On Sun, 23 Nov 2008, Duncan Coutts wrote:
On Sun, 2008-11-23 at 01:40 +0100, Henning Thielemann wrote:
On Sat, 22 Nov 2008, Thomas Schilling wrote:
It's a pattern match error, implemented by throwing an asynchronous
exception. The idea being, that we only have one mechanism (well, an
synchronous exceptions, thrown via throwIO).
Yes, I know that there's a difference between "error" and "exception",
but I would argue that which is which depends on the program. For
example, for most programs a pattern match error is a fatal condition,
there's no sane recovery from it. OTOH, in a program like GHCi, a
pattern match error in an executed statement is an exceptional
condition, which we want to catch, so it doesn't kill GHCi.
It's completely ok to run something in a sandbox and try to observe
errors. But that's debugging and I think there is no need to do this in
many places of an application. In general handling errors automatically is
not possible, because an error might also be if a program loops
infinitely. Thus one should not generally handle an error like an
exception.
In general I agree. I would advise against explicitly catching such
exceptions just in the region where one is expecting them. That seems
like bad design.
On the other hand "top level" catch-all handlers that also catch such
logic errors sometimes make sense. For example in a Haskell web server
where we generate a page dynamically it makes a lot of sense to catch
errors in the page-generation function, including pattern match errors,
and produce a 500 error code response and log the error message.
That's a case, rather like ghci, where some flaw in the program can and
should be compartmentalised. There's no attempt to clean up the error
but it is a modular system and there is a clear boundary where failures
can occur without bringing down the entire system.
full acknowledge
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