Maybe everytime you use 'cell' you tell haskell to create NEW cell.
Try this:
push' i cell = modifyIORef cell (++ [i])
main = do
cell <- newIORef []
push' "x" cell {- push' 3 cell will be incorrect in this case -}
push' "o" cell
readIORef cell >>= return
Why the original code porduces [(),()] but not [] I cannot undestand.
15.03.2012, 23:53, "gladst...@gladstein.com" <gladst...@gladstein.com>:
> Why does the following program compile and produce the results it does?
> It seems like 3 and "x" got interpreted as the same type, (). Thanks in
> advance for your help.
>
> import Data.IORef
> import System.IO.Unsafe
>
> cell = unsafePerformIO $ newIORef []
>
> push i = modifyIORef cell (++ [i])
>
> main = do
> push 3
> push "x"
> readIORef cell >>= return
>
> *Main> :browse
> cell :: GHC.IORef.IORef [a]
> push :: a -> IO ()
> main :: IO [a]
>
> *Main> main
> [(),()]
>
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