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nixpkgs/lib/asserts.nix
sternenseemann 48965506a1 lib/asserts: use throw to display message for assertMsg
`assert` has the annoying property that it dumps a lot of code at the
user without the built in capability to display a nicer message. We have
worked around this using `assertMsg` which would *additionally* display
a nice message. We can do even better: By using `throw` we can make
evaluation fail before assert draws its conclusions and prevent it from
displaying the code making up the assert condition, so we get the nicer
message of `throw` and the syntactical convenience of `assert`.

Before:

    nix-repl> python.override { reproducibleBuild = true; stripBytecode = false; }
    trace: Deterministic builds require stripping bytecode.
    error: assertion (((lib).assertMsg  (reproducibleBuild -> stripBytecode))  "Deterministic builds require stripping bytecode.") failed at /home/lukas/src/nix/nixpkgs/pkgs/development/interpreters/python/cpython/2.7/default.nix:45:1

After:

    nix-repl> python.override { reproducibleBuild = true; stripBytecode = false; }
    error: Deterministic builds require stripping bytecode.
2022-01-19 00:50:06 +01:00

43 lines
1.1 KiB
Nix

{ lib }:
rec {
/* Throw if pred is false, else return pred.
Intended to be used to augment asserts with helpful error messages.
Example:
assertMsg false "nope"
stderr> error: nope
assert assertMsg ("foo" == "bar") "foo is not bar, silly"; ""
stderr> error: foo is not bar, silly
Type:
assertMsg :: Bool -> String -> Bool
*/
# TODO(Profpatsch): add tests that check stderr
assertMsg = pred: msg:
pred || builtins.throw msg;
/* 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" ]
stderr> error: sslLibrary must be one of [
stderr> "openssl"
stderr> "bearssl"
stderr> ], 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}";
}