The (<-) symbol is syntax, so doesn't really have a type and probably shouldn't be thought of as having one. It's more like, given the expression that appears to its right of the type (m a) implies that the value to its left is of the type a. Unlike, (->) which has a kind (not a type), since it takes two type variables to produce a type e.g. (->) Int Int is a function taking Int to Int. Hope that helps. On 31/08/12 13:00, Patrick Redmond wrote:
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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