Re: [Bug-apl] Error handling and IO

[Juergen Sauermann]

Hi,

not sure what is meant by "having the ability to get suspension"?

My understanding of ⎕EA and ⎕EC is that their primary purpose is to prevent
suspension and instead to return to the caller.

Returning to the caller implies unwinding of the )SI stack, and a consequence
of unwinding )SI is that the statement in which the error has occurred has been
unwound with it. Unwinding the stack could very well remove the defined function
in which the error had occurred, for example if you did, for example ⎕FX of a local
variable in order to create a local function).

That is the main reason why lines 2 and 3 of ⎕EM are gone - there is no guarantee that
the offending statement still exists.

/// Jürgen

On 04/11/2016 09:24 AM, Elias Mårtenson wrote:

One option would be to have a way to install a callback that is called when a stack frame is unwound. This could be provided to the plugin API, providing the needed infrastructure to the FILE_IO module.

Regards,

Elias

On 11 April 2016 at 15:07, Kacper Gutowski <address@hidden> wrote:

On Mon, Apr 11, 2016 at 5:27 AM, Elias Mårtenson wrote:
> 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.

If you call some function inside ⎕EA or ⎕EC and then do the clean up, the
only thing you loose is the ability to get suspension inside it.  The
original error type and message is still available in ⎕ET and ⎕EM after
⎕EA or directly returned by ⎕EC.  Sadly, the second and third lines of ⎕EM
showing location where the error occured seem indeed to be left out.
Perhaps this could be improved.  But if you pass it to ⎕ES, it will simply
appear as if said function were fixed as not suspendable,* which makes
sense to some degree because it's not possible to suspend it under ⎕EA or
⎕EC.

Having ability to get suspension and then appropriate resources freed on
unwind regardless if it happens normally, after user's corrective action,
or by clearing stack indicator would be pretty cool, but that sounds like
a big leap in the way stack indicator works.

-k


*Compare:

      ⎕FX 'y F x' 'y+x'
F
      F 1
VALUE ERROR
F[1]  y+x
      ^
      →

      0 1 0 0 ⎕FX 'y F x' 'y+x'
F
      F 1
VALUE ERROR
      F 1
      ^ ^
      →

      ∇R←(F G) x
[1] ⎕ES 0 0 ⋄ ''⎕EA'R←F x'
[2] ⍝ cleanup here
[3] (,1↑[⎕IO]⎕EM) ⎕ES ⎕ET  ∇

      {⍺+⍵}G 1
VALUE ERROR
      G 1
      ^ ^
      →