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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,78 @@ | ||
| 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: rattrs: | ||
| fix' rattrs // { | ||
| ${extenderName} = f: makeExtensibleWithCustomName extenderName (extends f rattrs); | ||
| }; | ||
| } |
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