ch1: challenges 1 and 2

.gitignore

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+# Generated by Cargo
+# will have compiled files and executables
+target/
+
+# Remove Cargo.lock from gitignore if creating an executable, leave it for libraries
+# More information here http://doc.crates.io/guide.html#cargotoml-vs-cargolock
+Cargo.lock
+
+# These are backup files generated by rustfmt
+**/*.rs.bk
+

ch1/Cargo.toml

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+[package]
+name = "ch1"
+version = "0.1.0"
+authors = ["Erin <erin@hashbang.sh>"]
+
+[dependencies]

ch1/src/lib.rs

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+#![feature(conservative_impl_trait)]
+#![feature(box_syntax)]
+
+/// The identity function. Maps a categorical object onto itself.
+/// 
+/// # Haskell
+/// ```text
+/// id :: a -> a
+/// id x = x
+/// ```
+pub fn id<T>(x: T) -> T { x }
+
+/// The composition function. Chains two functions together.
+///
+/// # Haskell
+/// ```text
+/// compose :: (b -> c) -> (a -> b) -> (a -> c)
+/// compose f g = λx -> f $ g x
+/// ```
+/// 
+/// # Notes
+/// I don't like that this takes `Box`ed functions. Rust doesn't like parameterizing on functions
+/// with generic arguments, even if those arguments are sized
+/// (`rustc` complains that `std::ops::Fn(A) -> B + 'static` does not have a constant size known at compile-time)
+pub fn compose<A, B, C>(f: Box<Fn(B) -> C>, g: Box<Fn(A) -> B>) -> impl Fn(A) -> C {
+    move |x| { f(g(x)) }
+}
+
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn compose_smoketest() {
+        fn f(x: f64) -> f64 { x.powi(2) }
+        fn g(x: f64) -> f64 { x + 3. }
+        let h = compose(box g, box f);
+
+        assert_eq!(h(3.), g(f(3.)));
+    }
+}