lazyDerivation
lib.lazyDerivation
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 throughmkDerivation
. 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.