warn

lib.trivial.warn

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warn message value

Print a warning before returning the second argument.

See builtins.warn (Nix >= 2.23). On older versions, the Nix 2.23 behavior is emulated with builtins.trace, including the NIX_ABORT_ON_WARN behavior, but not the nix.conf setting or command line option.

Inputs

message (String): Warning message to print before evaluating value.
value (any value): Value to return as-is.

Type

String -> a -> a

Noogle detected

Aliases

lib.warn

Implementation

The following is the current implementation of this function.

warn =
    # Since Nix 2.23, https://github.com/NixOS/nix/pull/10592
    builtins.warn or (
      let mustAbort = lib.elem (builtins.getEnv "NIX_ABORT_ON_WARN") ["1" "true" "yes"];
      in
        # Do not eta reduce v, so that we have the same strictness as `builtins.warn`.
        msg: v:
          # `builtins.warn` requires a string message, so we enforce that in our implementation, so that callers aren't accidentally incompatible with newer Nix versions.
          assert isString msg;
          if mustAbort
          then builtins.trace "[1;31mevaluation warning:[0m ${msg}" (abort "NIX_ABORT_ON_WARN=true; warnings are treated as unrecoverable errors.")
          else builtins.trace "[1;35mevaluation warning:[0m ${msg}" v
    );