Passing Data — Advanced

In Quick Start, you learned the basics: tasks receive data through a context, and tags keep them decoupled. This guide goes deeper — covering the two ways to provide data to a task and when to choose each.

Via a Tag

Using a tag is the recommended default. A task declares what it needs through a tag, and the pipeline decides what fills it. The task has no knowledge of where the data comes from — it only knows the contract.

const userIdTag = createTag<number>({ name: "userId" })

const fetchUser = createTask({
  name: "fetch-user",
  run: {
    context: { userId: userIdTag.value },
    fn: ({ userId }) => { ... },
  },
})

The pipeline fills the tag using bind. The source can be a task result or a static value via literal — either way, fetchUser is not affected:

// From a task result
.stage({ steps: [bind(userIdTag, auth.result.userId)] })

// Or a static value — same tag, same task, different source
.stage({ steps: [bind(userIdTag, literal(42))] })

Tip

Passing a static value through a tag keeps the door open — swapping it for a real task result later requires no changes inside the task.

import { bind, compose, createTag, createTask } from "@grlt-hub/app-compose"

const userIdTag = createTag<number>({ name: "userId" })

const auth = createTask({
  name: "auth",
  run: {
    fn: () => {
      // 👇 Simulates a login result
      return { userId: 1 }
    },
  },
})

const fetchUser = createTask({
  name: "fetch-user",
  run: {
    // 👇 fetchUser only knows about the tag — not about auth
    context: { userId: userIdTag.value },
    fn: async ({ userId }) => {
      const response = await fetch(`https://jsonplaceholder.typicode.com/users/${userId}`)
      const result = await response.json()
      console.log(JSON.stringify(result, null, 2))
    },
  },
})

compose()
  .stage({ steps: [auth] })
  .stage({ steps: [bind(userIdTag, auth.result.userId)] }) // 👈 tag is filled here
  .stage({ steps: [fetchUser] })
  .run()

Directly

You can also provide data directly in the context — either a static value or another task’s result:

// static value baked into the task
context: {
  userId: literal(1)
}

// another task's result referenced directly
context: {
  userId: auth.result.userId
}

Both couple the task to something specific. The task can no longer be reused in a different context without modifying it. Treat this as a deliberate choice, not a default. When in doubt, prefer a tag.

import { compose, createTask, literal } from "@grlt-hub/app-compose"

const auth = createTask({
  name: "auth",
  run: {
    fn: () => ({ userId: 1 }),
  },
})

const fetchUser = createTask({
  name: "fetch-user",
  run: {
    // 👇 directly coupled to auth — fetchUser can't exist without it
    context: {
      userId: auth.result.userId,
      url: literal("https://jsonplaceholder.typicode.com/users/"),
    },
    fn: async (ctx) => {
      const response = await fetch(`${ctx.url}/${ctx.userId}`)
      const result = await response.json()
      console.log(JSON.stringify(result, null, 2))
    },
  },
})

compose()
  .stage({ steps: [auth] })
  .stage({ steps: [fetchUser] })
  .run()