variable replacement in equations

app/Main.hs

@@ -68,7 +68,7 @@ appEvent (T.VtyEvent (V.EvKey V.KBackTab [])) =
   focusRing %= F.focusPrev
 appEvent (T.VtyEvent (V.EvKey V.KEnter [])) = do
   s <- T.get
-  let ans = show $ calculate $ unlines $ E.getEditContents $ s ^. editEquation
+  let ans = show $ calculateWithVar (s ^. previousAnswers) $ unlines $ E.getEditContents $ s ^. editEquation
   T.put $
     St
       (F.focusRing [EditEquation])

lib/Calc.hs

@@ -1,5 +1,19 @@
 module Calc where
 
+calculateWithVar :: [String] -> String -> Double
+calculateWithVar prevAns eq =
+  let variables = zip ['a' ..] prevAns
+      replacedString = foldl (\acc (c, prevAns) -> replaceCharWithString c prevAns acc) eq variables
+   in calculate replacedString
+
+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 -> Double -- (Num a, Read a, Show a) => String -> a
 calculate =

tests/BackendTests.hs

@@ -25,6 +25,9 @@ testNaturalLog = TestCase (assertEqual "Calculating: 2.718281828459045 ln" 1.0 $
 testSqrt :: Test
 testSqrt = TestCase (assertEqual "Calculating: 16 sqrt" 4.0 $ calculate "16 sqrt")
 
+testAdditionWithVar :: Test
+testAdditionWithVar = TestCase (assertEqual "Calculating: 1 a + where a = 10" 11.0 $ calculateWithVar ["10.0"] "1 a +")
+
 tests :: Test
 tests =
   TestList
@@ -34,7 +37,8 @@ tests =
       TestLabel "Test Division" testDivision,
       TestLabel "Test Exponentiation" testExponentiation,
       TestLabel "Test Natual Log" testNaturalLog,
-      TestLabel "Test Sqrt" testSqrt
+      TestLabel "Test Sqrt" testSqrt,
+      TestLabel "Test Addition w/ Var" testAdditionWithVar
     ]
 
 main :: IO ()