Part 6: Catch One Error, Not the World

try/catch is often too wide.

You want to handle one known failure. But the catch block can grab everything.

That is how bugs get hidden.

Effect gives you a smaller move: catchTag.

Handle one error

const program = fetchUser(1).pipe(
  Effect.catchTag("UserNotFound", () =>
    Effect.succeed({ id: 1, name: "Guest" }),
  ),
);

This handles UserNotFound.

It does not mean every possible error is gone.

The error slot gets smaller

Before:

Effect.Effect<User, UserNotFound | DatabaseDown, UserRepo>;

After handling UserNotFound:

Effect.Effect<User, DatabaseDown, UserRepo>;

That is the win.

The type changes because the program changed.

Do not catch what you cannot fix

This is the part I care about.

If I can turn UserNotFound into a guest user, fine.

If the database is down, I probably should not pretend everything is okay.