Hi. I want to compose two monads to build another monad where computations of the two monads can be used inside. I have: - MonadTypeInfer : interface (class) for TypeInfer monad - TypeInfer : a monad that has Map String Type (association of names and types) - TypeInferT : transformer of above monad - MonadEval : interface (class) for Eval monad - Eval : a monad that has Map String Expr (association of names and code/function body) - EvalT : transformer of Eval - tInfer :: Expr -> TypeInfer Type -- given expr, returns type of it in TypeInfer monad - eval :: Expr -> Eval Expr -- given expr, returns normalized expr in Eval monad Problem: in repl, when user defines a function, it should type check and register type of the function to TypeInfer monad's Map String Type. Also, it should store the expression of the function in Eval monad. I build REPL monad using TypeInferT and EvalT.
newtype REPL a = REPL { runREPL :: TypeInferT (EvalT IO) a } deriving(Monad, Functor, MonadIO, MonadTypeInfer, MonadEval) repl :: REPL () repl = do input <- prompt ">>> " case parse input of Left err -> -- handle error Right expr -> do t <- tInfer expr -- BAD!! tInfer :: TypeInfer Type println (show t) result <- eval expr -- BAD!! eval :: Eval Expr println (show result) repl
Should I make tInfer :: REPL Type, eval :: REPL Expr? Is there a way to build a monad where you could use sub-monads' (monads used to build current monad) computations? I prefer keeping tInfer :: TypeInfer Type, eval :: Eval Expr because tInfer never uses actions in Eval monad and vice versa. It seems like what I am asking is to break the type system. Maybe I should just make them run in REPL monad. Thank you. Sam.