feat: QuickCheck testing

test/Arbitrary/InvalidBrainfuckProgram.hs

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+{-# 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]
+

test/Arbitrary/ValidBrainfuckProgram.hs

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+{-# LANGUAGE InstanceSigs #-}
+{-# LANGUAGE DerivingStrategies #-}
+module Arbitrary.ValidBrainfuckProgram (ValidBrainfuckProgram(..)) where
+import Data.Text (Text)
+import Test.QuickCheck.Arbitrary (Arbitrary (arbitrary))
+import Test.QuickCheck.Gen (Gen, sized, frequency, oneof, resize)
+import Data.Coerce (coerce)
+
+import qualified Data.Text as Text
+
+newtype ValidBrainfuckProgram = ValidBrainfuckProgram { get :: Text }
+  deriving stock Show
+
+instance Arbitrary ValidBrainfuckProgram where
+  arbitrary :: Gen ValidBrainfuckProgram
+  arbitrary = coerce . fmap Text.pack $ character []
+    where
+      character rest = do
+        body <- frequency
+          [ (1, comment)
+          , (9, operator)
+          ]
+
+        sized $ \ size -> frequency
+          [ (1   , pure body)
+          , (size, character body)
+          ]
+
+        where
+          operator = let
+
+            simpleOperator = oneof $ map (pure . (:rest)) "+-<>,."
+            loopOperator = fmap ('[':) . sized $ \ s -> resize (s `div` 8) (character (']':rest))
+
+            in frequency 
+              [ (3, simpleOperator)
+              , (1, loopOperator)
+              ]
+
+          comment = do
+            commentChar <- oneof $ map pure "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
+            pure $ commentChar : rest
+
+-- >>> import Test.QuickCheck.Gen (generate)
+-- >>> generate $ (arbitrary :: Gen ValidBrainfuckProgram)
+-- ValidBrainfuckProgram {get = "<-"}
+