From cdaf2e7013b0017228aba328f4a9bc511f79ad38 Mon Sep 17 00:00:00 2001
From: Camden Dixie O'Brien <cdo@wip.sh>
Date: Sat, 26 Sep 2026 11:49:20 +0100
Subject: [PATCH] Implement evaluator composition

---
 src/eval.rs         |   1 +
 src/eval/compose.rs | 148 ++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 149 insertions(+)
 create mode 100644 src/eval/compose.rs

diff --git a/src/eval.rs b/src/eval.rs
index 4cd38b6..00a85f2 100644
--- a/src/eval.rs
+++ b/src/eval.rs
@@ -1,5 +1,6 @@
 pub mod normalise;
 pub mod const_fold;
+pub mod compose;
 
 #[derive(Debug)]
 pub enum Error {
diff --git a/src/eval/compose.rs b/src/eval/compose.rs
new file mode 100644
index 0000000..6791d3b
--- /dev/null
+++ b/src/eval/compose.rs
@@ -0,0 +1,148 @@
+use crate::eval::Evaluator;
+
+pub struct Composition<E1, E2> {
+	first: E1,
+	second: E2,
+}
+
+impl<E1, E2> Composition<E1, E2> {
+	pub fn new(first: E1, second: E2) -> Self {
+		Self { first, second }
+	}
+}
+
+impl<E1, E2, Expr, Error> Evaluator for Composition<E1, E2>
+where
+	E1: Evaluator<Expr = Expr, Error = Error>,
+	E2: Evaluator<Expr = Expr, Error = Error>
+{
+	type Expr = Expr;
+	type Error = Error;
+
+	fn eval(&mut self, input: Expr) -> Result<Expr, Error> {
+		self.second.eval(self.first.eval(input)?)
+	}
+}
+
+#[macro_export]
+macro_rules! pipeline {
+	($last:expr) => {
+		$last
+	};
+	($first:expr, $($rest:expr),+) => {
+		$crate::eval::compose::Composition::new(
+			$first, $crate::pipeline!($($rest),+))
+	};
+}
+
+#[cfg(test)]
+mod tests {
+	use super::*;
+
+	#[derive(Debug, PartialEq)]
+	enum Expr {
+		Foo,
+		Bar,
+		Baz,
+		Quux,
+	}
+
+	#[derive(Debug, PartialEq)]
+	enum Err {
+		First,
+		Second,
+		Third,
+	}
+
+	struct First;
+	struct Second;
+	struct Third;
+
+	impl Evaluator for First {
+		type Expr = Expr;
+		type Error = Err;
+
+		fn eval(&mut self, input: Expr) -> Result<Expr, Err> {
+			match input {
+				Expr::Foo => Ok(Expr::Bar),
+				Expr::Baz => Ok(Expr::Foo),
+				Expr::Quux => Ok(Expr::Baz),
+				_ => Err(Err::First),
+			}
+		}
+	}
+
+	impl Evaluator for Second {
+		type Expr = Expr;
+		type Error = Err;
+
+		fn eval(&mut self, input: Expr) -> Result<Expr, Err> {
+			match input {
+				Expr::Bar => Ok(Expr::Baz),
+				Expr::Baz => Ok(Expr::Foo),
+				_ => Err(Err::Second),
+			}
+		}
+	}
+
+	impl Evaluator for Third {
+		type Expr = Expr;
+		type Error = Err;
+
+		fn eval(&mut self, input: Expr) -> Result<Expr, Err> {
+			match input {
+				Expr::Baz => Ok(Expr::Quux),
+				_ => Err(Err::Third),
+			}
+		}
+	}
+
+	#[test]
+	fn composition_chains_evaluators_in_order() {
+		let mut comp = Composition::new(First {}, Second {});
+		let result = comp.eval(Expr::Foo).unwrap();
+		assert_eq!(result, Expr::Baz);
+	}
+
+	#[test]
+	fn error_in_first_of_composition_is_returned() {
+		let mut comp = Composition::new(First {}, Second {});
+		let err = comp.eval(Expr::Bar).unwrap_err();
+		assert_eq!(err, Err::First);
+	}
+
+	#[test]
+	fn error_in_second_of_composition_is_returned() {
+		let mut comp = Composition::new(First {}, Second {});
+		let err = comp.eval(Expr::Baz).unwrap_err();
+		assert_eq!(err, Err::Second);
+	}
+
+	#[test]
+	fn pipeline_chains_evaluators_in_order() {
+		let mut pipe = pipeline!(First {}, Second {}, Third {});
+		let result = pipe.eval(Expr::Foo).unwrap();
+		assert_eq!(result, Expr::Quux);
+	}
+
+	#[test]
+	fn error_in_first_of_pipeline_is_returned() {
+		let mut pipe = pipeline!(First {}, Second {}, Third {});
+		let err = pipe.eval(Expr::Bar).unwrap_err();
+		assert_eq!(err, Err::First);
+	}
+
+	#[test]
+	fn error_in_second_of_pipeline_is_returned() {
+		let mut pipe = pipeline!(First {}, Second {}, Third {});
+		let err = pipe.eval(Expr::Baz).unwrap_err();
+		assert_eq!(err, Err::Second);
+	}
+
+	#[test]
+	fn error_in_third_of_pipeline_is_returned() {
+		let mut pipe = pipeline!(First {}, Second {}, Third {});
+		let err = pipe.eval(Expr::Quux).unwrap_err();
+		assert_eq!(err, Err::Third);
+	}
+}