added list len, front, and back

src/stl/append.rs

@@ -49,3 +49,57 @@ pub fn append_callback(ast: &Seg, _syms: &mut SymTable) -> Result<Ctr, String> {
         Ok(Ctr::Seg(temp))
     }
 }
+
+pub const LEN_DOCSTRING: &str = "Takes a single argument, expected to be a list.
+Returns the length of said list.
+Interpreter will panic if the length of the list is greater than the max value of a signed 128 bit integer";
+
+pub fn len_callback(ast: &Seg, _syms: &mut SymTable) -> Result<Ctr, String> {
+    if let Ctr::Seg(ref s) = *ast.car {
+        if let Ctr::None = *s.car {
+            Ok(Ctr::Integer(0))
+        } else {
+            Ok(Ctr::Integer(s.len() as i128))
+        }
+    } else {
+        Err("impossible condition: argument to len not a list".to_string())
+    }
+}
+
+pub const CAR_DOCSTRING: &str = "Takes a single argument, expected to be a list.
+Returns a copy of the first value in that list.
+If the list is empty, returns an err to avoid passing back a null value.";
+
+pub fn car_callback(ast: &Seg, _syms: &mut SymTable) -> Result<Ctr, String> {
+    if let Ctr::Seg(ref s) = *ast.car {
+        if let Ctr::None = *s.car {
+            Err("argument is empty".to_string())
+        } else {
+            Ok(*s.car.clone())
+        }
+    } else {
+        Err("impossible condition: argument to car not a list".to_string())
+    }
+}
+
+pub const CDR_DOCSTRING: &str = "Takes a single argument, expected to be a list.
+Returns a copy of the last value in that list.
+If the list is empty, returns an err to avoid passing back a null value.";
+
+pub fn cdr_callback(ast: &Seg, _syms: &mut SymTable) -> Result<Ctr, String> {
+    if let Ctr::Seg(ref s) = *ast.car {
+        let mut ret = &s.car;
+        let mut iter = &s.cdr;
+        while let Ctr::Seg(ref next) = **iter {
+            ret = &next.car;
+            iter = &next.cdr;
+        }
+        if let Ctr::None = **ret {
+            Err("argument is empty".to_string())
+        } else {
+            Ok(*ret.clone())
+        }
+    } else {
+        Err("impossible condition: argument to cdr not a list".to_string())
+    }
+}