2.4 Debugging Nix with trace and breakpoints

Sooner or later a value buried in nested let blocks is not what you expect, and you need a way to look inside. This lesson covers debugging Nix expressions with builtins.trace and the built-in breakpoint debugger, the two tools you will reach for whenever a Nix expression misbehaves.

In this video

• builtins.trace: printing values as a side effect while the program behaves as if nothing happened
• A quiz about trace output order, and what the answer reveals about lazy evaluation
• Why leftover trace calls annoy everyone who imports your code, and the quieter alternative
• Setting breakpoints with builtins.break and activating them with --debugger
• Inspecting variable scopes, stepping, continuing and reading backtraces from a REPL inside the REPL

After this lesson

When a value deep in a Nix expression looks wrong, you can print it or pause right there and poke around, in the REPL and in nix build alike.