Why does sequence (map print [1, 2, 3, 4, 5]) produce [(), (), (), (), ()] at the end?
Hello list, The question says it all. BTW, I'm studying LYH and I'm on the chapter on IO. The book offers and explanation but its not very clear -- to me at least. Thanks, - Olumide
map :: (a -> b) -> [a] -> [b] print :: Show a => a -> IO () map print :: Show a => [a] -> [IO ()] print takes a showable a and creates a procedure that prints it out (IO ()). So therefore map print causes each element in the array to become a procedure that prints out its element, thus the return value is [IO()]. Note that it has not actually printed them out. It merely has an array of as yet unexecuted actions. To print them you'd go like sequence (map print) [1,2,3], or better yet, sequence_ which returns () instead of [()] On Mon, Feb 1, 2016 at 12:28 PM, Olumide <50295@web.de> wrote:
Hello list,
The question says it all.
BTW, I'm studying LYH and I'm on the chapter on IO. The book offers and explanation but its not very clear -- to me at least.
Thanks,
- Olumide
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On 01/02/2016 17:39, David McBride wrote:
Note that it has not actually printed them out.
I hope you don't mind me asking but why then does sequence (map print [1,2,3] ) return the numbers 1, 2, 3 (albeit followed by [(),(),()]).
It merely has an array of as yet unexecuted actions. To print them you'd go like sequence (map print) [1,2,3] ...
Erm ... sequence (map print) [1,2,3] returns an error. - Olumide
In ghci, any statement that returns Show a => IO a (that is IO a where the a is showable) will be run in IO and the result a will be shown once it finishes. If a is not showable it will simply run the IO action, but not show the result.
:t sequence (map print [1,2,3]) sequence (map print [1,2,3]) :: IO [()] :i Show instance Show a => Show [a] -- Defined in ‘GHC.Show’ instance Show () -- Defined in ‘GHC.Show’
On Wed, Feb 3, 2016 at 5:35 AM, Olumide <50295@web.de> wrote:
On 01/02/2016 17:39, David McBride wrote:
Note that it has not actually printed them out.
I hope you don't mind me asking but why then does sequence (map print [1,2,3] ) return the numbers 1, 2, 3 (albeit followed by [(),(),()]).
It merely has an array of as yet unexecuted actions. To print them
you'd go like sequence (map print) [1,2,3] ...
Erm ... sequence (map print) [1,2,3] returns an error.
- Olumide
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participants (3)
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David McBride -
Erlend Hamberg -
Olumide