mirror of
https://github.com/NixOS/nixpkgs.git
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22d7c08dc5
This is meant to be used by packages who often re-generate their inputs. Producing valid JSON is easier than nix, and also garantees it's purity.
100 lines
3.3 KiB
Nix
100 lines
3.3 KiB
Nix
rec {
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# Identity function.
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id = x: x;
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# Constant function.
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const = x: y: x;
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# Named versions corresponding to some builtin operators.
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concat = x: y: x ++ y;
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or = x: y: x || y;
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and = x: y: x && y;
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mergeAttrs = x: y: x // y;
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# Compute the fixed point of the given function `f`, which is usually an
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# attribute set that expects its final, non-recursive representation as an
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# argument:
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#
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# f = self: { foo = "foo"; bar = "bar"; foobar = self.foo + self.bar; }
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#
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# Nix evaluates this recursion until all references to `self` have been
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# resolved. At that point, the final result is returned and `f x = x` holds:
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#
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# nix-repl> fix f
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# { bar = "bar"; foo = "foo"; foobar = "foobar"; }
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#
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# See https://en.wikipedia.org/wiki/Fixed-point_combinator for further
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# details.
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fix = f: let x = f x; in x;
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# A variant of `fix` that records the original recursive attribute set in the
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# result. This is useful in combination with the `extends` function to
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# implement deep overriding. See pkgs/development/haskell-modules/default.nix
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# for a concrete example.
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fix' = f: let x = f x // { __unfix__ = f; }; in x;
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# Modify the contents of an explicitly recursive attribute set in a way that
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# honors `self`-references. This is accomplished with a function
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#
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# g = self: super: { foo = super.foo + " + "; }
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#
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# that has access to the unmodified input (`super`) as well as the final
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# non-recursive representation of the attribute set (`self`). `extends`
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# differs from the native `//` operator insofar as that it's applied *before*
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# references to `self` are resolved:
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#
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# nix-repl> fix (extends g f)
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# { bar = "bar"; foo = "foo + "; foobar = "foo + bar"; }
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#
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# The name of the function is inspired by object-oriented inheritance, i.e.
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# think of it as an infix operator `g extends f` that mimics the syntax from
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# Java. It may seem counter-intuitive to have the "base class" as the second
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# argument, but it's nice this way if several uses of `extends` are cascaded.
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extends = f: rattrs: self: let super = rattrs self; in super // f self super;
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# Flip the order of the arguments of a binary function.
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flip = f: a: b: f b a;
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# Pull in some builtins not included elsewhere.
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inherit (builtins)
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pathExists readFile isBool isFunction
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isInt add sub lessThan
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seq deepSeq genericClosure;
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# Return the Nixpkgs version number.
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nixpkgsVersion =
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let suffixFile = ../.version-suffix; in
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readFile ../.version
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+ (if pathExists suffixFile then readFile suffixFile else "pre-git");
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# Whether we're being called by nix-shell.
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inNixShell = builtins.getEnv "IN_NIX_SHELL" == "1";
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# Return minimum/maximum of two numbers.
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min = x: y: if x < y then x else y;
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max = x: y: if x > y then x else y;
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/* Reads a JSON file. It is useful to import pure data into other nix
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expressions.
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Example:
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mkDerivation {
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src = fetchgit (importJSON ./repo.json)
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#...
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}
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where repo.json contains:
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{
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"url": "git://some-domain/some/repo",
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"rev": "265de7283488964f44f0257a8b4a055ad8af984d",
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"sha256": "0sb3h3067pzf3a7mlxn1hikpcjrsvycjcnj9hl9b1c3ykcgvps7h"
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}
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*/
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importJSON = path:
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builtins.fromJSON (builtins.readFile path);
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}
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