4 files changed, 61 insertions, 4 deletions
diff --git a/src/sexp/mod.rs b/src/sexp/mod.rs
index 4e3c11c..7aa517b 100644
--- a/src/sexp/mod.rs
+++ b/src/sexp/mod.rs
@@ -59,11 +59,19 @@ use SLeaf::*;
 
 
 impl SExp {
+    /// Quick test that lets are values
+    /// ```rust
+    /// use myslip::sexp::{SExp::*, SLeaf::*, util::*};
+    /// assert!(scons(Let, scons(var("a"), scons(5, Nil))).is_value())
+    /// ```
     pub fn is_value(&self) -> bool {
         match self {
             SCons(a, b) =>
-                (**a == Atom(Quote) || **a == Atom(Vector))
-                && b.consists_of_values(),
+                SCons(a.clone(), b.clone()).check_let().is_some() ||
+                (
+                    (**a == Atom(Quote) || **a == Atom(Vector))
+                    && b.consists_of_values()
+                ),
             Atom(Var(_)) => false,
             Atom(_) => true,
         }
diff --git a/src/sexp/step.rs b/src/sexp/step.rs
index e031fca..816582c 100644
--- a/src/sexp/step.rs
+++ b/src/sexp/step.rs
@@ -240,6 +240,8 @@ impl SExp {
     /// ```
     ///
     /// **Let-bindings**
+    ///
+    /// Basic operation:
     /// ```rust
     /// use myslip::sexp::{SExp::*, SLeaf::*, util::*};
     ///
@@ -261,7 +263,27 @@ impl SExp {
     ///     ).step(),
     ///     Ok(scons(
     ///         scons(Let, scons(var("x"), scons(5, Nil))), scons(
-    ///             scons(Add, scons(var("x"), scons(1, Nil))), Nil
+    ///             scons(Add, scons(var("x"), scons(4, Nil))), Nil
+    ///         )
+    ///     ))
+    /// );
+    /// ```
+    ///
+    /// Shadowing:
+    /// ```rust
+    /// use myslip::sexp::{SExp::*, SLeaf::*, util::*};
+    ///
+    /// assert_eq!(
+    ///     scons(
+    ///         scons(Let, scons(var("x"), scons(4, Nil))),
+    ///         scons(
+    ///             scons(Let, scons(var("x"), scons(5, Nil))),
+    ///             scons(scons(Add, scons(var("x"), scons(4, Nil))), Nil)
+    ///         )
+    ///     ).step(),
+    ///     Ok(scons(
+    ///         scons(Let, scons(var("x"), scons(5, Nil))), scons(
+    ///             scons(Add, scons(var("x"), scons(4, Nil))), Nil
     ///         )
     ///     ))
     /// );
@@ -278,6 +300,16 @@ impl SExp {
             SCons(op, l) if !(*op).is_value() => Ok(scons(op.step()?, l)),
 
 
+            // let binds
+            SCons(op, l) if op.clone().check_let().is_some() => match (*op).check_let() {
+                Some((varname, val)) => Ok(match *l {
+                    SCons(a, b) if *b == Atom(Nil) => *a,
+                    t => t,
+                }.subst(&varname, &val)),
+                None => panic!("unreachable as per match guard arm"),
+            },
+
+
             // op value and a1 .. an values, and b0, b1 .. bn not values
             // ---------------------------------------------------------
             //   (op a1 ... an b) -> (op a1 ... an b0.step() b1 .. bn)
diff --git a/src/sexp/subst.rs b/src/sexp/subst.rs
index 87f9b50..4cfa48d 100644
--- a/src/sexp/subst.rs
+++ b/src/sexp/subst.rs
@@ -17,7 +17,14 @@ impl SExp {
     /// ```
     pub fn subst(self, name: &str, value: &SExp) -> SExp {
         match self {
-            SCons(a, b) => scons((*a).subst(name, value), (*b).subst(name, value)),
+            SCons(a, b) => {
+                if let Some((varname, _)) = a.clone().check_let() {
+                    if &varname == name {
+                        return SCons(a, b);
+                    }
+                }
+                scons((*a).subst(name, value), (*b).subst(name, value))
+            },
             Atom(Var(x)) if x == name => value.clone(),
             t => t,
         }
diff --git a/src/sexp/util.rs b/src/sexp/util.rs
index b3edf19..a28aaad 100644
--- a/src/sexp/util.rs
+++ b/src/sexp/util.rs
@@ -76,4 +76,14 @@ impl SExp {
             },
         }
     }
+
+    pub fn check_let(self) -> Option<(String, SExp)> {
+        match &(self.parts())[..] {
+            [l, n, v] if *l == Atom(Let) => match (n.clone(), v.clone()) {
+                (Atom(Var(name)), val) => Some((name, val)),
+                _ => None
+            },
+            _ => None
+        }
+    }
 }