Let's say I have these two data records, X and Y and following two functions: f1 :: IO (Maybe [X]) f2 :: X -> IO (Maybe Y) I need to call the f1 first and then, for each element of returned l...

I've looked over https://www.fpcomplete.com/blog/2017/06/tale-of-two-brackets, though skimming some parts, and I still don't quite understand the core issue "StateT is bad, IO is OK", other than va...

So I have a certain function which I need to call only if a certain condition is true. If it's false, I consider it as Right. I would use EitherT.cond, but the thing is my function's return type ...

I'm refactoring some old code, which is in a polymorphic, but type-class constrained, monad: class ( MonadIO m , MonadLogger m , MonadLoggerIO m , MonadThrow m , MonadCatch m , MonadMask m ,...

In Adjoint functors determine monad transformers, but where's lift?, Simon C has shown us the construction... newtype Three u f m a = Three { getThree :: u (m (f a)) } ... which, as the answers ...

I'm trying to use Scalaz EitherT with a scala.concurrent.Future. When trying to use it in a for-comprehension: import scalaz._ import Scalaz._ val et1:EitherT[Future, String, Int] = EitherT(Futur...

I need to write a state monad that can also support error handling. I was thinking of using the Either monad for this purpose because it can also provide details about what caused the error. I foun...

While I was learning Composing Types chapter from Haskell Book, I was given tasks to write Functor and Applicative instances for the following type. newtype Compose f g a = Compose { getCompose :: ...

So far, every monad (that can be represented as a data type) that I have encountered had a corresponding monad transformer, or could have one. Is there such a monad that can't have one? Or do all m...

I was trying to write my own monad transformers where it would make sense to have multiple of the same monad transformer on the stack with different types. The issue can be illustrated with the rea...

The streaming package offers a zipsWith function zipsWith :: (Monad m, Functor h) => (forall x y. f x -> g y -> h (x, y)) -> Stream f m r -> Stream g m r -> Stream h m r and...

I understand (I think) that there is a close relationship between Either and Except in Haskell, and that it is easy to convert from one to the other. But I'm a bit confused about best practices for...

I have a Logger type of kind * -> * which can take any type and log the value in a file. I am trying to implement this in a monadic way so that I log and keep working the same. My code looks lik...

I am attempting to define an API to express a particular type of procedure in my program. newtype Procedure a = { runProcedure :: ? } There is state, consisting of a mapping of IDs to records: ...

The streaming package defines a Stream type that looks like the following: data Stream f m r = Step !(f (Stream f m r)) | Effect (m (Stream f m r)) | Return r There is a comment on the Stream...

In a previous question, I asked Why is there no MonadMask instance for ExceptT? and got a number of compelling answers, including from the library author, why there cannot exist a lawful instance...

Monad transformers are known for all standard monads (Reader, Writer, State, Cont, List, etc.), but each of these monad transformers works in a slightly different way. There is no general met...

In their paper on profunctor optics, Pickering et al. state that The term ‘profunctor’ comes from category theory, although much of the categorical structure gets lost in translation. That see...

I'm using a library that takes the results as two-track values (success and failure).In the Observable.map function bodies I often get an observable from success track of a function, and I don't kn...

I'm using the servant library for my JSON API. I need some help to get a ServerT MyAPI (ReaderT a IO) monad stack working. Here's an example using ReaderT, but without integrating it with servant...

I'm intrigued by the construction described here for determining a monad transformer from adjoint functors. Here's some code that summarizes the basic idea: {-# LANGUAGE MultiParamTypeClasses #-} ...

I'm currently stacking Futures and Eithers using EitherT: type ErrorOr[A] = Either[Error, A] def getAge: Future[ErrorOr[Int]] = ??? def getDob(age: Int): ErrorOr[LocalDate] = ??? for { age <...

I'm writing a very simple two-pass assembler in Haskell and I've come across a scenario that I don't yet have the experience to solve. I think the solution is likely to involve monad transformers, ...

Say I have a function function1 :: (MonadError String m, MonadIO m) => m Int function2 :: (MonadError Int m, MonadIO m) => m Int and a function to transform Int into a String renderExcep...

I can't grasp the correct way of interrupting lengthy pure computation on SIGINT signal. In the simple example below, I have slowFib function that simulates lengthy computation. When it is run ju...