mirror of
https://github.com/NixOS/nixpkgs.git
synced 2024-11-02 07:31:26 +00:00
106 lines
4.2 KiB
Nix
106 lines
4.2 KiB
Nix
{ ... }:
|
|
rec {
|
|
# Compute the fixed point of the given function `f`, which is usually an
|
|
# attribute set that expects its final, non-recursive representation as an
|
|
# argument:
|
|
#
|
|
# f = self: { foo = "foo"; bar = "bar"; foobar = self.foo + self.bar; }
|
|
#
|
|
# Nix evaluates this recursion until all references to `self` have been
|
|
# resolved. At that point, the final result is returned and `f x = x` holds:
|
|
#
|
|
# nix-repl> fix f
|
|
# { bar = "bar"; foo = "foo"; foobar = "foobar"; }
|
|
#
|
|
# Type: fix :: (a -> a) -> a
|
|
#
|
|
# See https://en.wikipedia.org/wiki/Fixed-point_combinator for further
|
|
# details.
|
|
fix = f: let x = f x; in x;
|
|
|
|
# A variant of `fix` that records the original recursive attribute set in the
|
|
# result. This is useful in combination with the `extends` function to
|
|
# implement deep overriding. See pkgs/development/haskell-modules/default.nix
|
|
# for a concrete example.
|
|
fix' = f: let x = f x // { __unfix__ = f; }; in x;
|
|
|
|
# Modify the contents of an explicitly recursive attribute set in a way that
|
|
# honors `self`-references. This is accomplished with a function
|
|
#
|
|
# g = self: super: { foo = super.foo + " + "; }
|
|
#
|
|
# that has access to the unmodified input (`super`) as well as the final
|
|
# non-recursive representation of the attribute set (`self`). `extends`
|
|
# differs from the native `//` operator insofar as that it's applied *before*
|
|
# references to `self` are resolved:
|
|
#
|
|
# nix-repl> fix (extends g f)
|
|
# { bar = "bar"; foo = "foo + "; foobar = "foo + bar"; }
|
|
#
|
|
# The name of the function is inspired by object-oriented inheritance, i.e.
|
|
# 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.
|
|
extends = f: rattrs: self: let super = rattrs self; in super // f self super;
|
|
|
|
# Compose two extending functions of the type expected by 'extends'
|
|
# into one where changes made in the first are available in the
|
|
# 'super' of the second
|
|
composeExtensions =
|
|
f: g: self: super:
|
|
let fApplied = f self super;
|
|
super' = super // fApplied;
|
|
in fApplied // g self super';
|
|
|
|
# Create an overridable, recursive attribute set. For example:
|
|
#
|
|
# nix-repl> obj = makeExtensible (self: { })
|
|
#
|
|
# nix-repl> obj
|
|
# { __unfix__ = «lambda»; extend = «lambda»; }
|
|
#
|
|
# nix-repl> obj = obj.extend (self: super: { foo = "foo"; })
|
|
#
|
|
# nix-repl> obj
|
|
# { __unfix__ = «lambda»; extend = «lambda»; foo = "foo"; }
|
|
#
|
|
# nix-repl> obj = obj.extend (self: super: { foo = super.foo + " + "; bar = "bar"; foobar = self.foo + self.bar; })
|
|
#
|
|
# nix-repl> obj
|
|
# { __unfix__ = «lambda»; bar = "bar"; extend = «lambda»; foo = "foo + "; foobar = "foo + bar"; }
|
|
makeExtensible = makeExtensibleWithCustomName "extend";
|
|
|
|
# Same as `makeExtensible` but the name of the extending attribute is
|
|
# customized.
|
|
makeExtensibleWithCustomName = extenderName: f: makeExtensibleWithInterface
|
|
(fixedPoint: extend: fixedPoint // { ${extenderName} = ext: extend (_: ext); })
|
|
(_: f);
|
|
|
|
# A version of `makeExtensible` that allows the function being fixed
|
|
# to return a different interface than the interface returned to the
|
|
# user. Along with `self` and `super` views of the internal
|
|
# interface, a `self` view of the output interface is also
|
|
# provided. `extend` is not added to the output by default. This is
|
|
# the job of the interface.
|
|
#
|
|
# nix-repl> foo = {a, b}: {c = a + b;}
|
|
#
|
|
# nix-repl> interface = {args, val, ...}: extend: val // {inherit extend;}
|
|
#
|
|
# nix-repl> obj = makeExtensibleWithInterface interface (output: self: { args = {a = 1; b = 2;}; val = foo self.args; })
|
|
#
|
|
# nix-repl> obj.c
|
|
# 3
|
|
#
|
|
# nix-repl> obj = obj.extend (output: self: super: { args = super.args // { b = output.d; }; })
|
|
#
|
|
# nix-repl> obj = obj.extend (output: self: super: { val = super.val // { d = 10; }; })
|
|
#
|
|
# nix-repl> { inherit (obj) c d; }
|
|
# { c = 11; d = 10; }
|
|
makeExtensibleWithInterface = interface: f: let i = interface
|
|
(fix' (f i))
|
|
(fext: makeExtensibleWithInterface interface (i': (extends (fext i') (f i'))));
|
|
in i;
|
|
}
|