Control.Exception

[David Menendez]

On Mon, Nov 3, 2008 at 7:27 PM, shelarcy <shelarcy at gmail.com> wrote:
> On Tue, 04 Nov 2008 07:40:50 +0900, David Menendez <dave at zednenem.com> wrote:
>>> ie:
>>>
>>> action
>>>  `catches`
>>>    [ \(e :: ExitCode) -> ...
>>>    , \(e :: PatternMatchFail) -> ...
>>>    ]
>>>
>>> or just by using multiple catch clauses:
>>>
>>> action
>>>  `catch` (\(e :: ExitCode) -> ...)
>>>  `catch` (\(e :: PatternMatchFail) -> ...)
>>
>> I don't think those are equivalent. In the second case, the
>> PatternMatchFail handler scopes over the ExitCode handler.
>
> I think Duncan forgot to write parens. According to Ian's example,
> here is an equivalent code.
>
> (action
>  `catch` (\(e :: ExitCode) -> ...))
>  `catch` (\(e :: PatternMatchFail) -> ...)
>
> http://www.haskell.org/pipermail/libraries/2008-July/010095.html

That's equivalent to the code without the parentheses, but it isn't
equivalent to the code using "catches".

Assume we have exitCodeHandler :: ExitCode -> IO () and
pattternMatchHandler :: PatternMatchFail -> IO (),

1. action `catches` [ Handler exitCodeHandler, Handler patternMatchHandler ]
2. (action `catch` exitCodeHandler) `catch` patternMatchHandler

Let's further assume that "action" throws an ExitCode exception and
"exitCodeHandler" throws a PatternMatchFail exception. In example 1,
the PatternMatchFail exception thrown by "exitCodeHandler" is not
caught by "patternMatchHandler", but it in example 2 it is caught.

In other words, patternMatchHandler is active during the evaluation of
exitCodeHandler in example 2, but not in example 1.

-- 
Dave Menendez <dave at zednenem.com>
<http://www.eyrie.org/~zednenem/>