Exercism

What it does

Pick a language track, solve small exercises in the browser editor or through the CLI on your own machine, then request a mentor who reads your solution and tells you what an experienced writer of that language would have done differently. That review loop is the point — it is aimed at people who can already program and want to stop writing Python in Java. It is a non-profit funded by donations, and mentoring is free but queued.

Best for

• Learning a second or third language
• Getting idiomatic code reviewed

Where it stops