Part 4: map vs flatMap

I kept mixing up map and flatMap.

The rule is simple.

Use map when the callback returns a plain value.

Use flatMap when the callback returns an Effect.

That is it.

map changes the value

const program = Effect.succeed("ada").pipe(
  Effect.map((name) => name.toUpperCase()),
);

toUpperCase() returns a plain string.

So this is map.

flatMap continues the program

const program = parseUserId("1").pipe(Effect.flatMap((id) => fetchUser(id)));

fetchUser(id) returns an Effect.

So this is flatMap.

The smell

If you use map with a function that returns an Effect, you get an Effect inside an Effect.

Effect.Effect<Effect.Effect<User, UserNotFound, never>, InvalidUserId, never>;

That type is trying to tell you something.

You probably wanted flatMap.

What I am taking with me

The question is not “which operator do I like?”

The question is:

What does my callback return?

Plain value: map.

Effect value: flatMap.