init

.gitignore

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+/target

Cargo.lock

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+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "statemonad"
+version = "0.1.0"

Cargo.toml

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+[package]
+name = "statemonad"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

src/main.rs

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+mod state {
+    // State monad
+    pub struct WithState<'c, Value, State>(Box<dyn FnOnce(State) -> (Value, State) + 'c>);
+
+    impl <'c, Value, State> WithState<'c, Value, State> {
+	pub fn of(f: impl FnOnce(State) -> (Value, State) + 'c) -> WithState<'c, Value, State> {
+	    WithState(Box::new(f))
+	}
+	
+	pub fn run(self, s: State) -> (Value, State) {
+	    self.0(s)
+	}
+
+	pub fn bind<'cn, NewValue>(self, f: impl FnOnce(Value) -> WithState<'cn, NewValue, State> + 'cn) -> WithState<'cn, NewValue, State>
+	where 'c: 'cn, State : 'cn, Value : 'cn {
+	    WithState::<'cn, NewValue, State>::of(move |n| {
+		let (x,sp) = self.run(n);
+		f(x).run(sp)
+	    })
+	}
+
+	pub fn zip<'cn, NewValue>(self, st: WithState<'cn, NewValue, State>) -> WithState<'cn, (Value,NewValue), State>
+	where 'c: 'cn, State: 'cn, Value: 'cn, NewValue: 'cn {
+	    self.bind(|aa| st.map(|bb| (aa,bb)))
+	}
+
+	pub fn map<'cn, NewValue>(self, f: impl FnOnce(Value) -> NewValue + 'cn) -> WithState<'cn, NewValue, State>
+	where 'c: 'cn, State : 'cn, Value : 'cn {
+	    WithState::<'cn, NewValue, State>::of(move |n| {
+		let (x, sp) = self.run(n);
+		( f(x), sp )
+	    })
+	}
+    }
+}
+
+
+mod labelgenerators {
+    use super::state::WithState;
+
+    // Generate a next label
+    pub trait States {
+	type Item;
+	fn init() -> Self::Item;
+	fn next<'c, Value>(f: impl FnOnce(Self::Item) -> Value + 'c) -> WithState<'c, Value, Self::Item>;
+    }
+
+    pub struct IntCounter;
+
+    impl States for IntCounter
+    {
+	type Item = i32;
+	
+	fn init() -> i32 { 0 }
+	
+	fn next<'c, Value>(f: impl FnOnce(i32) -> Value + 'c) -> WithState<'c, Value, i32> {
+	    WithState::of(move |n| { (f(n), n + 1) })
+	}
+    }
+
+
+    pub struct StrCat;
+
+    impl States for StrCat
+    {
+	type Item = String;
+
+	fn init() -> String { "a".to_string() }
+
+	fn next<'c, Value>(f: impl FnOnce(Self::Item) -> Value + 'c) -> WithState<'c, Value, Self::Item> {
+	    WithState::of(move |n: String| { let s = n.clone(); (f(n), format!("{}a", s)) })
+	}
+    }
+}
+
+use labelgenerators::*;
+use state::WithState;
+
+// Tree
+#[derive(Debug,Clone)]
+enum Tree<A> {
+    Leaf(A),
+    Node(Box<Tree<A>>, Box<Tree<A>>)
+}
+
+impl <A> Tree<A> {
+    fn node(l: Tree<A>, r: Tree<A>) -> Tree<A> {
+	Tree::Node(Box::new(l), Box::new(r))
+    }
+
+    fn label<NodeLabler : States>(self) -> Tree<NodeLabler::Item>
+    where NodeLabler::Item : Clone {
+	self.mlabel::<NodeLabler>().run(NodeLabler::init()).0
+    }
+
+    // Monadically label a Tree<A> to Tree<L>
+    fn mlabel<'c, NodeLabler: States>(self) -> WithState<'c, Tree<NodeLabler::Item>, NodeLabler::Item>
+    where NodeLabler::Item : Clone + 'c {
+	match self {
+	    Tree::Leaf(_) => NodeLabler::next(|n| Tree::Leaf(n)),
+	    Tree::Node(l, r) => {
+		let (l,r) = (*l, *r);
+
+		l.mlabel::<NodeLabler>().zip(r.mlabel::<NodeLabler>())
+		    .map(|(ll,rr)| Tree::node(ll,rr))
+	    }
+	}
+    }
+}
+
+fn main() {
+
+    let init = Tree::node(
+	Tree::node(
+	    Tree::Leaf('a'),
+	    Tree::Leaf('b')),
+	Tree::node(
+	    Tree::node(
+		Tree::Leaf('c'),
+		Tree::Leaf('d')),
+	    Tree::Leaf('e'))
+    );
+
+    let mapped = init.clone().label::<IntCounter>();
+
+    dbg!(init);
+    dbg!(mapped);
+}
+