rec { # Identity function. id = x: x; # Constant function. const = x: y: x; # Named versions corresponding to some builtin operators. concat = x: y: x ++ y; or = x: y: x || y; and = x: y: x && y; mergeAttrs = x: y: x // y; # 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"; } # # 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; # Flip the order of the arguments of a binary function. flip = f: a: b: f b a; # Pull in some builtins not included elsewhere. inherit (builtins) pathExists readFile isBool isFunction isInt add sub lessThan seq deepSeq genericClosure; inherit (import ./strings.nix) fileContents; # Return the Nixpkgs version number. nixpkgsVersion = let suffixFile = ../.version-suffix; in fileContents ../.version + (if pathExists suffixFile then fileContents suffixFile else "pre-git"); # Whether we're being called by nix-shell. inNixShell = builtins.getEnv "IN_NIX_SHELL" != ""; # Return minimum/maximum of two numbers. min = x: y: if x < y then x else y; max = x: y: if x > y then x else y; /* Reads a JSON file. It is useful to import pure data into other nix expressions. Example: mkDerivation { src = fetchgit (importJSON ./repo.json) #... } where repo.json contains: { "url": "git://some-domain/some/repo", "rev": "265de7283488964f44f0257a8b4a055ad8af984d", "sha256": "0sb3h3067pzf3a7mlxn1hikpcjrsvycjcnj9hl9b1c3ykcgvps7h" } */ importJSON = path: builtins.fromJSON (builtins.readFile path); }