OutputRange should be infinite?

[monarch_dodra]

On Tuesday, 9 October 2012 at 15:27:44 UTC, Steven Schveighoffer 
wrote:
> On Tue, 09 Oct 2012 09:39:32 -0400, monarch_dodra 
> <monarchdodra at gmail.com> wrote:
>
>> On Tuesday, 9 October 2012 at 13:22:28 UTC, Steven 
>> Schveighoffer wrote:
>>> [SNIP]
>>
>> I tend to disagree with your examples, because, you are mixing 
>> the notion of run-time failure with logic error.
>
> They are one and the same.
>
> Putting into a file that runs out of disk space, and putting 
> into an array that runs out of memory.

I'm not convinced.

A file running out of memory is an implementation defined 
limitation that is out of the field of control of the developer, 
just as much as an OutOfMemoryError.

An array that runs out of memory is predictable logic error. The 
problem is that we aren't giving the developer the tools required 
to predict it.

> Take the viewpoint of std.algorithm.copy.  It's been asked to 
> copy from A to B, and B cannot accept it.  What does it do?  
> Saying it has to just return success doesn't make any sense.

I never said copy should return success.

>> For example: "new" New can fail. And you don't know unless you 
>> try.
>> But new will throw an exception to tell you it failed..
>>
>> An appender, as you say, is finite in memory, and will end up 
>> throwing an exception, yes. You also have a chance to try to 
>> catch it and react.
>
> No, these are Errors, not (supposed to be) catchable.

Hum. Yes, but the point (IMO) remains that the error is not 
thrown by Appender itself, but by the underlying implementation, 
and by no fault of appender itself, nor the caller.

I mean, it is not the *appender* that is full. You are just 
running into out of memory on your machine...

Anyways, I don't think there is anything to be gained disagreeing 
on this point any longer, as it would seem the solution is going 
towards other paths anyways.

>> Over-putting into a finite slice, on the other end, will 
>> *assert*. Game over. It is a catch 22: You can't know unless 
>> you try, you crash if you do.
>
> I agree, this could have a better interface.  However, I think 
> in terms of what to do (assuming we add some way of checking 
> for fullness), if someone calls put on an output buffer and 
> that range is not able to handle it, it should be an 
> Error/assert as it is now, just like calling front on an empty 
> array is an assert.
>
>>
>>> I'm not against defining a standard way to say "I'm full", 
>>> but proposing it *can't* say that is not the solution.  
>>> Clearly, we could do better in defining a standard way to 
>>> test for fullness (full property akin to empty?).  Even so, 
>>> putting into a non-full range could generate an error.
>>
>> Hum... I'm just kind of wondering here: Couldn't we simply 
>> have put throw an actual exception? Something along the lines 
>> of "OutputRangFullException"? That would be a pretty good 
>> compromise.
>
> I think it would work, but I think we still need a way to check 
> for fullness.
>
> Here is what I propose:
>
> OutputRange is defined as an entity that consumes data.   If 
> you put data into an OutputRange that cannot accept the data, 
> the range has the option of asserting or throwing an exception.
>
> TerminatingOutputRange is an extension of OutputRange, but 
> defines bool @property full().  R.full returns true if it 
> cannot accept any new data.  It should assert if you try to put 
> data into a full TerminatingOutputRange.  In other words the 
> following sequence should always assert or not compile:
>
> static assert(isTerminatingOutputRange!(typeof(r)));
> assert(r.full);
> r.put(x);
>
> If you try and put into a TerminatingOutputRange that is *not* 
> full, behavior reverts to OutputRange (can either assert or 
> throw an exception), depending on the assumptions that can be 
> made for that condition.

I'll have to try to sleep on this before making any 
judgements/thoughts/comments.

But off the top of my head, you'll still run into the same 
problem of an output range becoming full *during* a put: if r 
accepts a T, then it accepts an input range of T.

>>>> However, I really don't like having a range tell me "yeah, 
>>>> I'm an Output Range", just to choke on the first call to put.
>>>
>>> What about an input range that is immediately empty?  These 
>>> are corner cases, but certainly valid.
>>
>> Wouldn't "empty" simply answer "true" before even starting? At 
>> least it is being honest.
>
> Right, but you seem to be saying the condition that an 
> OutputRange might throw on the first call to put is an invalid 
> reaction.  I don't think it is any less valid than throwing on 
> the first call to front on an empty range.
>
> -Steve

No, my problem is not one of "first call", it is one of answering 
not empty, but choking on a put(element) afterwards.

*Me "outputRange, are you an output range or int[] ?"
*outputRange: "Yes"
*Me: "outputRange are you empty?"
*outputRange: "No"
*Me: "then put this int[] _element_"
*outputRange: "OutOfRangeError"
*Me: "WTF?"

To me, this is not acceptable behavior.

----

Another solution could be something closer to my very first 
proposal of tightening the valid *ElementTypes* that are 
compatible with an output range (but not put itself).

For example, a delegate D that accepts a T (like a char) would be 
defined as return true to:
isOutputRange!(D, T)     //true
isOutputRange!(D, T[])   //true
isOutputRange!(D, T[][]) //true

An actual inputRange!T (IR) (such as int[]) that defines empty, 
though, would only be an output range for EXACTLY T:
isOutputRange!(IR, T)     //true
isOutputRange!(IR, T[])   //false
isOutputRange!(IR, T[][]) //false

This would nip the problem in the bud, as empty would *really* 
mean empty. If R says he's an outputRange of T, but not of T[], 
then don't trust it to not overflow if you feed it a T[]...

As for the delegates, well they don't have empty anyways, so you 
can go ahead and attempt to cram anything you want.



Unlike my very first proposal way back when, put would still work 
to copy several items at once, but at the caller's responsibility.