On Tue, 2006-11-21 at 09:53 +0100, Andreas Marth wrote:
Hi!
With the following code:
module Guess where
import Prelude hiding (catch) import Control.Exception (evaluate, catch)
guess :: [String] -> IO String guess sl = do res <- catch (evaluate (concat $ sl ++ [error "some error message", "blah blah blah"])) (\e -> return ("error#"++show e)) return res -- only for demonstration coded like this normally a call to sysalloc comes here
what will a) guess [] b) guess [""] c) guess [" "]
return?
(glossing over the issue of return in IO) a,b) "error#some error message" c) ' ' : error "error#some error message" note that c) is not _|_. What about this one: d) fmap head (guess [" "]) yep, it returns ' ', no error.
The idea was to return the string, catch every error if any occour, convert it into a string and prefix it with "error#" and return this string then. The reason to do this is to create a stable DLL with the error handling in non haskell land. At the moment every exception raised crashes the whole system, which is unacceptable. Unfortunately the c) case still raises an exception. I think at least the library description needs a hint that 'evaluate (" "++error "some error message")' does not raise the error (which I find strange!) and hence catch won't catch it. Is this behaivior really desired or should we consider it a bug?
The point is that: _|_ /= 1 : 2 : _|_ so doing evaluate (1 : error "argh") doesn't throw any error since (1 : error "argh") is not an error value (though it does contain one).
PS: I circumvent this issue by using 'rnf' and 'using' from Control.Parallel.Strategies
That's a sensible strategy since then you force the whole list and will uncover any _|_ values inside the data structure. Duncan