I think it's an interesting idea from a safety standpoint. Unfortunately,
as Ed pointed out to me in a similar context, `void` isn't always free.
On Mar 31, 2016 7:22 PM, "Erik de Castro Lopo"
Hi all,
I was recently faced with some unexpected behaviour from a piece of code that type checks and has zero warnings (even with -Wall). The code is below (and depends on the hashtables package).
The error was using the <$> operator instead of the =<< operator. Using the former, it just builds up a list of IO actions that never get run.
As pointed out to me on IRC (thanks pjdeport), chaning the type signature of `forM_` to
forM_' :: (Monad m, Foldable t) => t a -> (a -> m ()) -> m ()
would have resulted in an error.
Yes, this change would break existing code (breaking code would require an explicit `void $` inside the `forM_`) but does anyone else think this is a good idea?
Erik
import Control.Monad
import qualified Data.HashTable.IO as HT
type EvenCache = HT.BasicHashTable Int Bool
main :: IO () main = do ht <- buildTable xs <- HT.toList ht putStrLn $ "cache: length " ++ show (length xs)
buildTable :: IO EvenCache buildTable = do ht <- HT.new forM_ pairs $ \ (k,v) -> maybe (HT.insert ht k v) (const $ abort k) <$> HT.lookup ht k return ht where xs = [1 .. 10] :: [Int] pairs = map (\ i -> (i, even i)) xs abort k = error $ "cache: duplicate key " ++ show k ++ "."
-- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/ _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries