[Haskell-beginners] Why does sequence (map print [1, 2, 3, 4, 5]) produce [(), (), (), (), ()] at the end?

[David McBride]

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 at 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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