Adding a +

hal.cabal

@@ -30,6 +30,7 @@ library
       Expressions
       LispEnv
       LispParser
+      Maths
   other-modules:
       Paths_hal
   hs-source-dirs:

src/LispEnv.hs

@@ -1,6 +1,7 @@
 module LispEnv where
 import Expressions
 import Evaluator
+import Maths
 
 
 defaultEnv :: LispEnv
@@ -11,7 +12,7 @@ defaultEnv = [
         ("eq?", ABuiltin "eq?" evalEq),
         ("atom?", ABuiltin "atom?" evalAtom),
         ("cond", ABuiltin "cond" evalCond)
-    ]
+    ] ++ mathEnv
 
 evalCons :: [Statement] -> LispEnv -> Either String (Atom, LispEnv)
 evalCons [first, second] env = do

src/Maths.hs

@@ -0,0 +1,20 @@
+module Maths where
+import Expressions
+import Evaluator
+
+mathEnv :: LispEnv
+mathEnv = [
+        ("+", ABuiltin "+" evalPlus)
+    ]
+
+evalPlus :: [Statement] -> LispEnv -> Either String (Atom, LispEnv)
+evalPlus (x:xs) env = do
+    (value, nEnv) <- eval x env
+    (rest, nnEnv) <- evalPlus xs nEnv
+    case (value, rest) of
+         (AInt f, AInt s) -> Right (AInt $ f + s, nnEnv)
+         (AFloat f, AFloat s) -> Right (AFloat $ f + s, nnEnv)
+         (AFloat f, AInt s) -> Right (AFloat $ f + fromIntegral s, nnEnv)
+         (AInt f, AFloat s) -> Right (AFloat $ fromIntegral f + s, nnEnv)
+         (value, _) -> Left $ "**Error: \"" ++ show value ++"\" is not a number **"
+evalPlus [] env = Right (AInt 0, env)