[Haskell-beginners] Why can't I print an IO Integer?

[Petr Vápenka]

Using your wording, sequence makes from a `list of promises of integer` a
`promise of list of integers`.
You can get the value out of a promise using bind, ie using `<-` in do
notation as written below.

ioA :: IO Int
ioA = return 1

main :: IO ()
main = do
  a <- ioA
  print a

This may help: http://learnyouahaskell.com/input-and-output

On Tue, Oct 20, 2015 at 9:28 PM, Dan Stromberg <strombrg at gmail.com> wrote:

>
> Please correct my inference if I'm wrong:
>
> An IO Integer is not an integer, it's a promise to read an Integer later.
> The "sequence" function tells the runtime it's time to make good on that
> promise.
>
> Sound about right?
>
> Thanks!
>
> On Tue, Oct 20, 2015 at 9:15 AM, Petr Vápenka <petr.vapenka at gmail.com>
> wrote:
>
>> Hello Dan,
>>
>> `IO Integer` is something that, when executed, returns and `Integer` and
>> there is no instance of `Show` for `IO Integer` as the compiler says.
>>
>> You have to run the computations that will return the numbers and then
>> print them, like so:
>>
>> main :: IO ()
>> main = do
>>     let filenames = ["/etc/services"]
>>     let ioSizes = map get_size filenames :: [IO Integer]
>>     sizes <- sequence ioSizes
>>     mapM_ print sizes
>>
>> -- sequence :: Monad m => [m a] -> m [a]
>>
>> One important part is the use of sequence which transforms (ioSizes ::
>> [IO Integer]) to `IO [Integer]` that is run and the result bound to (sizes
>> : [Integer]).
>>
>> Hope that's clear enough to get the point :)
>>
>> Petr
>>
>> On Tue, Oct 20, 2015 at 5:58 PM, Dan Stromberg <strombrg at gmail.com>
>> wrote:
>>
>>>
>>> Here's a small program that replicates the compilation issue I'm seeing:
>>>
>>> import qualified System.Posix.Files
>>>
>>> get_size :: String -> IO Integer
>>> get_size filename = do
>>>     file_status <- System.Posix.Files.getFileStatus filename
>>>     let file_size = System.Posix.Files.fileSize file_status
>>>     let integer_file_size = fromIntegral file_size
>>>     return integer_file_size
>>>
>>> main :: IO ()
>>> main = do
>>>     let filenames = ["/etc/services"]
>>>     let sizes = map get_size filenames
>>>     mapM_ print sizes
>>>
>>> The compilation error I get is:
>>>
>>> ghc -Wall --make -o stat2 stat2.hs
>>> [1 of 1] Compiling Main             ( stat2.hs, stat2.o )
>>>
>>> stat2.hs:15:11:
>>>     No instance for (Show (IO Integer)) arising from a use of `print'
>>>     Possible fix: add an instance declaration for (Show (IO Integer))
>>>     In the first argument of `mapM_', namely `print'
>>>     In a stmt of a 'do' block: mapM_ print sizes
>>>     In the expression:
>>>       do { let filenames = ...;
>>>            let sizes = map get_size filenames;
>>>            mapM_ print sizes }
>>> make: *** [stat2] Error 1
>>>
>>> I've googled quite a bit, and guessed quite a bit, and added type
>>> declarations some, but I'm still not converging on a solution.
>>>
>>> Why can't I print an IO Integer?
>>>
>>> Thanks!
>>>
>>> --
>>> Dan Stromberg
>>>
>>> _______________________________________________
>>> Beginners mailing list
>>> Beginners at haskell.org
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>
>>>
>>
>> _______________________________________________
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>>
>>
>
>
> --
> Dan Stromberg
>
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