From 07195d4cf61f2cdc34355e5b52897072f60a576e Mon Sep 17 00:00:00 2001
From: Alex Bethel <abethel@cs.nmt.edu>
Date: Sun, 30 May 2021 13:26:10 -0500
Subject: [PATCH] Fix panic on arithmetic error

Divide by zero and add, subtract, or multiply with overflow are all
caught now and reported as an ArithmeticError, rather than causing the
interpreter to panic.
---
 src/interpret.rs | 49 ++++++++++++++++++++++++++++++++++--------------
 1 file changed, 35 insertions(+), 14 deletions(-)

diff --git a/src/interpret.rs b/src/interpret.rs
index 4f3dc4a..caf4b3a 100644
--- a/src/interpret.rs
+++ b/src/interpret.rs
@@ -110,21 +110,42 @@ impl ExecEnv {
         use Expr::*;
         use Value::*;
 
-        // NOTE(Alex): This is quite nasty, and should probably be
-        // re-done using macros or something.
+        // NOTE(Alex): This is really quite horrible. I think the only
+        // real way to clean it up would be to re-engineer the AST's
+        // representation to be more hierarchical: rather than having
+        // e.g. "Expr::Add" and "Expr::Subtract" each with a "left"
+        // and "right" struct member, have something like
+        // "Expr::Binary { oper: BinOp, left: Box<Expr>, right:
+        // Box<Expr> }". That way we could factor out a whole bunch of
+        // common code here.
+        //
+        // That work should probably wait for Ondra's new parser to
+        // come in, however.
         Ok(match expr {
-            Add { left, right } => {
-                Int(i32::try_from(self.eval_expr(left)?)? + i32::try_from(self.eval_expr(right)?)?)
-            }
-            Subtract { left, right } => {
-                Int(i32::try_from(self.eval_expr(left)?)? - i32::try_from(self.eval_expr(right)?)?)
-            }
-            Multiply { left, right } => {
-                Int(i32::try_from(self.eval_expr(left)?)? * i32::try_from(self.eval_expr(right)?)?)
-            }
-            Divide { left, right } => {
-                Int(i32::try_from(self.eval_expr(left)?)? / i32::try_from(self.eval_expr(right)?)?)
-            }
+            Add { left, right } => Int(i32::try_from(self.eval_expr(left)?)?
+                .checked_add(i32::try_from(self.eval_expr(right)?)?)
+                .ok_or(Error {
+                    kind: ErrorKind::ArithmeticError,
+                    position: 0..0,
+                })?),
+            Subtract { left, right } => Int(i32::try_from(self.eval_expr(left)?)?
+                .checked_sub(i32::try_from(self.eval_expr(right)?)?)
+                .ok_or(Error {
+                    kind: ErrorKind::ArithmeticError,
+                    position: 0..0,
+                })?),
+            Multiply { left, right } => Int(i32::try_from(self.eval_expr(left)?)?
+                .checked_mul(i32::try_from(self.eval_expr(right)?)?)
+                .ok_or(Error {
+                    kind: ErrorKind::ArithmeticError,
+                    position: 0..0,
+                })?),
+            Divide { left, right } => Int(i32::try_from(self.eval_expr(left)?)?
+                .checked_div(i32::try_from(self.eval_expr(right)?)?)
+                .ok_or(Error {
+                    kind: ErrorKind::ArithmeticError,
+                    position: 0..0,
+                })?),
             Lt { left, right } => {
                 Bool(i32::try_from(self.eval_expr(left)?)? < i32::try_from(self.eval_expr(right)?)?)
             }