It is a syntatic sugar that is expanded to getLine >>= \x -> putStrLn $ reverse x
= is defined in the typeclass for Monad.
In general, if something is using <- notation, it's type is Monad m => m a,
where m could be any of many monads, IO, Maybe, [] (lists), Parser or even
some type of yours that you made an instance of Monad, which you can do if
you would like to use that syntax.
On Thu, Aug 30, 2012 at 11:00 PM, Patrick Redmond
I'm reading "Learn You a Haskell for Great Good!", chapter 9, "Input and Output" http://learnyouahaskell.com/input-and-output.
IO actions are given liberal coverage throughout the chapter, however it is never mentioned whether the value-extractor syntax (<-) has a type or not.
main = do x <- getLine putStrLn $ reverse x
In this little program, getLine has type "IO String" and x has type "String". This implies to me that (<-) has type "IO a -> a". However, GHCI chokes on ":t (<-)" and Hoogle says it's just a syntactic element http://www.haskell.org/haskellwiki/Keywords#.3C-.
I guess I don't have a specific question, but I was kind of expecting it to be a function with a type because everything seems to be a function with a type in Haskell... Thanks for listening!
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