An high level example case of this problem occuring can be found below: nix-repl> lib.generators.toPretty {} (lib.concatStringsSep "\n") error: 'functionArgs' requires a function, at /home/lukas/src/nix/nixpkgs/lib/trivial.nix:334:42 However this does not happen on other partially applied functions: nix-repl> lib.generators.toPretty {} (lib.concatMapStringsSep "\n") "<function>" The issue, as it turns out is that while builtins are functions, builtins.functionArgs throws if is passed a builtin or a partially applied builtin: nix-repl> lib.generators.toPretty {} builtins.toString error: 'functionArgs' requires a function, at /home/lukas/src/nix/nixpkgs/lib/trivial.nix:334:42 nix-repl> lib.generators.toPretty {} (builtins.foldl' (a: b: a + b)) error: 'functionArgs' requires a function, at /home/lukas/src/nix/nixpkgs/lib/trivial.nix:334:42 I'm pretty sure this qualifies as a nix bug and should be filed accordingly, but we can work around it in lib.generators.toPretty by using tryEval and falling back to {} which functionArgs _should_ return for builtins. The nix behavior is inconsistent to say the least: nix-repl> builtins.functionArgs builtins.functionArgs error: 'functionArgs' requires a function, at (string):1:1 nix-repl> builtins.typeOf builtins.functionArgs "lambda" builtins.functionArgs (a: 1 + a) { } nix-repl> builtins.typeOf (a: 1 + a) "lambda"wip/yesman
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