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

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

src/stl/control.rs

@@ -0,0 +1,62 @@
+/* relish: versatile lisp shell
+ * 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::eval::eval;
+use crate::sym::SymTable;
+
+pub fn if_callback (ast: &Seg, syms: &mut SymTable) -> Result<Ctr, String> {
+    if let Ctr::Seg(ref cond_form) = *ast.car {
+        if let Ctr::Bool(cond) = *eval(cond_form, syms)? {
+            if let Ctr::Seg(ref first_form) = *ast.cdr {
+                if cond {
+                    match *first_form.car {
+                        Ctr::Seg(ref first_arg) => Ok(*eval(first_arg, syms)?),
+                        _ => Ok(*eval(&Seg::from_mono(first_form.car.clone()), syms)?),
+                    }
+                } else {
+                    if let Ctr::Seg(ref second_form) = *first_form.cdr {
+                        match *second_form.car {
+                            Ctr::Seg(ref second_arg) => Ok(*eval(second_arg, syms)?),
+                            _ => Ok(*eval(&Seg::from_mono(second_form.car.clone()), syms)?),
+                        }
+                    } else {
+                        Err("impossible condition: args not in standard form".to_string())
+                    }
+                }
+            } else {
+                Err("impossible condition: not 3 args to if".to_string())
+            }
+        } else {
+            Err("first argument to if must evaluate to be a boolean".to_string())
+        }
+    } else {
+        Err("impossible condition: not 3 args to if".to_string())
+    }
+}
+
+fn let_callback (_ast: &Seg, _syms: &mut SymTable) -> Result<Ctr, String> {
+    todo!()
+}
+
+fn while_callback (_ast: &Seg, _syms: &mut SymTable) -> Result<Ctr, String> {
+    todo!()
+}
+
+fn map_callback (_ast: &Seg, _syms: &mut SymTable) -> Result<Ctr, String> {
+    todo!()
+}