[Haskell-beginners] Loop

[Sumit Sahrawat, Maths & Computing, IIT (BHU)]

putStrLn "Test" is a IO value, that prints the string "Test".

To sequence such monadic values, we use sequence or sequence_ (which
ignores the results).

"sequence [a, b, c, d]" is equivalent to "a >> b >> c >> d"
"sequence_ [a, b, c, d]" is equivalent to "a >> b >> c >> d >> return ()"

There are similar functions mapM and mapM_, which can be used as follows.

mapM f [a, b, c]
 == sequence (map f [a,b,c])
 == sequence [f a, f b, f c]
 == f a >> f b >> f c

and,

mapM_ f [a, b, c] == f a >> f b >> f c >> return ()

>From mapM_, we can create a convenience function forM_

forM_ = flip mapM_

All the above are exported by Control.Monad

On 1 May 2015 at 18:10, Shishir Srivastava <shishir.srivastava at gmail.com>
wrote:

> Hi,
>
> Please can anyone point out the correct way of doing this - I am simply
> trying to print "Test" in a loop based on the counter derived from the list
> -
>
> ----
>  do
>  _ <- [1..4]
>  b <- putStrLn "Test"
>  return b
> ---
>
> The intended output is to print 'Test' 4 times.
>
> Clearly there is a mismatch between the Monad types but I can't see how to
> achieve this without causing the conflict.
>
> Thanks,
> Shishir
>
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>


-- 
Regards

Sumit Sahrawat
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