diff --git a/lib/default.nix b/lib/default.nix index e692637abf1..3893e349db3 100644 --- a/lib/default.nix +++ b/lib/default.nix @@ -5,8 +5,9 @@ */ let - # trivial, often used functions + # often used, or depending on very little trivial = import ./trivial.nix; + fixedPoints = import ./fixed-points.nix; # datatypes attrsets = import ./attrsets.nix; @@ -42,7 +43,7 @@ let filesystem = import ./filesystem.nix; in - { inherit trivial + { inherit trivial fixedPoints attrsets lists strings stringsWithDeps customisation maintainers meta sources modules options types @@ -55,6 +56,7 @@ in } # !!! don't include everything at top-level; perhaps only the most # commonly used functions. - // trivial // lists // strings // stringsWithDeps // attrsets // sources + // trivial // fixedPoints + // lists // strings // stringsWithDeps // attrsets // sources // options // types // meta // debug // misc // modules // customisation diff --git a/lib/fixed-points.nix b/lib/fixed-points.nix new file mode 100644 index 00000000000..a11b5a6f4bd --- /dev/null +++ b/lib/fixed-points.nix @@ -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); + }; +} diff --git a/lib/trivial.nix b/lib/trivial.nix index ffbf96aa9bc..9ee0549fc0f 100644 --- a/lib/trivial.nix +++ b/lib/trivial.nix @@ -43,84 +43,6 @@ rec { */ 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"; } - # - # 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); - }; - # Flip the order of the arguments of a binary function. flip = f: a: b: f b a;