[Haskell-cafe] Extensible Exceptions
nominolo at googlemail.com
Sat Nov 22 10:27:54 EST 2008
2008/11/22 David F. Place <d at vidplace.com>:
> On Sat, 2008-11-22 at 11:33 +0000, Thomas Schilling wrote:
>> Be careful, though. This only works if there's a single constructor
>> for your exception type. If there are multiple, you should write it
>> like this:
>> thing_to_try `catch` \(e :: MyErrorType) -> case e of MyError1 _ ->
>> ..; MyError2 _ -> ...
>> If you write `catch` (MyError1 ...) and a MyError2 is thrown, you will
>> get a pattern match error exception.
> Since I am trying to replicate the behavior of errorCalls, I have only
> the single constructor ErrorCall to worry about. I don't understand
> though, how this works: I have a predicate
> errorCalls e@(ErrorCall _) = Just e
> In the following transcript it seems to work correctly even if something
> beside ErrorCall is thrown. Passing NonTermination to errorCalls get a
> type error as expected. Why does it work inside tryJust?
> *Main> tryJust errorCalls $ print $  !! 23
> tryJust errorCalls $ print $  !! 23^JLeft Prelude.(!!): index
> too large
> *Main> tryJust errorCalls $ print $ throw NonTermination
> tryJust errorCalls $ print $ throw NonTermination^J***
> Exception: <<loop>>
It doesn't. The last line is printed by GHCi.
Note the missing "Exception: " in the first call.
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