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use crate::frame::{expand_and_collapse, MappableFrame};
/// The ability to recursively expand a seed to construct a value of this type, frame by frame.
///
/// # Example: A tree of integers
///
/// Here's an example showing how to use [`Expandable`] to expand a binary tree of integers,
/// where nodes hold two subnodes and no data and leaves hold a single `usize` value
///
/// ```rust
/// # use recursion::*;
/// #[derive(Debug, PartialEq, Eq)]
/// enum IntTree {
/// Leaf { value: usize },
/// Node { left: Box<Self>, right: Box<Self> },
/// }
///
/// # impl IntTree {
/// # fn node(left: Self, right: Self) -> Self { Self::Node{left: Box::new(left), right: Box::new(right)}}
/// # fn leaf(value: usize) -> Self { Self::Leaf{value}}
/// # }
/// ```
///
/// ## Defining a frame type
///
/// For working with values of type `IntTree`, we'll define an `IntTreeFrame<A>` frame type
/// that represents a single layer of the `IntTree` structure, with `A` subbed in for `Box<Self>`
///
/// ```rust
/// # use recursion::*;
/// enum IntTreeFrame<A> {
/// Leaf { value: usize },
/// Node { left: A, right: A },
/// }
/// impl MappableFrame for IntTreeFrame<PartiallyApplied> { /*...*/
/// # type Frame<X> = IntTreeFrame<X>;
/// #
/// # fn map_frame<A, B>(input: Self::Frame<A>, mut f: impl FnMut(A) -> B) -> Self::Frame<B> {
/// # match input {
/// # IntTreeFrame::Leaf { value } => IntTreeFrame::Leaf { value },
/// # IntTreeFrame::Node { left, right } => IntTreeFrame::Node {
/// # left: f(left),
/// # right: f(right),
/// # },
/// # }
/// # }
/// }
/// ```
///
/// ## Implementing Expandable
///
/// Then we can define an [`Expandable`] instance for `IntTree`
///
/// ```rust
/// # use recursion::*;
/// # #[derive(Debug, PartialEq, Eq)]
/// # enum IntTree {
/// # Leaf { value: usize },
/// # Node { left: Box<Self>, right: Box<Self> },
/// # }
/// # enum IntTreeFrame<A> {
/// # Leaf { value: usize },
/// # Node { left: A, right: A },
/// # }
/// # impl MappableFrame for IntTreeFrame<PartiallyApplied> {
/// # type Frame<X> = IntTreeFrame<X>;
/// #
/// # fn map_frame<A, B>(input: Self::Frame<A>, mut f: impl FnMut(A) -> B) -> Self::Frame<B> {
/// # match input {
/// # IntTreeFrame::Leaf { value } => IntTreeFrame::Leaf { value },
/// # IntTreeFrame::Node { left, right } => IntTreeFrame::Node {
/// # left: f(left),
/// # right: f(right),
/// # },
/// # }
/// # }
/// # }
/// impl Expandable for IntTree {
/// type FrameToken = IntTreeFrame<PartiallyApplied>;
///
/// fn from_frame(val: <Self::FrameToken as MappableFrame>::Frame<Self>) -> Self {
/// match val {
/// IntTreeFrame::Leaf { value } => IntTree::Leaf { value },
/// IntTreeFrame::Node { left, right } => IntTree::Node {
/// left: Box::new(left),
/// right: Box::new(right),
/// },
/// }
/// }
/// }
/// ```
/// ## Expanding a value into a tree
///
/// Finally, we can use our [`Expandable`] instance to generate a tree
///
/// ```rust
/// # use recursion::*;
/// # #[derive(Debug, PartialEq, Eq)]
/// # enum IntTree {
/// # Leaf { value: usize },
/// # Node { left: Box<Self>, right: Box<Self> },
/// # }
/// # impl IntTree {
/// # fn node(left: Self, right: Self) -> Self { Self::Node{left: Box::new(left), right: Box::new(right)}}
/// # fn leaf(value: usize) -> Self { Self::Leaf{value}}
/// # }
/// # enum IntTreeFrame<A> {
/// # Leaf { value: usize },
/// # Node { left: A, right: A },
/// # }
/// # impl MappableFrame for IntTreeFrame<PartiallyApplied> {
/// # type Frame<X> = IntTreeFrame<X>;
/// #
/// # fn map_frame<A, B>(input: Self::Frame<A>, mut f: impl FnMut(A) -> B) -> Self::Frame<B> {
/// # match input {
/// # IntTreeFrame::Leaf { value } => IntTreeFrame::Leaf { value },
/// # IntTreeFrame::Node { left, right } => IntTreeFrame::Node {
/// # left: f(left),
/// # right: f(right),
/// # },
/// # }
/// # }
/// # }
/// # impl Expandable for IntTree {
/// # type FrameToken = IntTreeFrame<PartiallyApplied>;
/// #
/// # fn from_frame(val: <Self::FrameToken as MappableFrame>::Frame<Self>) -> Self {
/// # match val {
/// # IntTreeFrame::Leaf { value } => IntTree::Leaf { value },
/// # IntTreeFrame::Node { left, right } => IntTree::Node {
/// # left: Box::new(left),
/// # right: Box::new(right),
/// # },
/// # }
/// # }
/// # }
/// let depth = 2;
///
/// let expanded_tree = IntTree::expand_frames(depth, |n| {
/// if n <= 0 {
/// IntTreeFrame::Leaf { value: n }
/// } else {
/// IntTreeFrame::Node {
/// left: n - 1,
/// right: n - 1,
/// }
/// }
/// });
///
/// let expected = IntTree::node(
/// IntTree::node(IntTree::leaf(0), IntTree::leaf(0)),
/// IntTree::node(IntTree::leaf(0), IntTree::leaf(0)),
/// );
///
/// assert_eq!(expected, expanded_tree)
/// ```
pub trait Expandable
where
Self: Sized,
{
type FrameToken: MappableFrame;
/// Given a frame holding instances of `Self`, generate an instance of `Self`
fn from_frame(val: <Self::FrameToken as MappableFrame>::Frame<Self>) -> Self;
}
pub trait ExpandableExt: Expandable {
/// Given a value of type `In`, expand it to generate a value of type `Self` frame by frame,
/// using a function from `In -> Frame<In>`
fn expand_frames<In>(
input: In,
expand_frame: impl FnMut(In) -> <Self::FrameToken as MappableFrame>::Frame<In>,
) -> Self;
}
impl<X: Expandable> ExpandableExt for X {
fn expand_frames<In>(
input: In,
expand_frame: impl FnMut(In) -> <Self::FrameToken as MappableFrame>::Frame<In>,
) -> Self {
expand_and_collapse::<Self::FrameToken, In, Self>(input, expand_frame, Self::from_frame)
}
}