It's a fair point... as written, the example using either would get just as
bad as the example using try/catch once you start adding methods, the
comparison was unfairly biased.
It's also quite reasonable to object to the names.
Thankfully they're just a convention, and can be easily improved. We could
even create a dedicated FileOpenStatus and have a selection of subclasses.
So after fixing everything:
// using either
fileOpen(fileName) match {
case FileNotFound => ...
case IllegalFilename => ...
case FileOpened(handle) =>
doSomethingWithFile(handle)
moreFileOps(handle)
yetSomethingElse(handle)
.
.
keepGoing(handle)
.
.
promiseThisIsTheLastOne(handle)
}
//if there were other subclasses of FileOpenStatus, the compiler would
warn you at this point
//other unanticipated problems can still throw an unchecked exception
If anything, I prefer this layout. It makes it far more obvious where the
flow is likely to diverge.
I'll also hasten to add that I'd never write a monster method like this, in
any language. All that file handling behaviour must absolutely be extracted
into its own method - one that doesn't declare FileNotFoundException.
Of course you can go far deeper with a full implementation of pattern
matching. However, my intent isn't to bash Java or promote Scala - I simply
use Scala here as a Java-like language that's suitable for demonstrating
pattern matches. More significantly, I wish to demonstrate how Java's
exception handling mechanism might be lifted into a general-purpose
construct that's usable in situations unrelated to error handling.
In doing this, cases like FileNotFound could be moved back into the return
type of the function, where they rightly belong; and checked exceptions
could be removed from the language - thus getting rid of the shadow type
system that they represent.
What I think important behind all this is not the comparative likelihood of
a FileOpened vs FileNotFound, but whether or not FileNotFound is a truly
Exceptional occurrence instead of something to be reasonably expected.
On 22 September 2010 23:20, Reinier Zwitserloot <[email protected]> wrote:
> .... but the either is noise when "file found" and "file not found"
> are NOT equally likely. There's no way to know as an API designer. The
> one thing you can tell is that "file found" is pretty much always
> going to be a reasonable option. No one is going to call fileOpen when
> they know for sure it'll fail, there wouldn't be any point. Your
> further comment that the "catch" will start drifting away makes no
> sense to me. Let's look at the either example again:
>
> You're *calling a different method* to handle the actual result
> ("doSomethingWithFile"). If that's how we're going to handle it, we
> should be fair and let the try/catch example also use that. But, then
> the 'catch' for the fileOpen failure is NEVER going to drift away too
> far. If you're _not_ going to be calling a different method, the
> pattern matching version is going to make the case Right drift away
> just as far. This is yet another case where you see (and say, as if
> you're some sort of authority) that some way that java can't do is
> better, where its actually just personal preference.
>
> Then there's "Left" and "Right" which are just ugly, and which also
> suggest there's only 1 type of exception that fileOpen can throw. try/
> catch does not suffer from any of these problems.
>
> I don't understand why this thread has drifted into "try/catch itself"
> is bad. It started with "forcing onto a programmer the need to check
> certain exceptions based on method signatures is not a good idea"
> which most seem to agree with. That's entirely different from the idea
> that try/catch itself is bad.
>
> On Sep 22, 2:34 pm, Kevin Wright <[email protected]> wrote:
> > Lets compare...
> >
> > Apologies for using Scala, my intent here is to demonstrate the
> differences
> > in the techniques using a language that supports both styles, not
> > specifically to advocate Scala.
> >
> > val fileName = """c:\autoexec.bat"""
> >
> > // using either
> > fileOpen(fileName) match {
> > case Left(handle) => doSomethingWithFile(handle)
> > case Right(error) => logError(error)
> > }
> >
> > //using try/catch
> > try {
> > val handle = fileOpen(fileName)
> > doSomethingWithFile(handle)
> > } catch {
> > case Exception(e) => logError(e)
> > }
> >
> > The try/catch example has a couple of extra lines, but that's hardly
> > significant. More importantly, as the amount of code grows between the
> try
> > and the catch, possible points of divergence for control flow become
> > increasingly unclear. This is high-risk for
> > causing maintenance difficulties in the future. using Either, on the
> other
> > hand, suggests that "file found" and "file not found" are equally valid
> > non-exceptional outcomes, and places them on a level footing as regards
> the
> > flow of control.
> >
> > On 22 September 2010 13:19, Ricky Clarkson <[email protected]>
> wrote:
> >
> >
> >
> >
> >
> > > The point is that it's your choice what to do. Using Either does not
> mean
> > > you have to write lots of if statements, though you can if you like.
> >
> > > On Wed, Sep 22, 2010 at 12:22 PM, Miroslav Pokorny <
> > > [email protected]> wrote:
> >
> > >> How is either any better than letting catching an exception or letting
> the
> > >> code continue in the original spot. One gets a split off into a
> everythings
> > >> ok here a file, or jump to there and process the problem ? Using
> Either ends
> > >> up being "more" code because we get the branch for free with
> > >> exceptions...And given FileCreation failed is an exception the flow
> will be
> > >> most likely at least a bit different. Continuing on and checking later
> does
> > >> not seem to make much sense most of the time.
> >
> > >> --
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> >
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> > --
> > Kevin Wright
> >
> > mail / gtalk / msn : [email protected]
> > pulse / skype: kev.lee.wright
> > twitter: @thecoda
>
> --
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>
--
Kevin Wright
mail / gtalk / msn : [email protected]
pulse / skype: kev.lee.wright
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