Debug a Nix expression with debug/trace statements?

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1

Is it possible to print debug/trace statements from a Nix expression? This could be useful when trying to fix a broken derivation.

For example, I was trying to write a shell.nix that overrides an R package, and part of my file contained:

rPackageOverrides = old: {
  jqr = old.jqr.overrideDerivation (attrs: {
    preConfigure = "patchShebangs configure";
  });
};
pkgs = import <nixpkgs> { config = { inherit rPackageOverrides; }; };

Running nix-shell would complain that old had no jqr attribute: error: attribute 'jqr' missing.

It would be helpful if I could inspect some of old’s attributes. Is there a way to do that?

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2

Use builtins.trace (string to output) (expr to return).
You probably also don’t want to inherit rPackageOverrides, but use packageOverrides instead.

nix-build --no-out-link - <<'EOF'
let
  packageOverrides = old: {
    jqr = builtins.trace ''jq: ${old.jq}, jqr: ${old.jqr or "unavailable"}'' old.hello;
  };
  pkgs = import <nixpkgs> { config = { inherit packageOverrides; }; };
in
  pkgs.jqr
EOF

What are you looking for in the old attributes?

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3

builtins.trace looks quite helpful, thanks earvstedt! Is there something like a builtins.breakpoint that drops you into nix repl when it evaluates?

4

Thanks, builtins.trace is exactly what I was looking for.

I just wanted to poke around to see if old was the top-level pkgs set, or something else.

I’m using rPackageOverrides because I’m trying to modify the jqr derivation produced by r-modules by leveraging the overrides argument that r-modules/default.nix accepts. In top-level/all-packages.nix, rPackages is set like:

rPackages = callPackage ../development/r-modules {
  overrides = (config.rPackageOverrides or (p: {})) pkgs;
};

I can’t figure out how to actually accomplish this (referencing old.rPackages.jqr leads to infinite recursion), but builtins.trace is an invaluable lesson. Thanks!

5

No, that’s not yet possible.

6

Related helpers can be found in https://github.com/NixOS/nixpkgs/blob/master/lib/debug.nix

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