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

[Petr Vápenka]

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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