array style rules

This commit adds two new rules: array-style-complex-type and
array-style-simple-type. In Flow, there are two array notation styles,
verbose (Array<Type>) and shorthand (Type[]). These rules enforce
consistent notation. Array element types are also divided into two
categories: complex and simple. This is useful for e.g. using shorthand
notation for simple types and and verbose notation for complex types.
What "simple" and "complex" means is explained in rules' documentation.

src/index.js

@@ -1,5 +1,7 @@
 import _ from 'lodash';
 import recommended from './configs/recommended.json';
+import arrayStyleComplexType from './rules/arrayStyleComplexType';
+import arrayStyleSimpleType from './rules/arrayStyleSimpleType';
 import booleanStyle from './rules/booleanStyle';
 import defineFlowType from './rules/defineFlowType';
 import delimiterDangle from './rules/delimiterDangle';
@@ -26,6 +28,8 @@ import validSyntax from './rules/validSyntax';
 import {checkFlowFileAnnotation} from './utilities';
 
 const rules = {
+  'array-style-complex-type': arrayStyleComplexType,
+  'array-style-simple-type': arrayStyleSimpleType,
   'boolean-style': booleanStyle,
   'define-flow-type': defineFlowType,
   'delimiter-dangle': delimiterDangle,

src/rules/arrayStyle/index.js

@@ -0,0 +1,34 @@
+import isSimpleType from './isSimpleType';
+
+const schema = [
+  {
+    enum: ['verbose', 'shorthand'],
+    type: 'string'
+  }
+];
+
+export default (defaultConfig, shorthandHandler, verboseHandler) => {
+  const create = (context) => {
+    const verbose = (context.options[0] || defaultConfig) === 'verbose';
+
+    return {
+      // shorthand
+      ArrayTypeAnnotation (node) {
+        shorthandHandler(isSimpleType(node.elementType), verbose, context, node);
+      },
+      // verbose
+      GenericTypeAnnotation (node) {
+        if (node.id.name === 'Array') {
+          if (node.typeParameters.params.length === 1) {
+            verboseHandler(isSimpleType(node.typeParameters.params[0]), verbose, context, node);
+          }
+        }
+      }
+    };
+  };
+
+  return {
+    create,
+    schema
+  };
+};

src/rules/arrayStyle/isSimpleType.js

@@ -0,0 +1,30 @@
+/**
+ * Types considered simple:
+ *
+ *  - primitive types
+ *  - literal types
+ *  - mixed and any types
+ *  - generic types (such as Date, Promise<string>, $Keys<T>, etc.)
+ *  - array type written in shorthand notation
+ *
+ * Types not considered simple:
+ *
+ *  - maybe type
+ *  - function type
+ *  - object type
+ *  - tuple type
+ *  - union and intersection types
+ *
+ * Reminder: if you change these semantics, don't forget to modify documentation of `array-style-...` rules
+ */
+
+const simpleTypePatterns = [
+  /^(?:Any|Array|Boolean|Generic|Mixed|Number|String|Void)TypeAnnotation$/,
+  /.+LiteralTypeAnnotation$/
+];
+
+export default (node) => {
+  return simpleTypePatterns.some((pattern) => {
+    return pattern.test(node.type);
+  });
+};

src/rules/arrayStyleComplexType.js

@@ -0,0 +1,21 @@
+import makeArrayStyleRule from './arrayStyle';
+
+const shorthandHandler = (isSimpleType, verbose, context, node) => {
+  if (!isSimpleType && verbose) {
+    context.report({
+      message: 'Use "Array<ComplexType>", not "ComplexType[]"',
+      node
+    });
+  }
+};
+
+const verboseHandler = (isSimpleType, verbose, context, node) => {
+  if (!isSimpleType && !verbose) {
+    context.report({
+      message: 'Use "ComplexType[]", not "Array<ComplexType>"',
+      node
+    });
+  }
+};
+
+export default makeArrayStyleRule('verbose', shorthandHandler, verboseHandler);

src/rules/arrayStyleSimpleType.js

@@ -0,0 +1,21 @@
+import makeArrayStyleRule from './arrayStyle';
+
+const shorthandHandler = (isSimpleType, verbose, context, node) => {
+  if (isSimpleType && verbose) {
+    context.report({
+      message: 'Use "Array<SimpleType>", not "SimpleType[]"',
+      node
+    });
+  }
+};
+
+const verboseHandler = (isSimpleType, verbose, context, node) => {
+  if (isSimpleType && !verbose) {
+    context.report({
+      message: 'Use "SimpleType[]", not "Array<SimpleType>"',
+      node
+    });
+  }
+};
+
+export default makeArrayStyleRule('shorthand', shorthandHandler, verboseHandler);

