Trying out the newest snapshot 2016.10.04 (git 7e0935d) and found these new
features: lc-each, lc-else and lc-for-c

Can't seem to find docs/examples about them in  list comprehension
https://en.wikibooks.org/wiki/OpenSCAD_User_Manual/List_Comprehensions
doc.

The lc-else is easy to understand:

But I am a bit wondering what's benefit of it over this shorter one:

I can't figure out the other two and will need help.

$  Runsun Pan, PhD $ libs: doctest , faces ( git ), offline doc ( git ), runscad.py ( 2 , git ), synwrite ( 2 );   $ tips: hash ( 2 ), matrix ( 2 , 3 ), sweep ( 2 , 3 ), var ( 2 ), lerp , animation ( gif , prodVid , animlib ), precision ( 2 ), xl-control , type , rounded polygon , chfont , tailRecur ( 2, 3 ), isosphere ( 2 ), area , vol/center , RGB , CurvedImg , tests ( 2 ); $ Apps: rollApp , blockscad , openjscad , on AWS ( pdf )

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On 10/30/2016 11:50 AM, runsun wrote:

See https://en.wikibooks.org/wiki/OpenSCAD_User_Manual/WIP

ciao,
Torsten.

runsun wrote

There is benefit in some nested tests and filters:

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Presumably, by using each you could add a different numbers of elements in
the else part, which you can't do with a ? : expression.

On 30 October 2016 at 13:45, Ronaldo rcmpersiano@gmail.com wrote:

Thx guys for the help. I tried out the examples in the WIP and you guys
provided. These are all very powerful additions. The for-else example in the
WIP was very confusing to me at first, until I tried the example Ronaldo
gave.

I think an easy way of explanation would be that a "cond?a:b" format handles
AND RETURN every single item, but "if" or "if-else" allows users to discard
some. With this in mind, the example in WIP,

can be read as:

Emphasizing the "discard" part helps me understand it better.

In my opinion, this would be the same as if we allow an "if" existing in the
"cond?a:b" format;

Or, for Ronado's example:

I'd like this form better, because it sort of focusing the feature of "if"
as a filter that discards something, but not "sometimes it discards,
sometimes it picks it back up when else is added".

But, at least it's there when we need it. Thx to those who puts them up.
Great additions.

$  Runsun Pan, PhD $ libs: doctest , faces ( git ), offline doc ( git ), runscad.py ( 2 , git ), synwrite ( 2 );   $ tips: hash ( 2 ), matrix ( 2 , 3 ), sweep ( 2 , 3 ), var ( 2 ), lerp , animation ( gif , prodVid , animlib ), precision ( 2 ), xl-control , type , rounded polygon , chfont , tailRecur ( 2, 3 ), isosphere ( 2 ), area , vol/center , RGB , CurvedImg , tests ( 2 ); $ Apps: rollApp , blockscad , openjscad , on AWS ( pdf )

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I transcribe here an example of the benefits of the if-else I had used a few
months http://forum.openscad.org/List-comprehension-filters-tc16069.html
ago to argue its need. The following list comprehension filter in the new
sintax:

could be expressed in the old one by:

which is rather cryptic and hard to write. Besides, if we change the first
code eliminating the 'else z' part, the standard code will change to:

which is far from an obvious change.

runsun wrote

I agree that the list comprehension 'if' is not always a filter in the new
syntax. But in your alternative the construct ?: would not always be an
expression if it is composed with a filter 'if'. If we extend the usage of
conditional expressions as you propose why not in other expressions too?

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On 10/31/2016 4:03 PM, Ronaldo wrote:

I believe such syntax should be forbidden because it is not obvious what
is else part of if (C).

Do you mean
A ? (B ? x : (C ? y : undef)) : z

Oh, no, it's something different.

I am quite doubtful if this is really benefit.

Just my $0.02
Tim

Ronaldo wrote

I think you made it too complicated. It should be expressed as follows
instead :

which, in my opinion, is a lot easier to both read and write than the 3
forms you presented.

