Implemented let-bindings

1 files changed, 33 insertions, 1 deletions

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)