feat: add null decoder

.changeset/wild-olives-exercise.md

@@ -0,0 +1,5 @@
+---
+"@tsplus/runtime": patch
+---
+
+Add Encoder<null>, Guard<null> and Decoder<null>

packages/runtime/_src/Decoder.ts

@@ -132,6 +132,17 @@ export class DecoderErrorLiteral implements Decoder.Error {
   }
 }
 
+export class DecoderErrorNull implements Decoder.Error {
+  constructor(
+    readonly value: unknown
+  ) {}
+  render = () => {
+    return Tree(
+      `Expected null instead received one of type "${typeof this.value}"`
+    )
+  }
+}
+
 export class DecoderErrorStructMissingField implements Decoder.Error {
   render = () => Tree(`Missing`)
 }
@@ -250,6 +261,15 @@ export const number: Decoder<number> = Decoder((u) =>
     Result.fail(new DecoderErrorPrimitive(u, "number"))
 )
 
+/**
+ * @tsplus implicit
+ */
+export const _null: Decoder<null> = Decoder((u) =>
+  Derive<Guard<null>>().is(u) ?
+    Result.success(u) :
+    Result.fail(new DecoderErrorNull(u))
+)
+
 /**
  * @tsplus implicit
  */

packages/runtime/_src/Encoder.ts

@@ -52,6 +52,13 @@ export const boolean: Encoder<boolean> = Encoder((u) => u)
  */
 export const number: Encoder<number> = Encoder((u) => u)
 
+/**
+ * Encoder for null
+ *
+ * @tsplus implicit
+ */
+export const _null: Encoder<null> = Encoder((u) => u)
+
 /**
  * Encoder for a string
  *

packages/runtime/_src/Guard.ts

@@ -56,6 +56,13 @@ export const boolean: Guard<boolean> = Guard((u): u is boolean => typeof u === "
  */
 export const number: Guard<number> = Guard((u): u is number => typeof u === "number")
 
+/**
+ * Guard for null
+ *
+ * @tsplus implicit
+ */
+export const _null: Guard<null> = Guard((u): u is null => u === null)
+
 /**
  * Guard for a string
  *

packages/runtime/_test/Decoder.test.ts

@@ -92,6 +92,18 @@ describe.concurrent("Decoder", () => {
       `Expected literal "1" of type "number" instead received "200"`
     )
   })
+  it("null", () => {
+    const decoder: Decoder<null> = Derive()
+    assert.deepEqual(decoder.decodeJSON(JSON.stringify(null)).right.value, null)
+    assert.deepEqual(
+      decoder.decodeJSON(JSON.stringify("no-hello")).left.value?.message,
+      `Expected null instead received one of type "string"`
+    )
+    assert.deepEqual(
+      decoder.decodeJSON(JSON.stringify(200)).left.value?.message,
+      `Expected null instead received one of type "number"`
+    )
+  })
   it("struct", () => {
     interface Person {
       firstName: string

packages/runtime/_test/Encoder.test.ts

@@ -12,6 +12,10 @@ describe.concurrent("Encoder", () => {
     const number: Encoder<number> = Derive()
     assert.deepEqual(number.encode(1), 1)
   })
+  it("null", () => {
+    const { encode }: Encoder<null> = Derive()
+    assert.deepEqual(encode(null), null)
+  })
   it("date", () => {
     const string: Encoder<Date> = Derive()
     const date = new Date()