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

[Dan Stromberg]

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
>>
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>>
>>
>
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-- 
Dan Stromberg
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