I hope you're not building some unneeded "rules" in your head. There is no reason to believe there is something to be remembered about whether or not "types can change along a >>= chain". That chain has nothing special in Haskell. >>= is just an operator, much like ++, ! or .

ghci> :t (>>=)
(>>=) :: (Monad m) => m a -> (a -> m b) -> m b

This says that, you provide an a and you get a b. Nothing says the a and b have to be the same upon successive uses.

Likewise,

ghci> :t (+)
(+) :: (Num a) => a -> a -> a

fromIntegral ((1 :: Int) + 2) + (3 :: Integer)
6

This shows clearly that the types are not the same along the "+ chain".


2009/10/12 michael rice <nowgate@yahoo.com>
Dumb! I just figured out I was entering the input string in quotes.

So, I suppose the answer to my question is yes, type CAN be changed along a >>= chain. I was having trouble doing it in a different problem, created this small example to illustrate the problem, and then screwed it up putting quotes around my input string.

Thanks!

Michael

--- On Mon, 10/12/09, Niklas Broberg <niklas.broberg@gmail.com> wrote:

From: Niklas Broberg <niklas.broberg@gmail.com>
Subject: Re: [Haskell-cafe] Can type be changed along a >>= chain?
To: "michael rice" <nowgate@yahoo.com>
Cc: haskell-cafe@haskell.org
Date: Monday, October 12, 2009, 12:43 PM

On Mon, Oct 12, 2009 at 6:37 PM, michael rice <nowgate@yahoo.com> wrote:
transform :: IO ()
transform = putStrLn "What is your digit string?"
         >> getLine
         >>= \str -> return ('9':str)
         >>= \str -> return (read str :: Int)
         >>= \i -> putStrLn $ "The number is " ++ show i
This code works perfectly for me. What problem are you seeing specifically?
Cheers,
/Niklas


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