Re: Exception handling (was Re: ...add to Lua...)

[nobody <nobody+lua-list@...>]

On 2018-09-20 22:50, Ivan Krylov wrote:
> On Thu, 20 Sep 2018 00:12:21 +0200 nobody 
> <nobody+lua-list@afra-berlin.de> wrote:
> > From the user code, you call `hopefully( desc, params... )` (could
> > also be hidden inside some constructor) and if the action fails, 
> > that tries the handlers in order (with the stack still intact – so
> > this is a normal function, no language support needed etc.).
>
> I'm not sure I understand how it's implemented -- can you somehow 
> protect a Lua call in such a way that your handler gets executed at 
> the same place in the stack where the original error() happened? -- 
> but the R language has a similar construct: the tryCatch function.

This doesn't (necessarily) involve pcall/error – just plain call/return.
`hopefully` is just a wrapper that iterates through ways to do the
requested thing, returns the result on success, or throws a real error
when nothing worked.  (And that wrapper just stays in the same place
until it's done, calling functions as it goes.)

A crappy diagram:

  user code
   \- hopefully
       \- default action
       / (reports failure)
       \- handler #1
           \- (does some stuff)
           /
           \- re-tries default action (and succeeds)
        /--/ (passes through the result)
   /---/ (return to user code)

You essentially take the familiar error handling if-then-else, put it in
a function (`hopefully`), and then wrap the code for the different
branches as functions and put those in a list.  This abstracts away a
repeated pattern and at the same time opens it up for later extension.
Internally (hopefully <-> handlers), you can communicate by `nil,errmsg`
or error/pcall (arbitrary choice, doesn't really matter for external
behavior).

-- nobody