add Config.literal (#1795)

.changeset/hot-dryers-push.md

@@ -0,0 +1,5 @@
+---
+"effect": patch
+---
+
+add Config.literal

packages/effect/src/Config.ts

@@ -151,6 +151,21 @@ export const number: (name?: string) => Config<number> = internal.number
  */
 export const integer: (name?: string) => Config<number> = internal.integer
 
+/**
+ * Constructs a config for a literal value.
+ *
+ * @example
+ * import { Config } from "effect"
+ *
+ * const config = Config.literal("http", "https")("PROTOCOL")
+ *
+ * @since 2.0.0
+ * @category constructors
+ */
+export const literal: <Literals extends ReadonlyArray<internal.LiteralValue>>(...literals: Literals) => (
+  name?: string
+) => Config<Literals[number]> = internal.literal
+
 /**
  * Constructs a config for a `LogLevel` value.
  *

packages/effect/src/internal/config.ts

@@ -255,6 +255,30 @@ export const integer = (name?: string): Config.Config<number> => {
   return name === undefined ? config : nested(config, name)
 }
 
+/** @internal */
+export type LiteralValue = string | number | boolean | null | bigint
+
+/** @internal */
+export const literal = <Literals extends ReadonlyArray<LiteralValue>>(...literals: Literals) =>
+(
+  name?: string
+): Config.Config<Literals[number]> => {
+  const valuesString = literals.map(String).join(", ")
+  const config = primitive(`one of (${valuesString})`, (text) => {
+    const found = literals.find((value) => String(value) === text)
+    if (found === undefined) {
+      return Either.left(
+        configError.InvalidData(
+          [],
+          `Expected one of (${valuesString}) but received ${text}`
+        )
+      )
+    }
+    return Either.right(found)
+  })
+  return name === undefined ? config : nested(config, name)
+}
+
 /** @internal */
 export const logLevel = (name?: string): Config.Config<LogLevel.LogLevel> => {
   const config = mapOrFail(string(), (value) => {

packages/effect/test/Config.test.ts

@@ -93,6 +93,35 @@ describe("Config", () => {
     })
   })
 
+  describe("literal", () => {
+    it("name = undefined", () => {
+      const config = Config.array(Config.literal("a", "b")(), "ITEMS")
+      assertSuccess(config, [["ITEMS", "a"]], ["a"])
+      assertFailure(
+        config,
+        [["ITEMS", "value"]],
+        ConfigError.InvalidData(["ITEMS"], "Expected one of (a, b) but received value")
+      )
+    })
+
+    it("name != undefined", () => {
+      const config = Config.literal("a", 0, -0.3, BigInt(5), false, null)("LITERAL")
+      assertSuccess(config, [["LITERAL", "a"]], "a")
+      assertSuccess(config, [["LITERAL", "0"]], 0)
+      assertSuccess(config, [["LITERAL", "-0.3"]], -0.3)
+      assertSuccess(config, [["LITERAL", "5"]], BigInt(5))
+      assertSuccess(config, [["LITERAL", "false"]], false)
+      assertSuccess(config, [["LITERAL", "null"]], null)
+
+      assertFailure(config, [], ConfigError.MissingData(["LITERAL"], "Expected LITERAL to exist in the provided map"))
+      assertFailure(
+        config,
+        [["LITERAL", "value"]],
+        ConfigError.InvalidData(["LITERAL"], "Expected one of (a, 0, -0.3, 5, false, null) but received value")
+      )
+    })
+  })
+
   describe("date", () => {
     it("name = undefined", () => {
       const config = Config.date()