[Haskell-cafe] Extensible Exceptions
David F. Place
d at vidplace.com
Sat Nov 22 09:49:34 EST 2008
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
*Main> tryJust errorCalls $ print $ throw NonTermination
tryJust errorCalls $ print $ throw NonTermination^J***
*Main> errorCalls (ErrorCall "What?")
errorCalls (ErrorCall "What?")^JJust What?
*Main> errorCalls NonTermination
Couldn't match expected type `ErrorCall'
against inferred type `NonTermination'
In the first argument of `errorCalls', namely
In the expression: errorCalls NonTermination
In the definition of `it': it = errorCalls NonTermination
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