nixpkgs/lib/asserts.nix

{ lib }:

rec {

  /* Print a trace message if pred is false.
     Intended to be used to augment asserts with helpful error messages.

     Example:
       assertMsg false "nope"
       => false
       stderr> trace: nope

       assert (assertMsg ("foo" == "bar") "foo is not bar, silly"); ""
       stderr> trace: foo is not bar, silly
       stderr> assert failed at …

     Type:
       assertMsg :: Bool -> String -> Bool
  */
  # TODO(Profpatsch): add tests that check stderr
  assertMsg = pred: msg:
    if pred
    then true
    else builtins.trace msg false;

  /* Specialized `assertMsg` for checking if val is one of the elements
     of a list. Useful for checking enums.

     Example:
       let sslLibrary = "libressl"
       in assertOneOf "sslLibrary" sslLibrary [ "openssl" "bearssl" ]
       => false
       stderr> trace: sslLibrary must be one of "openssl", "bearssl", but is: "libressl"

     Type:
       assertOneOf :: String -> ComparableVal -> List ComparableVal -> Bool
  */
  assertOneOf = name: val: xs: assertMsg
    (lib.elem val xs)
    "${name} must be one of ${
      lib.generators.toPretty {} xs}, but is: ${
        lib.generators.toPretty {} val}";

}