add reverse, snippet for prepend implementation

src/stl/append.rs

@@ -103,3 +103,84 @@ pub fn cdr_callback(ast: &Seg, _syms: &mut SymTable) -> Result<Ctr, String> {
         Err("impossible condition: argument to cdr not a list".to_string())
     }
 }
+
+pub const POP_DOCSTRING: &str = "Takes a single argument, expected to be a list.
+Returns a list containing the first element, and the rest of the elements.
+
+Example: (pop (1 2 3)) -> (1 (2 3)).
+
+The head can then be accessed by car and the rest can be accessed by cdr.";
+
+pub fn pop_callback(ast: &Seg, _syms: &mut SymTable) -> Result<Ctr, String> {
+    if let Ctr::Seg(ref s) = *ast.car {
+        if s.len() < 1 {
+            return Err("cannot pop from empty list".to_string());
+        }
+        let mut ret = Seg::new();
+        ret.append(s.car.clone());
+        if let Ctr::Seg(ref s2) = *s.cdr {
+            ret.append(Box::new(Ctr::Seg(s2.clone())));
+        } else {
+            ret.append(Box::new(Ctr::Seg(Seg::new())));
+        }
+        Ok(Ctr::Seg(ret))
+    } else {
+        Err("Impossible condition: arg not a list".to_string())
+    }
+}
+
+pub const DEQUEUE_DOCSTRING: &str = "Takes a single argument, expected to be a list.
+Returns a list containing the last element, and the rest of the elements.
+
+Example: (dq (1 2 3)) -> (3 (1 2)).
+
+The last element can then be accessed by car and the rest can be accessed by cdr.";
+
+pub fn dequeue_callback(ast: &Seg, _syms: &mut SymTable) -> Result<Ctr, String> {
+    if let Ctr::Seg(ref s) = *ast.car {
+        if s.len() < 1 {
+            return Err("cannot dequeue from empty list".to_string());
+        }
+        let mut rest = Seg::new();
+        let mut iter = s;
+        while *iter.cdr != Ctr::None {
+            rest.append(Box::new(*iter.car.clone()));
+            if let Ctr::Seg(ref next) = *iter.cdr {
+                iter = next;
+            } else {
+                break;
+            }
+        }
+
+        let mut ret = Seg::new();
+        match *iter.car {
+            Ctr::Seg(ref s) => {
+                ret.append(s.car.clone());
+            }
+            _ => ret.append(iter.car.clone()),
+        }
+
+        ret.append(Box::new(Ctr::Seg(rest)));
+
+        Ok(Ctr::Seg(ret))
+    } else {
+        Err("Impossible condition: arg not a list".to_string())
+    }
+}
+
+pub const REVERSE_DOCSTRING: &str = "Takes a single argument, expected to be a list.
+Returns the same list, but in reverse order.";
+
+pub fn reverse_callback(ast: &Seg, _syms: &mut SymTable) -> Result<Ctr, String> {
+    if let Ctr::Seg(ref s) = *ast.car {
+        let mut ret = Seg::new();
+        s.circuit_reverse(&mut |arg: &Ctr| -> bool {
+            ret.append(Box::new(arg.clone()));
+            true
+        });
+
+        Ok(Ctr::Seg(ret))
+    } else {
+        Err("Impossible condition: arg not a list".to_string())
+    }
+}