New Feature: better support for custom messages, closes #148

- add optional message field to ValidationError
- add message argument to failure
- PathReporter should account for the new field

Author: gcanti

README.md

@@ -149,6 +149,28 @@ const Person = t.type({
 console.log(getPaths(Person.decode({}))) // => [ '.name', '.age' ]
 ```
 
+# Custom error messages
+
+You can set your own error message by providing a `message` argument to `failure`
+
+Example
+
+```ts
+const NumberFromString = new t.Type<number, string, unknown>(
+  'NumberFromString',
+  t.number.is,
+  (u, c) =>
+    t.string.validate(u, c).chain(s => {
+      const n = +s
+      return isNaN(n) ? t.failure(u, c, 'cannot parse to a number') : t.success(n)
+    }),
+  String
+)
+
+console.log(PathReporter.report(NumberFromString.decode('a')))
+// => ['cannot parse to a number']
+```
+
 # Community
 
 - [io-ts-types](https://github.com/gcanti/io-ts-types) - A collection of codecs and combinators for use with

package.json

@@ -1,6 +1,6 @@
 {
   "name": "io-ts",
-  "version": "1.6.4",
+  "version": "1.7.0",
   "description": "TypeScript compatible runtime type system for IO validation",
   "files": [
     "lib"

src/PathReporter.ts

@@ -9,15 +9,17 @@ function getContextPath(context: Context): string {
   return context.map(({ key, type }) => `${key}: ${type.name}`).join('/')
 }
 
-function getMessage(v: any, context: Context): string {
-  return `Invalid value ${stringify(v)} supplied to ${getContextPath(context)}`
+function getMessage(e: ValidationError): string {
+  return e.message !== undefined
+    ? e.message
+    : `Invalid value ${stringify(e.value)} supplied to ${getContextPath(e.context)}`
 }
 
 /**
  * @since 1.0.0
  */
 export function failure(es: Array<ValidationError>): Array<string> {
-  return es.map(e => getMessage(e.value, e.context))
+  return es.map(getMessage)
 }
 
 /**

src/index.ts

@@ -25,6 +25,7 @@ export interface Context extends ReadonlyArray<ContextEntry> {}
 export interface ValidationError {
   readonly value: unknown
   readonly context: Context
+  readonly message?: string
 }
 
 /**
@@ -194,8 +195,8 @@ export const failures = <T>(errors: Errors): Validation<T> => new Left(errors)
 /**
  * @since 1.0.0
  */
-export const failure = <T>(value: unknown, context: Context): Validation<T> =>
-  failures([getValidationError(value, context)])
+export const failure = <T>(value: unknown, context: Context, message?: string): Validation<T> =>
+  failures([{ value, context, message }])
 
 /**
  * @since 1.0.0

test/PathReporter.ts

@@ -1,6 +1,7 @@
 import * as assert from 'assert'
 import * as t from '../src'
 import { PathReporter } from '../src/PathReporter'
+import { NumberFromString } from './helpers'
 
 describe('PathReporter', () => {
   it('should use the function name as error message', () => {
@@ -13,4 +14,8 @@ describe('PathReporter', () => {
   it('should say something whene there are no errors', () => {
     assert.deepEqual(PathReporter.report(t.number.decode(1)), ['No errors!'])
   })
+
+  it('should account for the optional message field', () => {
+    assert.deepEqual(PathReporter.report(NumberFromString.decode('a')), ['cannot parse to a number'])
+  })
 })

test/helpers.ts

@@ -39,23 +39,16 @@ export function assertFailure<T>(result: t.Validation<T>, errors: Array<string>)
   }
 }
 
-class NumberFromStringType extends t.Type<number, string, unknown> {
-  readonly _tag: 'NumberFromStringType' = 'NumberFromStringType'
-  constructor() {
-    super(
-      'NumberFromString',
-      t.number.is,
-      (u, c) =>
-        t.string.validate(u, c).chain(s => {
-          const n = +s
-          return isNaN(n) ? t.failure(u, c) : t.success(n)
-        }),
-      String
-    )
-  }
-}
-
-export const NumberFromString = new NumberFromStringType()
+export const NumberFromString = new t.Type<number, string, unknown>(
+  'NumberFromString',
+  t.number.is,
+  (u, c) =>
+    t.string.validate(u, c).chain(s => {
+      const n = +s
+      return isNaN(n) ? t.failure(u, c, 'cannot parse to a number') : t.success(n)
+    }),
+  String
+)
 
 export const HyphenatedString = new t.Type<string, string, unknown>(
   'HyphenatedString',

test/refinement.ts

@@ -52,7 +52,7 @@ describe('refinement', () => {
 
     it('should fail with the last deserialized value', () => {
       const T = IntegerFromString
-      assertFailure(T.decode('a'), ['Invalid value "a" supplied to : IntegerFromString'])
+      assertFailure(T.decode('a'), ['cannot parse to a number'])
       assertFailure(T.decode('1.2'), ['Invalid value 1.2 supplied to : IntegerFromString'])
     })
   })