[Haskell-cafe] IO Help

Stuart Cook scook0 at gmail.com
Thu May 8 09:46:03 EDT 2008

On Thu, May 8, 2008 at 10:59 PM, Mark Wallsgrove
<mark at simplycompute.co.uk> wrote:
> Thank you very much for your fast response!
> Ok, that is now changed, but everything else in my program is expecting
> Catalogue without IO. Is there a way to change IO Catalogue into Catalogue?

Suppose I have a function "readFile" of type String -> IO String, and
a function "read" of type String -> Catalogue. How can I feed the
output of the first function into the second function, when that pesky
"IO" is in my way? Perhaps I would write something that looks like

  loadData :: String -> IO Catalogue
  loadData fileName = do
      x <- readFile fileName
      return (read x :: Catalogue)

Even though the expression "readFile fileName" has type IO String, the
variable "x" bound on the third line has type String. This means that
I can pass it into "read" without any problems.

Eventually you'll learn about things like (>>=) and "liftM", which can
be used to write this in a simpler form, but hopefully this will get
you going for now.


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