nixpkgs/lib/fixed-points.nix
Graham Christensen 152c63c9ff
Convert libs to a fixed-point
This does break the API of being able to import any lib file and get
its libs, however I'm not sure people did this.

I made this while exploring being able to swap out docFn with a stub
in #2305, to avoid functor performance problems. I don't know if that
is going to move forward (or if it is a problem or not,) but after
doing all this work figured I'd put it up anyway :)

Two notable advantages to this approach:

1. when a lib inherits another lib's functions, it doesn't
   automatically get put in to the scope of lib
2. when a lib implements a new obscure functions, it doesn't
   automatically get put in to the scope of lib

Using the test script (later in this commit) I got the following diff
on the API:

  + diff master fixed-lib
  11764a11765,11766
  > .types.defaultFunctor
  > .types.defaultTypeMerge
  11774a11777,11778
  > .types.isOptionType
  > .types.isType
  11781a11786
  > .types.mkOptionType
  11788a11794
  > .types.setType
  11795a11802
  > .types.types

This means that this commit _adds_ to the API, however I can't find a
way to fix these last remaining discrepancies. At least none are
_removed_.

Test script (run with nix-repl in the PATH):

  #!/bin/sh

  set -eux

  repl() {
      suff=${1:-}
      echo "(import ./lib)$suff" \
          | nix-repl 2>&1
  }

  attrs_to_check() {
      repl "${1:-}" \
          | tr ';'  $'\n' \
          | grep "\.\.\." \
          | cut -d' ' -f2 \
          | sed -e "s/^/${1:-}./" \
          | sort
  }

  summ() {
      repl "${1:-}" \
          | tr ' ' $'\n' \
          | sort \
          | uniq
  }

  deep_summ() {
      suff="${1:-}"
      depth="${2:-4}"
      depth=$((depth - 1))
      summ "$suff"

      for attr in $(attrs_to_check "$suff" | grep -v "types.types"); do
          if [ $depth -eq 0 ]; then
              summ "$attr" | sed -e "s/^/$attr./"
          else
              deep_summ "$attr" "$depth" | sed -e "s/^/$attr./"
          fi
      done
  }

  (
      cd nixpkgs

      #git add .
      #git commit -m "Auto-commit, sorry" || true
      git checkout fixed-lib
      deep_summ > ../fixed-lib
      git checkout master
      deep_summ > ../master
  )

  if diff master fixed-lib; then
      echo "SHALLOW MATCH!"
  fi

  (
      cd nixpkgs
      git checkout fixed-lib
      repl .types
  )
2017-09-16 21:36:43 -04:00

80 lines
3.1 KiB
Nix

{ ... }:
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);
};
}