[Haskell-cafe] newbie - IO issues (of course)

Jason Dagit dagit at eecs.oregonstate.edu
Sat Jun 24 01:47:03 EDT 2006

On 6/23/06, Geoffrey King <lordgeoffrey at optushome.com.au> wrote:
> I have been work my way through "Haskell The Craft of Functional
> Programming", all was fine until IO (chapter 18). That is causing me
> bafflement.
> I am trying to write a totally trivial program that reads a series of
> integers from the console until it gets a zero, then returns the series
> of integers as a list.
> I am down to randomly editing my code, never a good sign. Any clues
> would be appreciated, My code so far, which clear doesn't work, is:
> getInt :: IO Int
> getInt = do
>             putStr "Enter number (zero to quit)"
>             line <- getLine
>             return (read line :: Int)

I haven't tested your code or my proposed changes, but let me step
through what you have and offer up explanations of what I think is
going wrong.

So here, things look pretty good with getInt.  getInt has type IO Int,
which makes sense because you do some work in the IO monad construct a
value and return it.  Since you can't escape the IO monad, getInt
returns an Int which is wrapped up in the IO monad.

> anIntList :: [Int]
> anIntList =
>         do
>             let n = getInt
>             if n == 0
>                 then return []
>                 else return (n : anIntList)

Okay, so lets start with the let.  What is the type of n?  Well, it's
IO Int because that's the type of getInt.  But, IO Int isn't a number
so you can't do the test in the if.  Otherwise, your code looks pretty

> ps: even a simple version that returns an Int i can't get to work.
> anInt :: Int
> anInt = do
>             n <- getInt
>             return n

Now you're getting closer.  But, you do work in the IO monad and
construct a value and return it, so your type signature must be wrong.
 It must be the case that anInt :: IO Int.  I see here that you know
how to grab 'unwrap' a value in the IO monad and work with it (the
line, n <- getInt), you should try doing this above where you have
'let n = getInt'.

You may also want to read these fine web pages:

You're so close, I expect you'll have it figured out before I can hit send :)


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