From cd3c54945d8bff57b121a2f834d165314e0f0738 Mon Sep 17 00:00:00 2001
From: Arnaud Bailly <arnaud.bailly@iohk.io>
Date: Tue, 1 Oct 2024 20:10:02 +0200
Subject: Rename eval() to be more specific

---
 rust/sample/test03/input  |  2 +-
 rust/sample/test03/output |  2 +-
 rust/src/io.rs            |  6 +++---
 rust/src/lambda.rs        | 23 ++++++++++++++---------
 4 files changed, 19 insertions(+), 14 deletions(-)

diff --git a/rust/sample/test03/input b/rust/sample/test03/input
index a23f8c6..6cff466 100644
--- a/rust/sample/test03/input
+++ b/rust/sample/test03/input
@@ -7,6 +7,6 @@
 (def four (succ three))
 (def five (succ four))
 
-(def plus (lam (a b) (a succ b)))
+(def plus (lam (a b f s) (a f (b f s))))
 
 (plus one three)
diff --git a/rust/sample/test03/output b/rust/sample/test03/output
index 578d298..fd56297 100644
--- a/rust/sample/test03/output
+++ b/rust/sample/test03/output
@@ -6,4 +6,4 @@ true
 true
 true
 true
-five
+four
diff --git a/rust/src/io.rs b/rust/src/io.rs
index 435f32f..8c628ba 100644
--- a/rust/src/io.rs
+++ b/rust/src/io.rs
@@ -5,7 +5,7 @@ use std::{
 
 use crate::{
     ast::Value,
-    lambda::{eval, eval_all, Environment},
+    lambda::{eval_all, eval_whnf, Environment},
     parser::parse,
 };
 
@@ -36,7 +36,7 @@ pub fn batch_eval<I: Read, O: Write>(inp: &mut I, outp: &mut O) {
         let values = parse(&input);
         let results = values
             .iter()
-            .map(|v| eval(v, &mut env))
+            .map(|v| eval_whnf(v, &mut env))
             .collect::<Vec<Value>>();
         for result in results {
             writeln!(outp, "{}", result).unwrap();
@@ -63,7 +63,7 @@ pub fn repl<I: Read, O: Write>(inp: &mut I, outp: &mut O) {
         let values = parse(&input);
         let results = values
             .iter()
-            .map(|v| eval(v, &mut env))
+            .map(|v| eval_whnf(v, &mut env))
             .collect::<Vec<Value>>();
         for result in results {
             writeln!(outp, "{}", result).unwrap();
diff --git a/rust/src/lambda.rs b/rust/src/lambda.rs
index f69fb5b..6f394d8 100644
--- a/rust/src/lambda.rs
+++ b/rust/src/lambda.rs
@@ -44,24 +44,29 @@ impl<'a> Default for Environment<'a> {
 
 pub fn eval_all(values: &[Value]) -> Vec<Value> {
     let mut env = Environment::new();
-    values.iter().map(|v| eval(v, &mut env)).collect()
+    values.iter().map(|v| eval_whnf(v, &mut env)).collect()
 }
 
-pub fn eval(arg: &Value, env: &mut Environment) -> Value {
+/// Reduce the given value to weak head normal form using call-by-name
+/// evaluation strategy.
+///
+/// call-by-name reduces the leftmost outermost redex first, which is
+/// not under a lambda abstraction.
+pub fn eval_whnf(arg: &Value, env: &mut Environment) -> Value {
     match arg {
         Value::Def(var, value) => {
             env.bind(var, value);
-            Value::Bool(true)
+            Value::Bool(true) // TODO: return a more meaningful value?
         }
         Value::Let(var, value, expr) => {
             let mut newenv = env.extends();
             newenv.bind(var, value);
-            eval(expr, &mut newenv)
+            eval_whnf(expr, &mut newenv)
         }
-        Value::App(l, r) => match eval(l, env) {
-            Value::Lam(v, body) => eval(&subst(&v, &body, r), env),
+        Value::App(l, r) => match eval_whnf(l, env) {
+            Value::Lam(v, body) => eval_whnf(&subst(&v, &body, r), env),
             Value::Sym(var) => match env.lookup(&var) {
-                Some(val) => eval(&Value::App(Box::new(val.clone()), r.clone()), env),
+                Some(val) => eval_whnf(&Value::App(Box::new(val.clone()), r.clone()), env),
                 None => arg.clone(),
             },
             other => Value::App(Box::new(other), r.clone()),
@@ -96,14 +101,14 @@ fn gensym() -> String {
 mod lambda_test {
     use crate::parser::parse;
 
-    use super::{eval, eval_all, Environment, Value};
+    use super::{eval_all, eval_whnf, Environment, Value};
 
     fn parse1(string: &str) -> Value {
         parse(string).pop().unwrap()
     }
 
     fn eval1(value: &Value) -> Value {
-        eval(value, &mut Environment::new())
+        eval_whnf(value, &mut Environment::new())
     }
 
     #[test]
-- 
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