Re: Exceptuations, fatality, resumption, locality, and the with keyword; was Re: use fatal err fail
Yuval Kogman wrote:
On Thu, Sep 29, 2005 at 13:52:54 -0400, Austin Hastings wrote:
[Bunches of stuff elided.]
A million years ago, $Larry pointed out that when we were able to use
'is just a' classifications on P6 concepts, it indicated that we were
making good forward progress. In that vein, let me propose that:
* Exception handling, and the whole try/catch thing, IS JUST An awkward
implementation of (late! binding) run-time return-type MMD.
Exception handling is just continuation passing style with sugar.
Have a look at haskell's either monad. It has two familiar keywords
- return and fail.
Every statement in a monadic action in haskell is sequenced by using
the monadic bind operator.
The implementation of =, the monadic bind operator, on the Either
type is one that first check to see if the left statement has
failed. If it does, it returns it. If it doesn't it returns the
evaluation of the right hand statement.
Essentially this is the same thing, just formalized into a type
Internally, it may be the same. But with exceptions, it's implemented by
someone other than the victim, and leveraged by all. That's the kind of
abstraction I'm looking for. My problem with the whole notion of Either
errorMessage resultingValue in Haskell is that we _could_ implement it
in perl as Exception|Somevalue in millions of p6 function signatures.
But I don't _want_ to. I want to say MyClass and have the IO subsystem
throw the exception right over my head to the top-level caller.
I guess that to me, exceptions are like aspects in they should be
handled orthogonally. Haskell's Either doesn't do that -- it encodes a
union return type, and forces the call chain to morph whenever
alternatives are added. The logical conclusion to that is that all subs
return Either Exception or Value, so all types should be implicitly
Either Exception or {your text here}. If that's so, then it's a
language feature and we're right back at the top of this thread.
Specifically, if I promise you:
sub foo() will return Dog;
and later on I actually wind up giving you:
sub foo() will return Exception::Math::DivisionByZero;
In haskell:
foo :: Either Dog Exception::Math::DivisionByZero
e.g., it can return either the expected type, or the parameter.
Haskell is elegant in that it compromises nothing for soundness, to
respect referential integrity and purity, but it still makes thing
convenient for the programmer using things such as monads
For appropriate definitions of both 'elegant' and 'convenient'. Java
calls this 'checked exceptions', and promises to remind you when you
forgot to type throws Exception::Math::DivisionByZero in one of a
hundred places. I call it using a word to mean its own opposite: having
been exposed to roles and aspects, having to code for the same things in
many different places no longer strikes me as elegant or convenient.
the try/catch paradigm essentially says:
I wanted to call csub Dog foo()/c but there may be times when I
discover, after making the call, that I really needed to call an anonymous
csub { $inner::= sub Exception foo(); $e = $inner(); given $e {...} }/c.
Yes and no.
The try/catch mechanism is not like the haskell way, since it is
purposefully ad-hoc. It serves to fix a case by case basis of out
of bounds values. Haskell forbids out of bound values, but in most
programming languages we have them to make things simpler for the
maintenance programmer.
Right. At some level, you're going to have to do that. This to me is
where the err suggestion fits the most comfortably: err (or doh!
:) is a keyword aimed at ad-hoc fixes to problems. It smooths away the
horrid boilerplate needed for using exceptions on a specific basis.
do_something() err fix_problem();
is much easier to read than the current
{ do_something(); CATCH { fix_problem(); }}
by a lot. But only in two conditions: first that all exceptions are
identical, and second that the correct response is to suppress the
exception.
To me that fails because it's like Candy Corn: you only buy it at
Halloween, and then only to give to other people's kids.
As syntactic sugar goes, it's not powerful enough yet.
We're conditionally editing the return stack. This fits right in with
the earlier thread about conditionally removing code from the inside of
loops, IMO. Once you open this can, you might as well eat more than one
worm. Another conceptually similar notion is that of AUTOLOAD. As a perl
coder, I don't EVER want to write
say Hello, world
or die Write to stdout failed.;
-- it's correct. It's safe coding. And it's stupid for a
whole bunch of reasons, mostly involving the word yucky.
