Nothing happens to them. They are still in there. Whereas logNumber always returns the number you give it, multiWithLog always returns 15 and appends two strings into its state ("got number 3 and 5"). You can return whatever value you want from a Monad. In this case he's doing a * b, but you could do 2 * b, Just (a + b) or anything else. As an example, here I return a tuple instead. multWithLogTuple :: Writer [String] (Int,Int,Int) multWithLogTuple = do a <- logNumber 3 b <- logNumber 5 return (a,b,2*b)
runWriter multWithLogTuple ((3,5,10),["Got number: 3","Got number: 5"])
On Thu, Jan 26, 2017 at 10:41 AM, Olumide <50295@web.de> wrote:
Hello List,
Chapter 14 of LYH ( http://learnyouahaskell.com/for-a-few-monads-more#reader ) has the following bit of code:
import Control.Monad.Writer
logNumber :: Int -> Writer [String] Int logNumber x = Writer (x, ["Got number: " ++ show x])
multWithLog :: Writer [String] Int multWithLog = do a <- logNumber 3 b <- logNumber 5 return (a*b)
I have a fair grasp of what's going bu the last line return(a*b) eludes me. Specifically its the a*b part that baffles me. I think 3 is bound to 'a' and 5 to 'b', then return(a*b) would put the value 15 in a context ... So what becomes of the string parts of the Writer monads created by logNumber 3 and logNumber 5 respectively.
Regards,
- Olumide _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners