Haskell for Imperative Programmers #37 - Arrows 

i know I am quite randomly asking but do anyone know of a good website to watch new tv shows online?

Monad Transformers will be the topic of the next video! Traversels will also be talked about at some point probably. Arrows are very helpful when the dataflow of your algorithm has to split at certain points (e.g. boolean circuits or a bayesian networks). John Hughs presents an interesting example of a data stream impremented with arrows and gives a nice explanation on the limitations of monads: "But whereas monadic computations are parameterised over the type of their output, but not their input, arrow computations are parameterised over both. The way monadic programs take input cannot be varied by varying the monad, but arrow programs, in contrast, can take their input in many different ways depending on the particular arrow used. The stream function example above illustrates an arrow which takes its input in a different way, as a stream of values rather than a single value, so this is an example of a kind of computation which cannot be represented as a monad." ( www.cse.chalmers.se/~rjmh/afp-arrows.pdf ) What is "best" for a given situation is a hard question to answer. If composition is very important, I think that arrows are better suited than monads. There are very exotic examples where applicatives are actually the way to go ( wiki.haskell.org/Applicative_functor ). In this case conditional initialization cannot be done by a monad but CAN be done by an applicative.

Thanks for the references and looking forward to the Transformers video!

It's unlikely since I don't really know much about these topics!

This series is aimed at programmers with an imperative mindset wanting to learn Haskell. I think a big part of learning how Haskell works (and differs from many other languages) is the sometimes daunting amount of abstraction. If a viewer has stuck around for 37 episodes I think they are ready to go down the rabbit hole. ;) Atleast that's what I'm hoping! The next two or three videos will probably close this excursion into abstract / formal topics and come back to lighter topics.