path: root/lib/fixed-points.nix

{ lib, ... }:
rec {
  /*
    `fix f` computes the fixed point of the given function `f`. In other words, the return value is `x` in `x = f x`.

    `f` must be a lazy function.
    This means that `x` must be a value that can be partially evaluated,
    such as an attribute set, a list, or a function.
    This way, `f` can use one part of `x` to compute another part.

    **Relation to syntactic recursion**

    This section explains `fix` by refactoring from syntactic recursion to a call of `fix` instead.

    For context, Nix lets you define attributes in terms of other attributes syntactically using the [`rec { }` syntax](https://nixos.org/manual/nix/stable/language/constructs.html#recursive-sets).

    ```nix
    nix-repl> rec {
      foo = "foo";
      bar = "bar";
      foobar = foo + bar;
    }
    { bar = "bar"; foo = "foo"; foobar = "foobar"; }
    ```

    This is convenient when constructing a value to pass to a function for example,
    but an equivalent effect can be achieved with the `let` binding syntax:

    ```nix
    nix-repl> let self = {
      foo = "foo";
      bar = "bar";
      foobar = self.foo + self.bar;
    }; in self
    { bar = "bar"; foo = "foo"; foobar = "foobar"; }
    ```

    But in general you can get more reuse out of `let` bindings by refactoring them to a function.

    ```nix
    nix-repl> f = self: {
      foo = "foo";
      bar = "bar";
      foobar = self.foo + self.bar;
    }
    ```

    This is where `fix` comes in, it contains the syntactic recursion that's not in `f` anymore.

    ```nix
    nix-repl> fix = f:
      let self = f self; in self;
    ```

    By applying `fix` we get the final result.

    ```nix
    nix-repl> fix f
    { bar = "bar"; foo = "foo"; foobar = "foobar"; }
    ```

    Such a refactored `f` using `fix` is not useful by itself.
    See [`extends`](#function-library-lib.fixedPoints.extends) for an example use case.
    There `self` is also often called `final`.

    Type: fix :: (a -> a) -> a

    Example:
      fix (self: { foo = "foo"; bar = "bar"; foobar = self.foo + self.bar; })
      => { bar = "bar"; foo = "foo"; foobar = "foobar"; }

      fix (self: [ 1 2 (elemAt self 0 + elemAt self 1) ])
      => [ 1 2 3 ]
  */
  fix = f: let x = f x; in x;

  /*
    A variant of `fix` that records the original recursive attribute set in the
    result, in an attribute named `__unfix__`.

    This is useful in combination with the `extends` function to
    implement deep overriding.
  */
  fix' = f: let x = f x // { __unfix__ = f; }; in x;

  /*
    Return the fixpoint that `f` converges to when called iteratively, starting
    with the input `x`.

    ```
    nix-repl> converge (x: x / 2) 16
    0
    ```

    Type: (a -> a) -> a -> a
  */
  converge = f: x:
    let
      x' = f x;
    in
      if x' == x
      then x
      else converge f x';

  /*
    Extend a function using an overlay.

    Overlays allow modifying and extending fixed-point functions, specifically ones returning attribute sets.
    A fixed-point function is a function which is intended to be evaluated by passing the result of itself as the argument.
    This is possible due to Nix's lazy evaluation.


    A fixed-point function returning an attribute set has the form

    ```nix
    final: { # attributes }
    ```

    where `final` refers to the lazily evaluated attribute set returned by the fixed-point function.

    An overlay to such a fixed-point function has the form

    ```nix
    final: prev: { # attributes }
    ```

    where `prev` refers to the result of the original function to `final`, and `final` is the result of the composition of the overlay and the original function.

    Applying an overlay is done with `extends`:

    ```nix
    let
      f = final: { # attributes };
      overlay = final: prev: { # attributes };
    in extends overlay f;
    ```

    To get the value of `final`, use `lib.fix`:

    ```nix
    let
      f = final: { # attributes };
      overlay = final: prev: { # attributes };
      g = extends overlay f;
    in fix g
    ```

    :::{.note}
    The argument to the given fixed-point function after applying an overlay will *not* refer to its own return value, but rather to the value after evaluating the overlay function.

    The given fixed-point function is called with a separate argument than if it was evaluated with `lib.fix`.
    :::

    :::{.example}

    # Extend a fixed-point function with an overlay

    Define a fixed-point function `f` that expects its own output as the argument `final`:

    ```nix-repl
    f = final: {
      # Constant value a
      a = 1;

      # b depends on the final value of a, available as final.a
      b = final.a + 2;
    }
    ```

    Evaluate this using [`lib.fix`](#function-library-lib.fixedPoints.fix) to get the final result:

    ```nix-repl
    fix f
    => { a = 1; b = 3; }
    ```

    An overlay represents a modification or extension of such a fixed-point function.
    Here's an example of an overlay:

    ```nix-repl
    overlay = final: prev: {
      # Modify the previous value of a, available as prev.a
      a = prev.a + 10;

      # Extend the attribute set with c, letting it depend on the final values of a and b
      c = final.a + final.b;
    }
    ```

    Use `extends overlay f` to apply the overlay to the fixed-point function `f`.
    This produces a new fixed-point function `g` with the combined behavior of `f` and `overlay`:

    ```nix-repl
    g = extends overlay f
    ```

    The result is a function, so we can't print it directly, but it's the same as:

    ```nix-repl
    g' = final: {
      # The constant from f, but changed with the overlay
      a = 1 + 10;

      # Unchanged from f
      b = final.a + 2;

      # Extended in the overlay
      c = final.a + final.b;
    }
    ```

    Evaluate this using [`lib.fix`](#function-library-lib.fixedPoints.fix) again to get the final result:

    ```nix-repl
    fix g
    => { a = 11; b = 13; c = 24; }
    ```
    :::

    Type:
      extends :: (Attrs -> Attrs -> Attrs) # The overlay to apply to the fixed-point function
              -> (Attrs -> Attrs) # A fixed-point function
              -> (Attrs -> Attrs) # The resulting fixed-point function

    Example:
      f = final: { a = 1; b = final.a + 2; }

      fix f
      => { a = 1; b = 3; }

      fix (extends (final: prev: { a = prev.a + 10; }) f)
      => { a = 11; b = 13; }

      fix (extends (final: prev: { b = final.a + 5; }) f)
      => { a = 1; b = 6; }

      fix (extends (final: prev: { c = final.a + final.b; }) f)
      => { a = 1; b = 3; c = 4; }
  */
  extends =
    # The overlay to apply to the fixed-point function
    overlay:
    # The fixed-point function
    f:
    # Wrap with parenthesis to prevent nixdoc from rendering the `final` argument in the documentation
    # The result should be thought of as a function, the argument of that function is not an argument to `extends` itself
    (
      final:
      let
        prev = f final;
      in
      prev // overlay final prev
    );

  /*
    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: final: prev:
      let fApplied = f final prev;
          prev' = prev // fApplied;
      in fApplied // g final prev';

  /*
    Compose several extending functions of the type expected by 'extends' into
    one where changes made in preceding functions are made available to
    subsequent ones.

    ```
    composeManyExtensions : [packageSet -> packageSet -> packageSet] -> packageSet -> packageSet -> packageSet
                              ^final        ^prev         ^overrides     ^final        ^prev         ^overrides
    ```
  */
  composeManyExtensions =
    lib.foldr (x: y: composeExtensions x y) (final: prev: {});

  /*
    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' (self: (rattrs self) // {
      ${extenderName} = f: makeExtensibleWithCustomName extenderName (extends f rattrs);
    });
}