在Haskell中，与作家Monad的木匠比赛有吗？

Question

What is correct way to do pattern match with Writer Monad in Haskell?

I want to do like following...

import Control.Monad.Writer
hoge :: Writer [Int] (Int, String) -> String
hoge (Writer (_, (_, s))) = s

Actually, I don't really know difference between Writer and WriterT and Writer.Identity , etc...

Any guidance would be appreciated, Thank you for your reading.

Answer 1

If I've understood correctly what you're trying to do, you can use runWriter to get ((Int, String), [Int]) out of your Writer , and then extract the string from that. Something like snd . fst . runWriter ... snd . fst . runWriter ... snd . fst . runWriter ... , perhaps.

WriterT is what you use when you need to combine a Writer with some other monad. Rather than writing separate-but-similar implementations for Writer and WriterT , it was easier to write just the implementation for WriterT , and define Writer as a WriterT that uses the Identity monad. The Identity monad doesn't really do anything; you can simply pretend it's not there.