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

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

@@ -13,4 +13,17 @@ 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', () => {
+    const UpperCase = new t.Type<string, string, unknown>(
+      'UpperCase',
+      (u): u is string => t.string.is(u) && u.toLocaleUpperCase() === u,
+      (u, c) =>
+        t.string
+          .validate(u, c)
+          .chain(s => (s.toLocaleUpperCase() === s ? t.success(s) : t.failure(u, c, 'should be upper case'))),
+      t.string.encode
+    )
+    assert.deepEqual(PathReporter.report(UpperCase.decode('a')), ['should be upper case'])
+  })
 })