HOF docs

README.md

@@ -184,7 +184,23 @@ fn main() {
 
 ## Higher order functions
 
-TODO: Coming soon
+You can define functions in a context based on a previously parsed expression.
+
+```rust
+use xprs::{xprs_fn, Context, Parser, Xprs};
+
+fn main() {
+    let xprs_hof = Xprs::try_from("2x + y").unwrap();
+    let fn_hof = xprs_hof.bind2("x", "y").unwrap();
+    let hof = xprs_fn!("hof", dyn fn_hof, 2);
+    let ctx = Context::default().with_fn(hof);
+    let parser = Parser::new_with_ctx(ctx);
+
+    let xprs = parser.parse("hof(2, 3)").unwrap();
+
+    println!("hof(2, 3) = {}", xprs.eval_no_vars().unwrap());
+}
+```
 
 These examples and others can be found in the [examples](./examples) directory.