It's incorrect because it's distracting and tedious.
http://c2.com/cgi/wiki?IntentionNotAlgorithm
Code which does it is, IMHO bad code because obviously the author
does not know where to draw the line and say this is good enough,
anything more would only make it worse.
For instance, some
Re: Exceptuations, fatality, resumption, locality, and the with keyword; was Re: use fatal err fail
TSa wrote: The view I believe Yuval is harboring is the one examplified in movies like The Matrix or The 13th Floor and that underlies the holodeck of the Enterprise: you can leave the intrinsic causality of the running program and inspect it. Usually that is called debugging. But this implies the programmer catches a breakpoint exception or some such ;) Exception handling is the programmatic automatisation of this process. As such it works the better the closer it is in time and context to the cause and the more information is preserved. But we all know that a usefull program is lossy in that respect. It re-uses finite resources during its execution. In an extreme setting one could run a program *backwards* if all relevant events were recorded! The current state of the art dictates that exceptions are to be avoided when it is possible to handle the error in-line. That exceptions should only be used for exceptional cases, and anything you encounter in the manual pages is not exceptional. I don't agree with this, because it is IMO effectively saying We had this powerful notion, but it turned out to be difficult to integrate post-hoc into our stack-based languages, so we're going to avoid it. Rather than admitting defeat, though, we're going to categorize it as some kind of marginal entity. I don't see exceptions as necessarily being outside the intrinsic causality of the running program. They are non-traditional forms of flow control: event-based programming, if you will, in an otherwise sequential program. We do much the same thing when we talk about coroutines: violate the traditional stack model. We do the same thing again when we talk about aspects: de-localize processing of certain (ahem) aspects of the problem domain. The telling part of aspects, though, was the the first popular implementation (AspectJ) required a preprocessor and a special markup language to implement. Why? Because nobody uses extensibility and Java in the same sentence. I guess aspects are traditional in that regard, though: remember CFront. Perl, OTGH, doesn't have the poor body-image or whatever it is that keeps people afraid to change the syntax. It can't be a method because it never returns to it's caller - it's It beeing the CATCH block? Ahh, no. It in this case is the .resume call. My question was is cresume/c a multi, an object method, or what? This is because the types of exceptions I would want to resume are ones that have a distinct cause that can be mined from the exception object, and which my code can unambiguously fix without breaking the encapsulation of the code that raised the exception. Agreed. I tried to express the same above with my words. The only thing that is a bit underspecced right now is what exactly is lost in the process and what is not. My guiding theme again is the type system where you leave information about the things you need to be preserved to handle unusual cicumstances gracefully---note: *not* successfully, which would contradict the concept of exceptions! This is the classical view of exceptions, and so it is subject to the classical constraints: you can't break encapsulation, so you can't really know what's going when the exception occurs. The reason I like the with approach is that it lets us delocalize the processing, but does _not_ keep the exceptions are violent, incomprehensible events which wrench us from our placid idyll mentality. In that regard, exceptuations are resumable gotos. =Austin
Re: Exceptuations, fatality, resumption, locality, and the with keyword; was Re: use fatal err fail
On Sat, Oct 01, 2005 at 05:57:54 -0400, Austin Hastings wrote:
Internally, it may be the same. But with exceptions, it's implemented by
someone other than the victim, and leveraged by all. That's the kind of
abstraction I'm looking for. My problem with the whole notion of Either
errorMessage resultingValue in Haskell is that we _could_ implement it
in perl as Exception|Somevalue in millions of p6 function signatures.
But I don't _want_ to. I want to say MyClass and have the IO subsystem
throw the exception right over my head to the top-level caller.
In haskell it's the job of the Either monad to let you pretend you
aren't doing Exception|Somevalue everywhere.
