Add Context.mergeAll (#4135)

.changeset/rare-eagles-jog.md

@@ -0,0 +1,5 @@
+---
+"effect": minor
+---
+
+Add Context.mergeAll to combine multiple Contexts into one.

packages/effect/src/Context.ts

@@ -428,6 +428,36 @@ export const merge: {
   <Services, R1>(self: Context<Services>, that: Context<R1>): Context<Services | R1>
 } = internal.merge
 
+/**
+ * Merges any number of `Context`s, returning a new `Context` containing the services of all.
+ *
+ * @param ctxs - The `Context`s to merge.
+ *
+ * @example
+ * ```ts
+ * import { Context } from "effect"
+ *
+ * const Port = Context.GenericTag<{ PORT: number }>("Port")
+ * const Timeout = Context.GenericTag<{ TIMEOUT: number }>("Timeout")
+ * const Host = Context.GenericTag<{ HOST: string }>("Host")
+ *
+ * const firstContext = Context.make(Port, { PORT: 8080 })
+ * const secondContext = Context.make(Timeout, { TIMEOUT: 5000 })
+ * const thirdContext = Context.make(Host, { HOST: "localhost" })
+ *
+ * const Services = Context.mergeAll(firstContext, secondContext, thirdContext)
+ *
+ * assert.deepStrictEqual(Context.get(Services, Port), { PORT: 8080 })
+ * assert.deepStrictEqual(Context.get(Services, Timeout), { TIMEOUT: 5000 })
+ * assert.deepStrictEqual(Context.get(Services, Host), { HOST: "localhost" })
+ * ```
+ *
+ * @since 3.12.0
+ */
+export const mergeAll: <T extends Array<unknown>>(
+  ...ctxs: [...{ [K in keyof T]: Context<T[K]> }]
+) => Context<T[number]> = internal.mergeAll
+
 /**
  * Returns a new `Context` that contains only the specified services.
  *

packages/effect/src/internal/context.ts

@@ -295,6 +295,19 @@ export const merge = dual<
   return makeContext(map)
 })
 
+/** @internal */
+export const mergeAll = <T extends Array<unknown>>(
+  ...ctxs: [...{ [K in keyof T]: C.Context<T[K]> }]
+): C.Context<T[number]> => {
+  const map = new Map()
+  for (const ctx of ctxs) {
+    for (const [tag, s] of ctx.unsafeMap) {
+      map.set(tag, s)
+    }
+  }
+  return makeContext(map)
+}
+
 /** @internal */
 export const pick =
   <Services, S extends Array<C.ValidTagsById<Services>>>(...tags: S) =>

packages/effect/test/Context.test.ts

@@ -256,6 +256,39 @@ describe("Context", () => {
     expect(result.pipe(Context.get(A))).toEqual({ a: 0 })
   })
 
+  it("mergeAll", () => {
+    const env = Context.mergeAll(
+      Context.make(A, { a: 0 }),
+      Context.make(B, { b: 1 }),
+      Context.make(C, { c: 2 })
+    )
+
+    const pruned = pipe(
+      env,
+      Context.pick(A, B)
+    )
+
+    expect(pipe(
+      pruned,
+      Context.get(A)
+    )).toEqual({ a: 0 })
+
+    expect(pipe(
+      pruned,
+      Context.getOption(B)
+    )).toEqual(O.some({ b: 1 }))
+
+    expect(pipe(
+      pruned,
+      Context.getOption(C)
+    )).toEqual(O.none())
+
+    expect(pipe(
+      env,
+      Context.getOption(C)
+    )).toEqual(O.some({ c: 2 }))
+  })
+
   it("isContext", () => {
     expect(Context.isContext(Context.empty())).toEqual(true)
     expect(Context.isContext(null)).toEqual(false)