Lesson 8 — Organizing programs (modules)

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What are we learning?

A module is one .echo file.

You can export names from one file and import them in another, so a program can live in more than one file.

Why split a program?

• Keep related code together (math in one file, the starting file in another).
• Reuse the same function from more than one place.
• Make large programs easier to read.

Two files in the same folder

Save both files in the same folder.

math.echo

export fn add(a: int, b: int) -> int {
    return a + b;
}

export means: other files are allowed to import add.

app.echo

import add from "math";

say(add(2, 3));

import add from "math"; means: use add from the file math.echo next to this one.

Which file do you run?

Run the file that starts the program — here, app.echo:

elang app.echo

Run this command from the folder that contains both files.

Output

5

You do not run math.echo by itself for this example. It provides add; app.echo uses it.

Try it

Add export fn double(n: int) -> int { return n * 2; } to math.echo. Import it in app.echo and print double(4).

Predict it

If app.echo imports add and prints add(10, 1), what prints?

Show answer

11

Fix it — forgot export

If add is not marked export, importing it fails. Add export in front of the function in math.echo.

Deeper (later)

Relative paths, std/… imports, and full rules: Modules (full) and Module Semantics.

Playground note

The browser playground runs one editor buffer. Multi-file modules need the installed elang CLI (or a clone with examples).

Next

→ Lesson 9 — Classes