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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.
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@ -2,35 +2,33 @@
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rec {
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/* Print a trace message if pred is false.
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/* Throw if pred is false, else return pred.
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Intended to be used to augment asserts with helpful error messages.
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Example:
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assertMsg false "nope"
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=> false
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stderr> trace: nope
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stderr> error: nope
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assert (assertMsg ("foo" == "bar") "foo is not bar, silly"); ""
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stderr> trace: foo is not bar, silly
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stderr> assert failed at …
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assert assertMsg ("foo" == "bar") "foo is not bar, silly"; ""
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stderr> error: foo is not bar, silly
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Type:
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assertMsg :: Bool -> String -> Bool
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*/
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# TODO(Profpatsch): add tests that check stderr
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assertMsg = pred: msg:
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if pred
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then true
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else builtins.trace msg false;
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pred || builtins.throw msg;
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/* Specialized `assertMsg` for checking if val is one of the elements
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of a list. Useful for checking enums.
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Example:
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let sslLibrary = "libressl"
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let sslLibrary = "libressl";
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in assertOneOf "sslLibrary" sslLibrary [ "openssl" "bearssl" ]
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=> false
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stderr> trace: sslLibrary must be one of "openssl", "bearssl", but is: "libressl"
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stderr> error: sslLibrary must be one of [
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stderr> "openssl"
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stderr> "bearssl"
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stderr> ], but is: "libressl"
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Type:
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assertOneOf :: String -> ComparableVal -> List ComparableVal -> Bool
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@ -408,6 +408,18 @@
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configuration.
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</para>
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</listitem>
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<listitem>
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<para>
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<literal>lib.assertMsg</literal> and
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<literal>lib.assertOneOf</literal> no longer return
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<literal>false</literal> if the passed condition is
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<literal>false</literal>, <literal>throw</literal>ing the
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given error message instead (which makes the resulting error
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message less cluttered). This will not impact the behaviour of
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code using these functions as intended, namely as top-level
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wrapper for <literal>assert</literal> conditions.
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</para>
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</listitem>
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</itemizedlist>
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</section>
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<section xml:id="sec-release-22.05-notable-changes">
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@ -132,6 +132,8 @@ In addition to numerous new and upgraded packages, this release has the followin
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[settings-style](https://github.com/NixOS/rfcs/blob/master/rfcs/0042-config-option.md)
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configuration.
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- `lib.assertMsg` and `lib.assertOneOf` no longer return `false` if the passed condition is `false`, `throw`ing the given error message instead (which makes the resulting error message less cluttered). This will not impact the behaviour of code using these functions as intended, namely as top-level wrapper for `assert` conditions.
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<!-- To avoid merge conflicts, consider adding your item at an arbitrary place in the list instead. -->
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## Other Notable Changes {#sec-release-22.05-notable-changes}
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