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base repository: NixOS/nixpkgs
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  • 2 commits
  • 1 file changed
  • 2 contributors

Commits on Nov 21, 2018

  1. lib/fixed-points.nix: add an example for extends 

     - helped me understand how extends works, hopefully it can help others too
    @typetetris
    typetetris committed Nov 21, 2018

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    Mic92 Jörg Thalheim
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  2. Merge pull request #50532 from typetetris/add-extends-example 

    lib/fixed-points.nix: add an example for extends
    @roberth
    roberth authored Nov 21, 2018
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Showing with 12 additions and 0 deletions.
  1. +12 −0 lib/fixed-points.nix
12 changes: 12 additions & 0 deletions lib/fixed-points.nix
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Original file line number Diff line number Diff line change
@@ -41,6 +41,18 @@ rec {
# think of it as an infix operator `g extends f` that mimics the syntax from
# 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.
#
# 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`
# unfolds with `g` and `f` defined above:
#
# extends g f = self: let super = f self; in super // g self super;
# = self: let super = { foo = "foo"; bar = "bar"; foobar = self.foo + self.bar; }; in super // g self super
# = self: { foo = "foo"; bar = "bar"; foobar = self.foo + self.bar; } // g self { foo = "foo"; bar = "bar"; foobar = self.foo + self.bar; }
# = self: { foo = "foo"; bar = "bar"; foobar = self.foo + self.bar; } // { foo = "foo" + " + "; }
# = self: { foo = "foo + "; bar = "bar"; foobar = self.foo + self.bar; }
#
extends = f: rattrs: self: let super = rattrs self; in super // f self super;

# Compose two extending functions of the type expected by 'extends'