implement basic control flow, error handling from functions, many tests

Signed-off-by: Ava Hahn <ava@aidanis.online>

src/stl/append.rs

@@ -0,0 +1,52 @@
+/* Copyright (C) 2021 Aidan Hahn
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+use crate::segment::{Ctr, Seg};
+use crate::sym::{SymTable};
+
+// Some essential operations below
+pub fn append_callback (ast: &Seg, _syms: &mut SymTable) -> Result<Ctr, String> {
+    // if car is a list, append cdr
+    // otherwise create a list out of all arguments
+    if let Ctr::Seg(ref s) = *ast.car {
+        let mut temp = s.clone();
+        temp.append(ast.cdr.clone());
+        Ok(Ctr::Seg(temp))
+    } else {
+        let mut temp = Seg::new();
+        let mut car_iter = &ast.car;
+        let mut cdr_iter = &ast.cdr;
+        loop {
+            temp.append(car_iter.clone());
+            if let Ctr::Seg(ref s) = **cdr_iter {
+                car_iter = &s.car;
+                cdr_iter = &s.cdr;
+            } else {
+                break;
+            }
+        }
+
+        Ok(Ctr::Seg(temp))
+    }
+}
+
+pub fn expand_callback (ast: &Seg, _syms: &mut SymTable) -> Result<Ctr, String> {
+    if let Ctr::Seg(_) = *ast.car {
+        Ok(*ast.car.clone())
+    } else {
+        Err("non list passed to expand!".to_string())
+    }
+}