From 34074287861b3ef6c9ee89195056d20ae1603cfc Mon Sep 17 00:00:00 2001
From: Joel Kronqvist <joel.kronqvist@iki.fi>
Date: Sat, 26 Jul 2025 18:34:44 +0300
Subject: Added variables and tests for their substitution

---
 src/sexp/mod.rs   |  5 +++--
 src/sexp/subst.rs | 21 +++++++++++++++++++++
 src/sexp/util.rs  |  5 +++++
 3 files changed, 29 insertions(+), 2 deletions(-)
 create mode 100644 src/sexp/subst.rs

diff --git a/src/sexp/mod.rs b/src/sexp/mod.rs
index 087cb67..d866be5 100644
--- a/src/sexp/mod.rs
+++ b/src/sexp/mod.rs
@@ -1,12 +1,12 @@
 
 pub mod step;
 pub mod util;
+pub mod subst;
 
 /// A leaf node for S-Expressions.
 ///
 /// May represent built-in operators,
-/// variables (to be added), functions
-/// (to be added) or values.
+/// variables, functions (to be added) or values.
 #[derive(Debug)]
 #[derive(PartialEq)]
 pub enum SLeaf {
@@ -15,6 +15,7 @@ pub enum SLeaf {
     Mul,
     Div,
     Int(i32),
+    Var(String),
 }
 
 /// An S-Expression; the defining structure of the language.
diff --git a/src/sexp/subst.rs b/src/sexp/subst.rs
new file mode 100644
index 0000000..7cfa180
--- /dev/null
+++ b/src/sexp/subst.rs
@@ -0,0 +1,21 @@
+
+use crate::sexp::{SExp, SExp::*, SLeaf::*, util::*};
+
+impl SExp {
+    /// Performs substitution on the s-expression.
+    ///
+    /// Replaces all free occurrences of variables
+    /// named `name` with the given `value`
+    /// in this s-expression.
+    /// ```rust
+    /// use melisp::sexp::{SExp::*, SLeaf::*, util::*};
+    /// 
+    /// assert_eq!(
+    ///     scons(Add, scons(var("a"), var("b"))).subst("b", scons(Sub, scons(2, 1))),
+    ///     scons(Add, scons(var("a"), scons(Sub, scons(2, 1))))
+    /// );
+    /// ```
+    pub fn subst(self, name: &str, value: SExp) -> SExp {
+        todo!();
+    }
+}
diff --git a/src/sexp/util.rs b/src/sexp/util.rs
index c31358f..ce6de5d 100644
--- a/src/sexp/util.rs
+++ b/src/sexp/util.rs
@@ -25,3 +25,8 @@ impl From<i32> for SLeaf {
 pub fn scons(x: impl Into<Box<SExp>>, y: impl Into<Box<SExp>>) -> SExp {
     SCons(x.into(), y.into())
 }
+
+pub fn var(name: &str) -> SExp {
+    Atom(Var(name.to_string()))
+}
+
-- 
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