Comment by sold

14 years ago

- I made many mistakes in the learning process. Monads should be done after you know core language (higher order functions, ADTs and pattern matching, typeclasses). I was attempting to learn many things at once and perfectly, instead of climbing Wittgenstein's ladders.

- Getting used to nested functions, \x -> f >>= (\y -> g >>= h x y) is not that easy.

- The type of bind, m a -> (a -> m b) -> m b is rather complex for a noob. You've got polymorphism, and m can be different depending on the monad. Monads form a higher-kinded typeclass * -> * , unlike most other typeclasses * . I discovered Functor rather late. You need to realize Reader r is a monad, not Reader, not Reader r a. Partial application on types.

- It took me some time to read and rederive all standard monads - [], Maybe, Reader, Writer, State, Cont. Those things enter the mind slowly, especially state and continuations. Now I can reimplement all of them given a scratch of paper.

- If you do not know IO, you can only use GHCi to test expressions. I thought I need to know monads very well to do IO. This is not true, but I survived a month by loading modules and interacting with GHCi only. On the other side, after all this trouble IO clicked immediately. Fortunately Haskell has enough concepts you can learn for a month without writing a complete interactive program. Look at http://www.haskell.org/haskellwiki/Blow_your_mind for example.

- After you know (>>=), return you need to learn standard library functions such as join, mapM, sequence, forever etc. Then transformers.

- When I was learning, there was no Learn You a Haskell or Typeclassopedia yet. I found sigfpe's famous http://blog.sigfpe.com/2006/08/you-could-have-invented-monad... very late. I found a lot of useless buzz and opinions but rarely with details or instructive code.