You can sequence operations in a deeply nested manner, and then
'fail' at some point. Then control flow will just pop back up all
the way with the error, instead of trying to continue.
You don't really need to say 'Either ... ...', you just use do
notation.
For appropriate definitions of both 'elegant' and 'convenient'. Java
calls this 'checked exceptions', and promises to remind you when you
forgot to type throws Exception::Math::DivisionByZero in one of a
hundred places. I call it using a word to mean its own opposite: having
been exposed to roles and aspects, having to code for the same things in
many different places no longer strikes me as elegant or convenient.
I agree with that wholeheartedly, but in haskell you are making no
obligation towards the shape of an exception - it can be 'Either
thing Error' where Error is any data type you like.
In this sense haskell is just as flexible but requires more
abstraction than perl etc. It has it's merits - it's safer, and more
reusable. It tends to win ICFP contests, and so forth.
However, to just get that thing working real fast, without having
to pay too much when the context becomes maintenance instead of
development, i think Perl 6 will be the most suitable language in
the world.
Right. At some level, you're going to have to do that. This to me is
where the err suggestion fits the most comfortably: err (or doh!
:) is a keyword aimed at ad-hoc fixes to problems. It smooths away the
horrid boilerplate needed for using exceptions on a specific basis.
Which is why it's such a nice propsal =)
As syntactic sugar goes, it's not powerful enough yet.
err next # catch all
err Pattern, next # catch some
Putting my code where my mouth is:
sub infix:err (lhs is delayed, Pattern ?$match = Any, rhs is
delayed) {
lhs;
CATCH {
when $match { rhs }
default { die }
}
}
Ofcourse, these can also stack:
my $fh = open file
err rx/Permission Denied/, next
err rx/No such file/, die;
But i don't think this is very useful for one or at the very most
two catches - for anything else it's ad-hoc nature just doesn't
scale as nicely as CATCH blocks, which can be applied to several
error generating blocks of code at once.
Ofcourse, true acid heads can always say:
do {
...;
...;
} err ..., ...
err ..., ...;
but that's their problem. =)
The last sentence is telling, I think. The run-time system SHOULD take
as much care as possible. And rub my feet.
Yes =)
True for any method that invokes exit(), no? Or that says NEXT on a
label outside its scope.
Well, that's a semantic detail. The issue is that those methods
*can* return, but don't.
A continuation will never return - because it already has another
place to return - the place that created it.
This is ignoring CPS, ofcourse, in which every return and every call
is a continuation. While this may be true under the hood, this is
not what the average Perl 6 user can observe.
The scenario is that I try something (error_throwing_code) and catch an
exception. Then while showing a dialog box to the user, for example, I
get another exception: not enough handles or whatever. So someone higher
than me resolves that on my behalf, then resumes me. I'm still trying to
resume the error thrown earlier:
Yes, that should work.
Now I need to ask, what happens when show_dialog_box throws an
exception? Presumably, I don't catch it in this code path, or there will
be a stack fault shortly.
If the exception from show_dialog_box was thrown, and another CATCH
handler fixed it for you, you don't need to worry about it - you can
never know because you don't get access to that exception. It's as
if it was never propagated.
One possibility is that the catcher of an exception knows little or
nothing about the innards of the thrower.
It's the job of exception classes to bridge these - they have a
class, and any number of attributes.
Exception::IO::Open::PermissionDenied
In fact I suspect that Exception::IO::Open enforces a certain type of
fix, too:
class Exception::IO::Open is Exception {
has
Re: Exceptuations, fatality, resumption, locality, and the with keyword; was Re: use fatal err fail
HaloO,
Yuval Kogman wrote:
The try/catch mechanism is not like the haskell way, since it is
purposefully ad-hoc. It serves to fix a case by case basis of out
of bounds values. Haskell forbids out of bound values, but in most
programming languages we have them to make things simpler for the
maintenance programmer.
My view here is that the parameters in the code pick a point in
the range from free on the one end and bound at the other end.
