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

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

tests/test_lib_control.rs

@@ -0,0 +1,64 @@
+mod control_lib_tests {
+    use relish::ast::{Ctr, eval, lex, SymTable};
+    use relish::stdlib::{static_stdlib, dynamic_stdlib};
+
+    #[test]
+    fn test_if_first_case_singlet() {
+        let document = "(if true 1 2)";
+        let result = 1;
+
+        let mut syms = SymTable::new();
+        static_stdlib(&mut syms).unwrap();
+        dynamic_stdlib(false, &mut syms).unwrap();
+
+        if let Ok(tree) = lex(&document.to_string()) {
+            if let Ctr::Integer(i) = *eval(&tree, &mut syms).unwrap() {
+                assert_eq!(i, result);
+            } else {
+                assert!(false);
+            }
+        } else {
+            assert!(false);
+        }
+    }
+
+    #[test]
+    fn test_if_second_case_singlet() {
+        let document = "(if false 1 2)";
+        let result = 2;
+
+        let mut syms = SymTable::new();
+        static_stdlib(&mut syms).unwrap();
+        dynamic_stdlib(false, &mut syms).unwrap();
+
+        if let Ok(tree) = lex(&document.to_string()) {
+            if let Ctr::Integer(i) = *eval(&tree, &mut syms).unwrap() {
+                assert_eq!(i, result);
+            } else {
+                assert!(false);
+            }
+        } else {
+            assert!(false);
+        }
+    }
+
+    #[test]
+    fn test_complex_case_call() {
+        let document = "(if true (append () 1) 2)";
+        let result = "(1)";
+
+        let mut syms = SymTable::new();
+        static_stdlib(&mut syms).unwrap();
+        dynamic_stdlib(false, &mut syms).unwrap();
+
+        if let Ok(tree) = lex(&document.to_string()) {
+            if let Ctr::Seg(ref i) = *eval(&tree, &mut syms).unwrap() {
+                assert_eq!(i.to_string(), result);
+            } else {
+                assert!(false);
+            }
+        } else {
+            assert!(false);
+        }
+    }
+}