Noogλe

Restrict a derivation to a predictable set of attribute names, so that the returned attrset is not strict in the actual derivation, saving a lot of computation when the derivation is non-trivial.

This is useful in situations where a derivation might only be used for its passthru attributes, improving evaluation performance.

The returned attribute set is lazy in derivation. Specifically, this means that the derivation will not be evaluated in at least the situations below.

For illustration and/or testing, we define derivation such that its evaluation is very noticeable.

let derivation = throw "This won't be evaluated.";

In the following expressions, derivation will not be evaluated:

(lazyDerivation { inherit derivation; }).type

attrNames (lazyDerivation { inherit derivation; })

(lazyDerivation { inherit derivation; } // { foo = true; }).foo

(lazyDerivation { inherit derivation; meta.foo = true; }).meta

In these expressions, derivation will be evaluated:

"${lazyDerivation { inherit derivation }}"

(lazyDerivation { inherit derivation }).outPath

(lazyDerivation { inherit derivation }).meta

And the following expressions are not valid, because the refer to implementation details and/or attributes that may not be present on some derivations:

(lazyDerivation { inherit derivation }).buildInputs

(lazyDerivation { inherit derivation }).passthru

(lazyDerivation { inherit derivation }).pythonPath

Inputs

Takes an attribute set with the following attributes

derivation

The derivation to be wrapped.

meta

Optional meta attribute. While this function is primarily about derivations, it can improve the meta package attribute, which is usually specified through mkDerivation.

passthru

Optional extra values to add to the returned attrset. This can be used for adding package attributes, such as tests.

outputs

Optional list of assumed outputs. Default: [ "out" ] This must match the set of outputs that the returned derivation has. You must use this when the derivation has multiple outputs.