The unifying concept is constraints. So I see the following boundary
equalities:
unconstraint = free # 0.0
contstraint # 0.0^..^1.0
fully constraint
as the black and white ends with shades of gray in the middle.
And it is the type system that guaranties the availability
of the required information e.g. in $!. In that sense a sub
with a CATCH block is a different type than one without. This
difference is taking into account when dispatching exceptions.
Reentrancy is an implementation detail best left unmentioned.
Uh ohh, in an imperative language with shared state outside
the unshared state of multiple invocations of one sub the reentrance
proplem is just there. Interestingly it is easily unifyable with
shared data.
Assume that every bit of code you can run in perl 6 is first class
code - it gets safety, calls, control flow, exceptions, and so
forth.
Just to synchronize our understanding, I see the following
equivalences from the data and code domains
datacode
class = sub
instance = invocation
To illustrate my view consider
class foo
{
has ...
method ...
}
and match with
sub foo
{
has ... # my named parameters defined in body proposal
BEGIN ...
CATCH ...
label:
}
What I want to say is that calling a sub means creating an
instance of the class that describes---or *constrains*---
the potential invocations. If such an invocation is left
lying in memory unfinished we have a continuation. How concurrent
these continuations are stepped in real time with respect to their
respective inner causal logic is irrelevant to the concept.
But *causality* is important!
The view I believe Yuval is harboring is the one examplified
in movies like The Matrix or The 13th Floor and that underlies
the holodeck of the Enterprise: you can leave the intrinsic
causality of the running program and inspect it. Usually that
is called debugging. But this implies the programmer catches
a breakpoint exception or some such ;)
Exception handling is the programmatic automatisation of this
process. As such it works the better the closer it is in time
and context to the cause and the more information is preserved.
But we all know that a usefull program is lossy in that respect.
It re-uses finite resources during its execution. In an extreme
setting one could run a program *backwards* if all relevant
events were recorded!
Yes, even signals and exceptions.
The runtime is responsible for making these as fast as possible
without being unsafe.
Hmm, I would see the type system in that role. It has all the
information of the interested parties in a longjump. If it knows
there are no potential handlers
It can't be a method because it never returns to it's caller - it's
It beeing the CATCH block? Then I think it *is* in a certain
way a method with $! as it's invocant. HiHi and here a locality
metric for dispatch applies. BTW, how is the signature of a CATCH
block given? Simply
CATCH SomeException {...}
or is inspection with cascaded when mandatory?
a continuation because it picks up where the exception was thrown,
I would say it is given information about. In a way an exception
handler is dispatched on the type of exception.
and returns not to the code which continued it, but to the code that
would have been returned to if there was no exception.
This is the thing that I see is hardly possible from a far away scope.
But fair enough for closely related code.
It is, IMHO a multi though. There is no reason that every
continuation cannot be a multi, because a continuation is just a
sub.
I don't know if there are method continuations - i guess there could
be, but there's no need to permutate all the options when the
options can compose just as well.
My view is that a (free) method type becomes a continuation
as follows:
1) the invocant type is determined
2) the dispatcher selects a matching target
3) this method maker object (like a class for constructing data objects)
is asked to create a not yet called invocation and bind it to the
invocant at hand
4) at that moment we have a not yet invoked sub instance, so
plain subs just start here
5) The signature is checked and bound in the environment of the calling
scope, the callers return continuation is one of the args
6) then this invocation is activated
7a) a return uses the return continuation in such a way that
the invocation is abandoned after the jump
b) a yield keeps the continuation just like a constructed object
and
Re: Exceptuations, fatality, resumption, locality, and the with keyword; was Re: use fatal err fail
On Thu, Sep 29, 2005 at 13:52:54 -0400, Austin Hastings wrote:
You already know that err is the low-precedence version of //, right?
What replaces that? I like default or defaults myself,
Yes, he proposed 'dor'.
