What's the next step to learning Haskell after monads?

Question

I\'ve been gradually learning Haskell, and even feel like I\'ve got a hang of monads. However, there\'s still a lot of more exotic stuff that I barely understand, like Arrows, A

Answer 1

Regarding type classes:

• Applicative is actually simpler than Monad. I've recently said a few things about it elsewhere, but the gist is that it's about enhanced Functors that you can lift functions into. To get a feel for Applicative, you could try writing something using Parsec without using do notation--my experience has been that applicative style works better than monadic for straightforward parsers.

• Arrows are a very abstract way of working with things that are sort of like functions ("arrows" between types). They can be difficult to get your mind around until you stumble on something that's naturally Arrow-like. At one point I reinvented half of Control.Arrow (poorly) while writing interactive state machines with feedback loops.

• You didn't mention it, but an oft-underrated, powerful type class is the humble Monoid. There are lots of places where monoid-like structure can be found. Take a look at the monoids package, for instance.

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Aside from type classes, I'd offer a very simple answer to your question: Write programs! The best way to learn is by doing, so pick something fun or useful and just make it happen.

In fact, many of the more abstract concepts--like Arrow--will probably make more sense if you come back to them later and find that, like me, they offer a tidy solution to a problem you've encountered but hadn't even realized could be abstracted out.

However, if you want something specific to shoot for, why not take a look at Functional Reactive Programming--this is a family of techniques that have a lot of promise, but there are a lot of open questions of what the best way to do it is.