Test fails #1 but seems to work in ui

app/Main.hs

@@ -58,7 +58,10 @@ drawUI st = maybe [ui] (:[]) (st ^. maybeError)
                   15
                   case st ^. previousAnswers of
                     [] -> str "                          "
-                    prevAns -> Brick.Widgets.Core.vBox $ reverse $ zipWith (\i e -> str ([i] ++ ":  " ++ e)) ['a' ..] $ map (reverse . merge (replicate 16 ' ') . reverse) prevAns
+                    prevAns -> Brick.Widgets.Core.vBox $
+                      reverse $
+                      zipWith (\i e -> str ("#" ++ [i] ++ " : " ++ e)) ['a' ..] $
+                      map (reverse . merge (replicate 15 ' ') . reverse) prevAns
               )
           )
           <=> Brick.Widgets.Border.border (str "= " <+> hLimit 48 e1)

lib/Calc.hs

@@ -4,17 +4,17 @@ import Text.Read
 
 calculateWithVar :: [String] -> String -> Maybe Double
 calculateWithVar prevAns eq =
-  let variables = zip ['a' ..] prevAns
-      replacedString = foldl (\acc (c, prevAns) -> replaceCharWithString c prevAns acc) eq variables
-   in calculate replacedString
+  let variables = zip (map (\x -> '#' :[x]) ['a' ..]) prevAns
+      replacedString = foldl (\acc (i, prevAns) -> replace i prevAns acc) eq variables
+  in calculate replacedString
+  where 
+    replace from to [] = []
+    replace from to s@(x:xs) = 
+          if take (length from) s == from 
+            then to ++ replace from to (drop (length from) s)
+            else x : replace from to xs 
+
 
-replaceCharWithString :: Char -> String -> String -> String
-replaceCharWithString c replacement [] =
-  []
-replaceCharWithString c replacement (firstChar : rest) =
-  if firstChar == c
-    then replacement ++ replaceCharWithString c replacement rest
-    else firstChar : replaceCharWithString c replacement rest
 
 -- TODO: Figure out why Num does not work
 calculate :: String -> Maybe Double -- (Num a, Read a, Show a) => String -> a

tests/BackendTests.hs

@@ -26,7 +26,7 @@ testSqrt :: Test
 testSqrt = TestCase (assertEqual "Calculating: 16 sqrt" (Just 4.0) $ calculate "16 sqrt")
 
 testAdditionWithVar :: Test
-testAdditionWithVar = TestCase (assertEqual "Calculating: 1 a + where a = 10" (Just 11.0) $ calculateWithVar ["10.0"] "1 a +")
+testAdditionWithVar = TestCase (assertEqual "Calculating: 1 #1 + where #1 = 10" (Just 11.0) $ calculateWithVar ["10.0"] "1 #1 +")
 
 testPointOne :: Test
 testPointOne = TestCase (assertEqual "Calculate: .1" (Just 0.1) $ calculate ".1")