As I see it err is like this:
sub infix:err ($lhs is delayed, $rhs is delayed) { # we really need
thunk parameters
return $lhs dor $rhs;
CATCH { return $rhs }
}
So really 'err' is just 'dor' with an added error silencer.
On the primary hand, I don't like the idea of using err as a try/catch
because that's putting exception handling in line with the primary code.
See FMTYEWT below.
Isn't
no fatal;
my $fh = open $file or die;
...
my $fh = open $file or next;
also putting exception handling in line with the primary code?
Isn't that useful?
Exception handling should be put in wherever there is well defined
behavior on how error conditions should be dealt with.
It's Perl's job to make this as easy as possible for the programmer.
If we have a module called File::Slurp::Some, which doesn't really
care if some of the files don't exist, it's code would, IMHO be
much more readable with
open $file err next;
than a catch block.
The reason for this is that a catch block is always detached from
the code.
However, if the error is part of the normal flow (it is no longer
exceptional for the file to not exist), what benefit does the added
detachment give?
A million years ago, $Larry pointed out that when we were able to use
'is just a' classifications on P6 concepts, it indicated that we were
making good forward progress. In that vein, let me propose that:
* Exception handling, and the whole try/catch thing, IS JUST An awkward
implementation of (late! binding) run-time return-type MMD.
Exception handling is just continuation passing style with sugar.
Have a look at haskell's either monad. It has two familiar keywords
- return and fail.
Every statement in a monadic action in haskell is sequenced by using
the monadic bind operator.
The implementation of =, the monadic bind operator, on the Either
type is one that first check to see if the left statement has
failed. If it does, it returns it. If it doesn't it returns the
evaluation of the right hand statement.
Essentially this is the same thing, just formalized into a type
Specifically, if I promise you:
sub foo() will return Dog;
and later on I actually wind up giving you:
sub foo() will return Exception::Math::DivisionByZero;
In haskell:
foo :: Either Dog Exception::Math::DivisionByZero
e.g., it can return either the expected type, or the parameter.
Haskell is elegant in that it compromises nothing for soundness, to
respect referential integrity and purity, but it still makes thing
convenient for the programmer using things such as monads
the try/catch paradigm essentially says:
I wanted to call csub Dog foo()/c but there may be times when I
discover, after making the call, that I really needed to call an anonymous
csub { $inner::= sub Exception foo(); $e = $inner(); given $e {...} }/c.
Yes and no.
The try/catch mechanism is not like the haskell way, since it is
purposefully ad-hoc. It serves to fix a case by case basis of out
of bounds values. Haskell forbids out of bound values, but in most
programming languages we have them to make things simpler for the
maintenance programmer.
We're conditionally editing the return stack. This fits right in with
the earlier thread about conditionally removing code from the inside of
loops, IMO. Once you open this can, you might as well eat more than one
worm. Another conceptually similar notion is that of AUTOLOAD. As a perl
coder, I don't EVER want to write
say Hello, world
or die Write to stdout failed.;
-- it's correct. It's safe coding. And it's stupid for a
whole bunch of reasons, mostly involving the word yucky.
It's incorrect because it's distracting and tedious.
http://c2.com/cgi/wiki?IntentionNotAlgorithm
Code which does it is, IMHO bad code because obviously the author
does not know where to draw the line and say this is good enough,
anything more would only make it worse.
But I acknowledge that through the miracle of broken pipes, it can
legitimately happen that stdout will fail while stderr is a viable
diagnostic mechanism.
the default $SIG{PIPE} handler is a wonderful example of how nice
exception handling is for naive code.
$SIG{PIPE}'s only problem is that it's completely non standard.
But the default behavior (modulo threads) is going to unlink all the
stack frame pages when the continuation is invoked.
Forget the stack... Perl is CPS under the hood, even if it's
optimized not to be that way.
So there has to be yet another copy of the links to the stack,
because the exception handling will want to call functions and
build who-knows-what elaborate skycastles. And it must be
reentrant because of
