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Re: [Bug-apl] Error handling and IO

From: Elias Mårtenson
Subject: Re: [Bug-apl] Error handling and IO
Date: Mon, 11 Apr 2016 11:27:45 +0800

Hello Alexey,

This seems similar to what GNU APL provides with the ⎕EA feature. The problem with this (and, as far as I can tell, also with this Dyalog feature) is that in an error handler, the original error has disappeared.

What is needed is a way to say: "call function X with file handle Y, ensuring that Y is closed after the X has finished, regardless of the way it finishes. Successfully or through an error".

I can sort of do with with ⎕EA. I used it in SQL∆WithTransaction, for example. However, if an error occurs anywhere inside the function, all you get is a generic "Transaction rolled back" error, with no indication of what happened or where it happened.

This is the functionality that is desperately needed.

I'd like to see a more powerful version of ⎕EA that is able to do this. I would also like to hear Jürgens opinions on this first.


On 10 April 2016 at 23:49, Alexey Veretennikov <address@hidden> wrote:

In Dyalog APL there is a mechanism similar to try/catch in other

R←f X
:Trap 0
    ⎕←'Error happened'

      f 0
Error happened

For details see the Mastering Dyalog Apl
(http://www.dyalog.com/uploads/documents/MasteringDyalogAPL.pdf) Chapter M: Event Handling

Elias Mårtenson <address@hidden> writes:

> Yesterday, I participated in the Google Code Jam (programming competition). For one of the tasks,
> APL was a very good fit. The question is here:
> https://code.google.com/codejam/contest/6254486/dashboard#s=p1
> My solution (which I will not post right now, since one of you might want to give it a shot first) was terse
> and simple. A very simple APL _expression_.
> However, reading the input a file and formatting the result took many lines of very ugly code.
> This attempt at using APL to solve a real-world programming problem illustrated two separate issues
> that, needs to be handled:
> Firstly, the FILE_IO library is way too low-level. For example, in the Codejam tasks, one usually have to
> read a whitespace-limited sequence of numbers. When I solve the problems in Lisp, all I need to do is
> to call READ. A flextible IO probrary that makes these kinds of this simple would be nice.
> I could (and indeed have considered to) write such functions in APL, but this causes a second problem:
> Error handling in GNU APL is very bad. In particular, there is nothing similar to UNWIND-PROTECT (or
> try/finally in Java). There is no way to safely write code that opens a file, works on it and then closes it.
> If an error occurs, there is no way to ensure that the filehandle is closed. When I developed my solution
> to the Codejam problem, I ended up leaking a lot of file handles.
> Does anyone know how Dyalog and other vendors handle this? Do they have a full exception system?
> Regards,
> Elias


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