Yet Another Monad Tutorial (part 1: basics)

[mvanier]

It's a standing joke in the Haskell community that every Haskell programmer

Comments

Thank you

[anonymous]

Absolutely wonderful introduction to monads. I really appreciate the time you take to explain things from scratch. Can't wait to read the next article :) !

Re: Thank you

[mvanier]

You're welcome! I'm always happy when people get something out of my blog posts.

Fantastic Tutorial

[ext_558365]

I have been struggling with Monads for weeks, and this is the tutorial that finally helped me understand them. Thanks a ton for writing this!

Re: Fantastic Tutorial

[mvanier]

Thanks a ton for reading it! :-) I'm glad it helped you understand monads. There will be more to come.

Very, very helpful

[anonymous]

Clarity like this is all too rare. Nice job - just the monad jumpstart I needed

Re: Very, very helpful

[mvanier]

Thanks! Clarity is my #1 priority.

Very nice introduction!

[anonymous]

I especially like the use of unit to explain an "action". Sure makes it clearer than some of those other tutorials.

Thanks!

Re: Very nice introduction!

[mvanier]

Thanks for your kind comments!

Nice instroduction

[mathk]

Nice introduction. That is very helpful.
Btw I am wondering why when applying getLine the "it" value is type as String not as IO String?

Prelude> getLine
ertrtet
"ertrtet"
it :: String
Prelude>

[mvanier]

Thanks for your comments, and sorry to be responding so long after you posted this!

Anyway, IO values and regular values are often conflated at the ghci toplevel in ways that would never happen in a running Haskell program. For instance:

Prelude> return 10
10
Prelude> :t it
10 :: Integer

Essentially, "it" must unpack the IO value "return 10" into just 10. The way to think about it is that all of ghci is running inside the IO monad. So you can do stuff like this:

Prelude> s <- getLine
foobar
Prelude> s
"foobar"

In a compiled Haskell program, this would have to be inside a do expression or it wouldn't compile.

Hope this helps!