Add withLazyFallback type

docs/modules/withLazyFallback.md

@@ -0,0 +1,38 @@
+---
+title: withLazyFallback.ts
+nav_order: 22
+parent: Modules
+---
+
+---
+
+<h2 class="text-delta">Table of contents</h2>
+
+- [withLazyFallback (function)](#withlazyfallback-function)
+
+---
+
+# withLazyFallback (function)
+
+Returns a clone of the given codec that always succeed using the given value `a` if the original codec fails
+
+**Signature**
+
+```ts
+export function withLazyFallback<C extends t.Any>(codec: C, a: () => t.TypeOf<C>, name = `withLazyFallback(${codec.name})`): C { ... }
+```
+
+**Example**
+
+```ts
+import { withLazyFallback } from 'io-ts-types/lib/withLazyFallback';
+import * as t from 'io-ts';
+import { right } from 'fp-ts/lib/Either';
+
+const T = withLazyFallback(t.number, () => -1);
+
+assert.deepStrictEqual(T.decode(1), right(1));
+assert.deepStrictEqual(T.decode('a'), right(-1));
+```
+
+Added in v0.5.0

docs/modules/withLazyFallback.ts.md

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+---
+title: withLazyFallback.ts
+nav_order: 23
+parent: Modules
+---
+
+---
+
+<h2 class="text-delta">Table of contents</h2>
+
+- [withLazyFallback (function)](#withlazyfallback-function)
+
+---
+
+# withLazyFallback (function)
+
+Returns a clone of the given codec that always succeed using the given value `a` if the original codec fails
+
+**Signature**
+
+```ts
+export function withLazyFallback<C extends t.Any>(
+  codec: C,
+  a: () => t.TypeOf<C>,
+  name = `withLazyFallback(${codec.name})`
+): C { ... }
+```
+
+**Example**
+
+```ts
+import { withLazyFallback } from 'io-ts-types/lib/withLazyFallback'
+import * as t from 'io-ts'
+import { right } from 'fp-ts/lib/Either'
+
+const T = withLazyFallback(t.number, () => -1)
+
+assert.deepStrictEqual(T.decode(1), right(1))
+assert.deepStrictEqual(T.decode('a'), right(-1))
+```
+
+Added in v0.5.0

docs/modules/withMessage.ts.md

@@ -1,6 +1,6 @@
 ---
 title: withMessage.ts
-nav_order: 23
+nav_order: 24
 parent: Modules
 ---
 

docs/modules/withValidate.ts.md

@@ -1,6 +1,6 @@
 ---
 title: withValidate.ts
-nav_order: 24
+nav_order: 25
 parent: Modules
 ---
 

src/withLazyFallback.ts

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+import * as t from 'io-ts'
+import { withValidate } from './withValidate'
+import { orElse } from 'fp-ts/lib/Either'
+
+/**
+ * Returns a clone of the given codec that always succeed using the given value `a` if the original codec fails
+ *
+ * @example
+ * import { withLazyFallback } from 'io-ts-types/lib/withLazyFallback'
+ * import * as t from 'io-ts'
+ * import { right } from 'fp-ts/lib/Either'
+ *
+ * const T = withLazyFallback(t.number, () => -1)
+ *
+ * assert.deepStrictEqual(T.decode(1), right(1))
+ * assert.deepStrictEqual(T.decode('a'), right(-1))
+ *
+ * @since 0.5.0
+ */
+export function withLazyFallback<C extends t.Any>(
+  codec: C,
+  a: () => t.TypeOf<C>,
+  name = `withLazyFallback(${codec.name})`
+): C {
+  return withValidate(codec, (u, c) => orElse(() => t.success(a()))(codec.validate(u, c)), name)
+}

test/withLazyFallback.ts

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+import * as assert from 'assert'
+import * as t from 'io-ts'
+import { withLazyFallback } from '../src/withLazyFallback'
+import { assertSuccess } from './helpers'
+
+describe('withLazyFallback', () => {
+  const zero: t.Int = 0 as any
+  const one: t.Int = 1 as any
+
+  const zeroFn: () => t.Int = () => 0 as any
+
+  describe('name', () => {
+    it('should assign a default name', () => {
+      const T = withLazyFallback(t.Int, zeroFn)
+      assert.strictEqual(T.name, 'withLazyFallback(Int)')
+    })
+
+    it('should accept a name', () => {
+      const T = withLazyFallback(t.Int, zeroFn, 'T')
+      assert.strictEqual(T.name, 'T')
+    })
+  })
+
+  describe('is', () => {
+    it('should check a isomorphic value', () => {
+      const T = withLazyFallback(t.Int, zeroFn)
+      assert.strictEqual(T.is(1), true)
+      assert.strictEqual(T.is(1.1), false)
+      assert.strictEqual(T.is('a'), false)
+    })
+  })
+
+  describe('decode', () => {
+    it('should decode a isomorphic value', () => {
+      const T = withLazyFallback(t.Int, zeroFn)
+      assertSuccess(T.decode(1), one)
+      assertSuccess(T.decode(1.2), zero)
+    })
+  })
+})