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, 29 insertions, 0 deletions

diff --git a/src/sexp/util.rs b/src/sexp/util.rs
index ce6de5d..70ac152 100644
--- a/src/sexp/util.rs
+++ b/src/sexp/util.rs
@@ -1,4 +1,6 @@
 
+use std::iter;
+
 use crate::sexp::SExp;
 use crate::sexp::SLeaf;
 use crate::sexp::SExp::*;
@@ -30,3 +32,30 @@ pub fn var(name: &str) -> SExp {
     Atom(Var(name.to_string()))
 }
 
+impl SExp {
+    /// Transforms this SExp into a Vec<SLeaf>.
+    ///
+    /// Errors if the left-hand side of SCons has an SCons,
+    /// or if the passed SExp is an Atom, or if the
+    /// lists aren't Nil-terminated.
+    ///
+    /// ```rust
+    /// use melisp::sexp::{SExp::*, SLeaf::*, util::*};
+    /// assert_eq!(
+    ///     scons(1, scons(2, scons(3, Nil))).into_vec(),
+    ///     Ok(vec![Int(1), Int(2), Int(3)])
+    /// );
+    /// assert!(scons(scons(1, Nil), Nil).into_vec().is_err());
+    /// assert!(scons(1, 2).into_vec().is_err());
+    /// ```
+    pub fn into_vec(self) -> Result<Vec<SLeaf>, String> {
+        match self {
+            SCons(a, b) => match (*a, *b) {
+                (Atom(a), Atom(Nil)) => Ok(iter::once(a).collect::<Vec<SLeaf>>()),
+                (Atom(a), b) => Ok(iter::once(a).chain(b.into_vec()?).collect::<Vec<SLeaf>>()),
+                (a, b) => Err(format!("invalid list structure: {:?}", scons(a, b)))
+            },
+            _ => Err("expected list, found atom".to_string()),
+        }
+    }
+}