Hello,
Just for curiosity, is x always the same in your recursive calls or you have
different x?
function withx(x, body)
if x == globalx then
return body()
end
local oldx = globalx
globalx = x
local result = {body()}
globalx = oldx
return unpack(result)
end
Maybe be an easy option…
Best,
Laurent.
--
Laurent Deniau http://cern.ch/mad
Accelerators Beam Physics mad@xxxxxxx
CERN, CH-1211 Geneva 23 Tel: +41 (0) 22 767 4647
On 04 Nov 2016, at 01:27, Brandon Bloom <brandon.d.bloom@xxxxxxxxx> wrote:
Of course, that example code should be:
function withx(x, body)
local oldx = globalx
globalx = x
local result = {body()}
globalx = oldx
return unpack(result)
end
On Thu, Nov 3, 2016 at 5:16 PM, Brandon Bloom <brandon.d.bloom@xxxxxxxxx>
wrote:
Hello,
Is it possible to accomplish dynamic scope in a way that respects tail calls
with luajit?
I've been approximating dynamic scope for a particular algorithm in the
classic set/restore manner:
function withx(x, body)
local oldx = globalx
local result = {body()}
globalx = oldx
return unpack(result)
end
Unfortunately, while simple, this approach evaluates the body function in a
non-tail position. Until now, this wasn't a problem, but I've run in to a
recursive use case which now suffers from stack overflows.
Racket accomplishes dynamic scope via "continuation marks" (see [1] and [2])
that, if manipulated in tail position, modify a cell in the current frame, so
these "marks" with dynamic extent can be used without affecting space
complexity. The documentation for the "parameterize" macro calls out the
body-in-tail-position behavior explicitly [3].
A blog post [4] suggests that dynamic scope can hack-ily be accomplished via
debug.getlocal, but the specific code posted there is incorrect in the
presence of tail calls (try adding a 'return' to the fn call in run_func). I
suppose Lua 5.2's getinfo with what='t' can return istailcall to workaround
this, but we've gotten pretty far in to hack territory and I'd like to stay
on luajit.
I don't care if the resulting code achieves normal variable syntax or not.
I'm fine with function call syntax to get the current dynamic value.
So to recap: Is there a way in luajit to accomplish tail-call-safe dynamic
scope or otherwise associate per-stack-frame data?
Thanks,
Brandon
[1] https://docs.racket-lang.org/reference/contmarks.html
[2] "Adding Delimited and Composable Control to a Production Programming
Environment" - Flatt, et al.
[3]
https://docs.racket-lang.org/reference/parameters.html?q=parameterize#%28form._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._parameterize%29%29
[4] http://leafo.net/guides/dynamic-scoping-in-lua.html
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