evaluate to lazy?

[Daniel Fischer]

Am Dienstag, 21. November 2006 09:53 schrieb Andreas Marth:
> 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?
>
> 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?

Well, evaluate doesn't _fully_ evaluate its argument and in case c) it sees 
that the argument of evaluate is of the form (_:_), so no cause to raise the 
exception, like

*Guess> (" " ++ error "Hah!") `seq` "Huh!"
"Huh!"
*Guess> ("" ++ error "Hah!") `seq` "Huh!"
"*** Exception: Hah!


Maybe this is what you want?


guess2 :: [String] -> IO String
guess2 sl
    = do res <- mapM (\s -> (catch (evaluate s) (\e -> return ("error#" ++ 
show e))))
                     (sl ++ [error "MyMess", "Blah blub"])
         return $ concat res

*Guess> guess2 [" "]
" error#MyMessBlah blub"

Cheers,
Daniel
>
> Thanks,
> Andreas
>
> PS: I circumvent this issue by using 'rnf' and 'using' from
> Control.Parallel.Strategies
>
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