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
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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