From 6abe90d7cc8e0b899ea97f3b1b4fd5138ced6167 Mon Sep 17 00:00:00 2001
From: Suguivy <suguivy@riseup.net>
Date: Wed, 24 Feb 2021 20:23:58 +0100
Subject: [PATCH] Added some error handling

---
 src/Evaluator.hs | 24 ++++++++++++------------
 src/Main.hs      |  7 ++++---
 2 files changed, 16 insertions(+), 15 deletions(-)

diff --git a/src/Evaluator.hs b/src/Evaluator.hs
index 3efc942..3a83a07 100644
--- a/src/Evaluator.hs
+++ b/src/Evaluator.hs
@@ -3,17 +3,17 @@ module Evaluator where
 import Expression
 import Primitives
 
-eval :: Expression -> Expression
-eval n@(Number _) = n
-eval s@(Symbol _) = s
-eval (SExpr es)   = apply $ SExpr (map eval es)
+eval :: Expression -> Either String Expression
+eval n@(Number _) = Right n
+eval s@(Symbol _) = Right s
+eval (SExpr es)   = mapM eval es >>= apply . SExpr
 
-apply :: Expression -> Expression
+apply :: Expression -> Either String Expression
 apply (SExpr (f:args)) = case f of
-  Symbol "+" -> pAdd args
-  Symbol "-" -> pSub args
-  Symbol "*" -> pMul args
-  Symbol "/" -> pDiv args
-  Symbol _     -> error "no primitive functions not supported"
-  _            -> error "object not applicable"
-apply _                = error "Object not applicable"
+  Symbol "+" -> Right $ pAdd args
+  Symbol "-" -> Right $ pSub args
+  Symbol "*" -> Right $ pMul args
+  Symbol "/" -> Right $ pDiv args
+  Symbol _     -> Left "no primitive functions not supported (only +, -, * and /)"
+  _            -> Left "object not applicable"
+apply _                = Left "object not applicable"
diff --git a/src/Main.hs b/src/Main.hs
index 8c72125..43d8779 100644
--- a/src/Main.hs
+++ b/src/Main.hs
@@ -16,6 +16,7 @@ main = runInputT defaultSettings repl
                 Left err -> do
                   outputStrLn $ show err
                   repl
-                Right e -> do
-                  outputStrLn . show $ eval e
-                  repl
+                Right e -> outputStrLn (case eval e of
+                  Right out -> show out
+                  Left err -> err)
+          repl