[Newbies] Modify block (closure) parameters

Casey Ransberger casey.obrien.r at gmail.com
Tue Aug 6 20:42:33 UTC 2013

I didn't read your post clearly enough. Yep, that would seem odd. You may
have a bug there. I'm not sure why that happens. It looks like the outer
context isn't taking the assignment, but hanging onto the initial value it
receives from #value:.

I'm not totally sure we should expect to be able to do what you're trying
to do in modern Squeak. I haven't assigned to a block arg in a long time (I
keep allow block assignments off unless I'm loading old code.) I don't know
what the status is there. I run a relatively recent Cog VM on a 10.7.5 Mac
and I'm seeing the same behavior in Squeak 4.4.

When I switch to an old VM (3.8.18Beta3U) and run Squeak 3.0, your snippet
works as expected. I'm not sure if this is in the image or the VM yet, or
whether it's expected behavior or not with allowBlockArgumentAssignment
(Again, I usually turn it off.)

The main take away here is, don't do that:) as it's a back-compat feature
and it really ought to have a big sign on it that says DEPRECATED. If
you're trying to load some older code and running into this, it might be
better to actually rewrite it not to assign to block arguments in my
opinion (and maybe I'm nuts.)

Can you tell me what VM you're using?

Smalltalk vmVersion "this will tell us"

And also which version of Squeak?

SmalltalkImage current systemInformationString "ditto"

Also, what's the OS of the host system?

On Mon, Aug 5, 2013 at 1:44 AM, psea <denis.lukichev at gmail.com> wrote:

> Hi Smalltalkers!
> Here is a question I can't find answer with google.
> In short: Does block parameters and block local variables are the same
> thing
> (semantically) inside closure? It looks like it's not. Below the
> explanation.
> ===================
> Here is the accumulator function (first attempt):
> makeAcc := [ :acc |
>         [:n | acc:=acc+n. acc]]
> It turns out it doesn't work as intended:
> a1 := makeAcc value: 10
> a1 value: 1 => 11
> a1 value: 1 => 11
> a1 value: 2 => 12
> ====================
> Here is the second attempt:
> makeAcc := [ :acc |
>         | total |
>         total:=acc.
>         [:n | total:=total+n. total]]
> And it does work as intended:
> a1 := makeAcc value: 10
> a1 value: 1 => 11
> a1 value: 1 => 12
> a1 value: 2 => 14
> So if we use the local variable to store accumulator it works as it should
> and remembers the value between function (block) calls. But if we use block
> parameter to store the value of accumulator it does not remembers the value
> between calls.
> Why is it so?
> P.S. Sorry for my English. I do all my best.
> --
> View this message in context:
> http://forum.world.st/Modify-block-closure-parameters-tp4702118.html
> Sent from the Squeak - Beginners mailing list archive at Nabble.com.
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Casey Ransberger
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