Nicer io-ts reporter

examples/io-ts/niceReporter.test.ts

@@ -0,0 +1,119 @@
+import { niceReporter } from './niceReporter'
+import * as t from 'io-ts'
+
+describe('NiceReporter', () => {
+  it('Basic failures', () => {
+    const result = t.number.decode('dog')
+    expect(niceReporter(result)).toEqual([
+      'Expected number, got string',
+    ])
+  })
+
+  it('Enum failure', () => {
+    const trafficLight = t.union([
+      t.literal('red'),
+      t.literal('yellow'),
+      t.literal('green'),
+    ])
+
+    const result = trafficLight.decode('dog')
+    expect(niceReporter(result)).toEqual([
+      'Expected ("red" | "yellow" | "green"), got string',
+    ])
+  })
+
+  it('Enum failure uses name', () => {
+    const trafficLight = t.union(
+      [t.literal('red'), t.literal('yellow'), t.literal('green')],
+      'TrafficLight'
+    )
+
+    const result = trafficLight.decode('dog')
+    expect(niceReporter(result)).toEqual([
+      'Expected TrafficLight, got string',
+    ])
+  })
+
+  it('Missing field in type', () => {
+    const userRecord = t.type({ name: t.string })
+
+    const result = userRecord.decode({})
+    expect(niceReporter(result)).toEqual([
+      'name: Expected string, got undefined',
+    ])
+  })
+
+  it('Multiple missing fields in type', () => {
+    const userRecord = t.type({ name: t.string, age: t.number })
+
+    const result = userRecord.decode({})
+    expect(niceReporter(result)).toEqual([
+      'name: Expected string, got undefined',
+      'age: Expected number, got undefined',
+    ])
+  })
+
+  it('One missing field in type', () => {
+    const userRecord = t.type({ name: t.string, age: t.number })
+
+    const result = userRecord.decode({ age: 123 })
+    expect(niceReporter(result)).toEqual([
+      'name: Expected string, got undefined',
+    ])
+  })
+
+  it('Error in nested type', () => {
+    const userRecord = t.type({
+      name: t.string,
+      pets: t.type({ dog: t.boolean }),
+    })
+
+    const result = userRecord.decode({ name: 'Dog', pets: {} })
+    expect(niceReporter(result)).toEqual([
+      'pets.dog: Expected boolean, got undefined',
+    ])
+  })
+
+  it('Multiple items missing in a record', () => {
+    const userRecord = t.type({
+      name: t.string,
+      pets: t.type({ dog: t.boolean }),
+    })
+
+    const result = userRecord.decode({})
+    expect(niceReporter(result)).toEqual([
+      'name: Expected string, got undefined',
+      'pets: Expected { dog: boolean }, got undefined',
+    ])
+  })
+
+  const maybe = (prop: t.Type<any>) =>
+    t.union([
+      t.type({ type: t.literal('Just'), value: prop }),
+      t.type({ type: t.literal('Nothing') }),
+    ])
+
+  it('Cannot match union type', () => {
+    const userRecord = t.type({
+      maybeThing: maybe(t.string),
+    })
+
+    const result = userRecord.decode({ maybeThing: 123 })
+    expect(niceReporter(result)).toEqual([
+      'maybeThing: Expected ({ type: "Just", value: string } | { type: "Nothing" }), got number',
+    ])
+  })
+
+  it('Partial match on union type', () => {
+    const userRecord = t.type({
+      maybeThing: maybe(t.string),
+    })
+
+    const result = userRecord.decode({
+      maybeThing: { type: 'Just', value: 123 },
+    })
+    expect(niceReporter(result)).toEqual([
+      'maybeThing.value: Expected string, got number',
+    ])
+  })
+})

examples/io-ts/niceReporter.ts

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+import * as t from 'io-ts'
+import * as E from 'fp-ts/Either'
+
+const getUniq = <A>(array: A[]): A[] => [...new Set(array)]
+
+export const niceReporter = <A>(
+  result: E.Either<t.Errors, A>
+): string[] | A => {
+  if (E.isRight(result)) {
+    return result.right
+  }
+
+  return getUniq(result.left.map((err) => renderContext(err.context)))
+}
+
+const labelForCodec = (codec: t.Decoder<any, any>) => codec.name
+
+const destroyNumbers = (str: string) => str.replace(/[0-9]/g, '')
+
+const renderContext = (context: t.Context): string => {
+  return (
+    contextString(context) +
+    renderContextItem(chooseContextEntry(context))
+  )
+}
+
+const last = <A>(arr: readonly A[], fromEnd: number) =>
+  arr[arr.length - fromEnd]
+
+// for t.type we want the last item
+// but for t.union we want the second last item because the specific one is
+// boring
+const chooseContextEntry = (context: t.Context): t.ContextEntry => {
+  const sndLast = last(context, 2)
+
+  if (sndLast && (sndLast.type as any)._tag === 'UnionType') {
+    return sndLast
+  }
+  return last(context, 1)
+}
+
+const renderContextItem = (contextEntry: t.ContextEntry): string => {
+  return `Expected ${labelForCodec(
+    contextEntry.type
+  )}, got ${typeof contextEntry.actual}`
+}
+
+const contextString = (context: t.Context): string => {
+  const ctx = context
+    .map((contextEntry) => contextEntry.key)
+    .filter((a) => destroyNumbers(a).length > 0)
+    .join('.')
+  return ctx.length > 0 ? `${ctx}: ` : ''
+}