Implemented substitution

2 files changed, 9 insertions, 3 deletions

diff --git a/src/sexp/mod.rs b/src/sexp/mod.rs
index d866be5..dee672f 100644
--- a/src/sexp/mod.rs
+++ b/src/sexp/mod.rs
@@ -9,6 +9,7 @@ pub mod subst;
 /// variables, functions (to be added) or values.
 #[derive(Debug)]
 #[derive(PartialEq)]
+#[derive(Clone)]
 pub enum SLeaf {
     Add,
     Sub,
@@ -24,6 +25,7 @@ pub enum SLeaf {
 /// IETF-standardized s-expressions; they are only inspiration.
 #[derive(Debug)]
 #[derive(PartialEq)]
+#[derive(Clone)]
 pub enum SExp {
     SCons(Box<SExp>, Box<SExp>),
     Atom(SLeaf),
diff --git a/src/sexp/subst.rs b/src/sexp/subst.rs
index 7cfa180..abab614 100644
--- a/src/sexp/subst.rs
+++ b/src/sexp/subst.rs
@@ -11,11 +11,15 @@ impl SExp {
     /// 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"), 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!();
+    pub fn subst(self, name: &str, value: &SExp) -> SExp {
+        match self {
+            SCons(a, b) => scons((*a).subst(name, value), (*b).subst(name, value)),
+            Atom(Var(x)) if x == name => value.clone(),
+            t => t,
+        }
     }
 }