View difference between Paste ID: JHKrCSHJ and KPZQDeyN

 -- Environment to map values to var names in intepreter
 Type Env = [(String, Integer)]
 
 
 -- Some different type of error (types doesn't matter)
 data SomeError = A String | B String
-data SomeError = A String' | B String''
+
 -- Test monad :
 -- The test monad does some computation in input Env.
 -- Returns either SomeError, or a result of type a.
 -- Side effect in form of output is added to [String]
 newtype Test a = Test {runTest :: Env -> (Either SomeError a, [String]) }
 
 instance Monad Test where
     return a = Test $ const (Right a, [])
     m >>= f = Test $ \e -> case runTest m e of
                                 (Left a, x) -> (Left a, x)
                                 (Right a, s) -> case runTest (f a) e of
                                                      (Left a,x) -> (Left a,x)
                                                      (Right a', s') -> (Right a', s++s')
 
     -- Monadic doesnt work
     -- m >>= f = Test $ \e ->
     --  runTest m e >>= \t ->
     --  runTest (f (first t)) >>= \t' ->
     --  return t'