tests/rules/assertions/arrayStyleComplexType.js

@@ -0,0 +1,95 @@
+export default {
+  invalid: [
+    {
+      code: 'type X = (?string)[]',
+      errors: [{message: 'Use "Array<ComplexType>", not "ComplexType[]"'}]
+    },
+    {
+      code: 'type X = (?string)[]',
+      errors: [{message: 'Use "Array<ComplexType>", not "ComplexType[]"'}],
+      options: ['verbose']
+    },
+    {
+      code: 'type X = Array<?string>',
+      errors: [{message: 'Use "ComplexType[]", not "Array<ComplexType>"'}],
+      options: ['shorthand']
+    },
+    {
+      code: 'type X = (string | number)[]',
+      errors: [{message: 'Use "Array<ComplexType>", not "ComplexType[]"'}]
+    },
+    {
+      code: 'type X = (string & number)[]',
+      errors: [{message: 'Use "Array<ComplexType>", not "ComplexType[]"'}]
+    },
+    {
+      code: 'type X = [string, number][]',
+      errors: [{message: 'Use "Array<ComplexType>", not "ComplexType[]"'}]
+    },
+    {
+      code: 'type X = ({foo: string})[]',
+      errors: [{message: 'Use "Array<ComplexType>", not "ComplexType[]"'}]
+    },
+    {
+      code: 'type X = (string => number)[]',
+      errors: [{message: 'Use "Array<ComplexType>", not "ComplexType[]"'}]
+    }
+  ],
+  misconfigured: [
+    {
+      errors: [
+        {
+          data: 'normal',
+          dataPath: '[0]',
+          keyword: 'enum',
+          message: 'should be equal to one of the allowed values',
+          params: {
+            allowedValues: [
+              'verbose',
+              'shorthand'
+            ]
+          },
+          parentSchema: {
+            enum: [
+              'verbose',
+              'shorthand'
+            ],
+            type: 'string'
+          },
+          schema: [
+            'verbose',
+            'shorthand'
+          ],
+          schemaPath: '#/items/0/enum'
+        }
+      ],
+      options: ['normal']
+    }
+  ],
+  valid: [
+    {
+      code: 'type X = Array<?string>'
+    },
+    {
+      code: 'type X = Array<?string>',
+      options: ['verbose']
+    },
+    {
+      code: 'type X = (?string)[]',
+      options: ['shorthand']
+    },
+    {
+      code: 'type X = Array<string>',
+      options: ['shorthand']
+    },
+    {
+      code: 'type X = Array<?string>',
+      options: ['shorthand'],
+      settings: {
+        flowtype: {
+          onlyFilesWithFlowAnnotation: true
+        }
+      }
+    }
+  ]
+};

tests/rules/assertions/arrayStyleSimpleType.js

@@ -0,0 +1,110 @@
+export default {
+  invalid: [
+    {
+      code: 'type X = Array<string>',
+      errors: [{message: 'Use "SimpleType[]", not "Array<SimpleType>"'}]
+    },
+    {
+      code: 'type X = string[]',
+      errors: [{message: 'Use "Array<SimpleType>", not "SimpleType[]"'}],
+      options: ['verbose']
+    },
+    {
+      code: 'type X = Array<string>',
+      errors: [{message: 'Use "SimpleType[]", not "Array<SimpleType>"'}],
+      options: ['shorthand']
+    },
+    {
+      code: 'type X = Array<Date>',
+      errors: [{message: 'Use "SimpleType[]", not "Array<SimpleType>"'}]
+    },
+    {
+      code: 'type X = Array<Promise<string>>',
+      errors: [{message: 'Use "SimpleType[]", not "Array<SimpleType>"'}]
+    },
+    {
+      code: 'type X = Array<$Keys<{ foo: string }>>',
+      errors: [{message: 'Use "SimpleType[]", not "Array<SimpleType>"'}]
+    },
+    {
+      code: 'type X = Array<any>',
+      errors: [{message: 'Use "SimpleType[]", not "Array<SimpleType>"'}]
+    },
+    {
+      code: 'type X = Array<mixed>',
+      errors: [{message: 'Use "SimpleType[]", not "Array<SimpleType>"'}]
+    },
+    {
+      code: 'type X = Array<void>',
+      errors: [{message: 'Use "SimpleType[]", not "Array<SimpleType>"'}]
+    },
+    {
+      code: 'type X = Array<null>',
+      errors: [{message: 'Use "SimpleType[]", not "Array<SimpleType>"'}]
+    },
+    {
+      code: 'type X = Array<string[]>',
+      errors: [{message: 'Use "SimpleType[]", not "Array<SimpleType>"'}]
+    }
+  ],
+  misconfigured: [
+    {
+      errors: [
+        {
+          data: 'normal',
+          dataPath: '[0]',
+          keyword: 'enum',
+          message: 'should be equal to one of the allowed values',
+          params: {
+            allowedValues: [
+              'verbose',
+              'shorthand'
+            ]
+          },
+          parentSchema: {
+            enum: [
+              'verbose',
+              'shorthand'
+            ],
+            type: 'string'
+          },
+          schema: [
+            'verbose',
+            'shorthand'
+          ],
+          schemaPath: '#/items/0/enum'
+        }
+      ],
+      options: ['normal']
+    }
+  ],
+  valid: [
+    {
+      code: 'type X = string[]'
+    },
+    {
+      code: 'type X = Array<string>',
+      options: ['verbose']
+    },
+    {
+      code: 'type X = string[]',
+      options: ['shorthand']
+    },
+    {
+      code: 'type X = string[][]'
+    },
+    {
+      code: 'type X = (?string)[]',
+      options: ['verbose']
+    },
+    {
+      code: 'type X = string[]',
+      options: ['verbose'],
+      settings: {
+        flowtype: {
+          onlyFilesWithFlowAnnotation: true
+        }
+      }
+    }
+  ]
+};

tests/rules/index.js

@@ -11,6 +11,8 @@ rules.importPlugin(plugin, 'flowtype');
 const ruleTester = new RuleTester();
 
 const reportingRules = [
+  'array-style-complex-type',
+  'array-style-simple-type',
   'boolean-style',
   'define-flow-type',
   'delimiter-dangle',