Implemented evaluation according to tests. Quite a bit of changes were required, see rest of commit message.

SExp::Quote was added to let the interpreter know whether
a list should be evaluated or treated as a literal list.
It still needs code to be added for parsing it successfully.

Some utility functions were needed:
* SExp::is_value
* SExp::consists_of_values
* SExp::into_vec

1 files changed, 21 insertions, 0 deletions

diff --git a/src/sexp/mod.rs b/src/sexp/mod.rs
index 3dbcbb5..f6c50d7 100644
--- a/src/sexp/mod.rs
+++ b/src/sexp/mod.rs
@@ -17,6 +17,7 @@ pub enum SLeaf {
     Div,
     Int(i32),
     Var(String),
+    Quote,
     Nil,
 }
 
@@ -31,3 +32,23 @@ pub enum SExp {
     SCons(Box<SExp>, Box<SExp>),
     Atom(SLeaf),
 }
+
+use SExp::*;
+use SLeaf::*;
+
+impl SExp {
+    pub fn is_value(&self) -> bool {
+        match self {
+            SCons(a, _) => **a == Atom(Quote),
+            Atom(Var(_)) => false,
+            Atom(_) => true,
+        }
+    }
+
+    pub fn consists_of_values(&self) -> bool {
+        self.is_value() || match self {
+            SCons(a, b) if (*a).is_value() => b.consists_of_values(),
+            _ => false
+        }
+    }
+}