all: add stub for if

src/BC/Eval.hs

@@ -2,10 +2,22 @@ module BC.Eval (eval) where
 
 import BC.Types
 
+truthy :: Value -> Bool
+truthy (BNum num) = num /= 0
+truthy (BBool x) = x
+truthy _ = False
+
+
 eval :: [Value] -> Value
 eval [x@(BNum _)] = x
 eval [x@(BBool _)] = x
 eval [x@(BErr _)] = x
+eval [x@(BIf cond body alt)] =
+  if truthy (eval cond)
+    then eval body
+    else case alt of
+          Just vals -> eval vals
+          Nothing -> BBool False
 eval [(BOp x)] = BErr ("operation " ++ x ++ " requires arguments")
 eval [] = BOp ""
 eval l = treeEval l [] []
@@ -13,6 +25,7 @@ eval l = treeEval l [] []
 treeEval :: [Value] -> [Value] -> [Value] -> Value
 treeEval [] [] (num:_) = num
 treeEval [] ops nums = handleOp [] ops nums
+treeEval (x@(BIf _ _ _):xy) ops nums = treeEval (eval [x]:xy) ops nums
 treeEval (x@(BNum _):xy) ops nums = treeEval xy ops (x:nums)
 treeEval ((BBool x):xy) ops nums =
     treeEval xy ops ((BNum $ BInt $ if x then 1 else 0):nums)
@@ -50,6 +63,6 @@ binOp :: Value -> Maybe (Number -> Number -> Number)
 binOp (BOp "*") = Just (*)
 binOp (BOp "/") = Just (/)
 binOp (BOp "+") = Just (+)
-binOp (BOp "-") = Just (-)
+binOp (BOp "-") = Just ( - )
 binOp (BOp "^") = Just (**)
 binOp _ = Nothing