lib: Move fixed-point combinators out of trivial

Trivia != prelude. This is a better organized and less likely to
scare off new contributors.
This commit is contained in:
John Ericson 2017-05-29 18:09:52 -04:00
parent b20f20d3eb
commit 87b4a91fc4
3 changed files with 83 additions and 81 deletions

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@ -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

78
lib/fixed-points.nix Normal file
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@ -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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@ -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;