feat: QuickCheck testing

test/Arbitrary/InvalidBrainfuckProgram.hs

@@ -0,0 +1,26 @@
+{-# LANGUAGE InstanceSigs #-}
+{-# LANGUAGE DerivingStrategies #-}
+module Arbitrary.InvalidBrainfuckProgram (InvalidBrainfuckProgram(..)) where
+
+import Data.Text (Text)
+import Test.QuickCheck.Gen (Gen)
+import Test.QuickCheck.Arbitrary (Arbitrary (arbitrary))
+
+import qualified Data.Text as Text
+import qualified Test.QuickCheck.Gen as Gen
+import qualified Arbitrary.ValidBrainfuckProgram as ValidBrainfuckProgram
+
+newtype InvalidBrainfuckProgram = InvalidBrainfuckProgram { get :: Text }
+  deriving stock Show
+
+instance Arbitrary InvalidBrainfuckProgram where
+  arbitrary :: Gen InvalidBrainfuckProgram
+  arbitrary = do
+    validProgram <- ValidBrainfuckProgram.get <$> arbitrary
+
+    index <- Gen.choose (0, Text.length validProgram)
+    badBrace <- Gen.oneof $ map pure "[]"
+    let (prefix, suffix) = Text.splitAt index validProgram
+    
+    pure . InvalidBrainfuckProgram $ Text.concat [prefix, Text.singleton badBrace, suffix]
+