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/
