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lib.fixedPoints.extends: document order of invocation #245825

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lib.fixedPoints.extends: document order of invocation #245825

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4 changes: 4 additions & 0 deletions lib/fixed-points.nix
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Expand Up @@ -75,6 +75,10 @@ rec {
Java. It may seem counter-intuitive to have the "base class" as the second
argument, but it's nice this way if several uses of `extends` are cascaded.

In other words, the argument order for `extends g f` is the same as `g ∘ f`
in [mathematics](https://en.wikipedia.org/wiki/Function_composition) -- in
both cases the resulting function will perform `f` first, then perform `g`.

To get a better understanding how `extends` turns a function with a fix
point (the package set we start with) into a new function with a different fix
point (the desired packages set) lets just see, how `extends g f`
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