Note that I highlight the if(C)y part to emphasize that "an if w/o else
is a filter", and "an if with an else is the same as cond?A:B".

runsun wrote

Because it can be easily achieved with current stable release:

$  Runsun Pan, PhD $ libs: doctest , faces ( git ), offline doc ( git ), runscad.py ( 2 , git ), synwrite ( 2 );   $ tips: hash ( 2 ), matrix ( 2 , 3 ), sweep ( 2 , 3 ), var ( 2 ), lerp , animation ( gif , prodVid , animlib ), precision ( 2 ), xl-control , type , rounded polygon , chfont , tailRecur ( 2, 3 ), isosphere ( 2 ), area , vol/center , RGB , CurvedImg , tests ( 2 ); $ Apps: rollApp , blockscad , openjscad , on AWS ( pdf )

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Tim, I had the similar confusion, took me a lot of "brain twists" to
understand it. Now that after I keep talking to myself for hours:

"an if w/o else is a filter", and "an if with an else is the same as
cond?A:B".

I am able to decode it and I think once you keep reminding yourself this
principle, it'd be easier for you, too.

So the example:

has 3 if's: the first and 2nd ones, if(A) and if(B), are followed by an
"else", so are the same as A?~:~ and B?~:~, respectively

The 3rd one, if(C), has no "else", so it is a filter. So it can be
translated to the "imagined workable code":

A? ( B?x: if(C)y) : z

Once this principle is embedded in mind, it becomes easy.

In the mean time, however, I believe this construct will continue to
frustrate future (or even current) users.

$  Runsun Pan, PhD $ libs: doctest , faces ( git ), offline doc ( git ), runscad.py ( 2 , git ), synwrite ( 2 );   $ tips: hash ( 2 ), matrix ( 2 , 3 ), sweep ( 2 , 3 ), var ( 2 ), lerp , animation ( gif , prodVid , animlib ), precision ( 2 ), xl-control , type , rounded polygon , chfont , tailRecur ( 2, 3 ), isosphere ( 2 ), area , vol/center , RGB , CurvedImg , tests ( 2 ); $ Apps: rollApp , blockscad , openjscad , on AWS ( pdf )

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runsun wrote

We are discussing two different matters: the benefits of the if-else filter
in list comprehension and your alternative sintax of it. The first point of
my last message was about the benefits. So, your recoding above mix the two
things and did not help.

The second issue (your alternative to if-else filter) has it own drawbacks.
With my trivial example:

I meant:

which is a filter, and not your code:

which is not. The /if/ in the first expression imposes a filter as in your
alternative sintax. That is, if a filter /if/ is allowed as a filter in
conditional expression why wouldn't it be allowed in any expression (keeping
its effect of filtering) ?

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Ronaldo wrote

I'd strongly suggest not to categorize "if-else" construct as a "filter". A
filter means pick something and discard the rest based on a condition. With
the "if-else", nothing can be discarded (just like cond?a:b), so it
shouldn't be considered a filter. Treating it a filter only add more
confusion 'cos it's the role "if" (w/o else) plays.

Sorry I don't get this part.

Re-arranging "if(a==0) a+1" to "a+(if (a==0)1)" and suggesting they are the
same doesn't make any sense at all. If "a" is subject to the conditional
pick/drop, why would it be placed BEFORE if ? I couldn't ever imagine an
"if" can simultaneously decide the fate of the code before it (a) and after
it (1). That is a mess-up of scopes and only makes it more confusing.

If you ask "why not 'if' shows up here / there if it is allowed in
cond?if(...):if(...)", then with the same logic, since "if" shows up in

why not it is allowed in in

?
To me that's simply a choice of language design.

$  Runsun Pan, PhD $ libs: doctest , faces ( git ), offline doc ( git ), runscad.py ( 2 , git ), synwrite ( 2 );   $ tips: hash ( 2 ), matrix ( 2 , 3 ), sweep ( 2 , 3 ), var ( 2 ), lerp , animation ( gif , prodVid , animlib ), precision ( 2 ), xl-control , type , rounded polygon , chfont , tailRecur ( 2, 3 ), isosphere ( 2 ), area , vol/center , RGB , CurvedImg , tests ( 2 ); $ Apps: rollApp , blockscad , openjscad , on AWS ( pdf )

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runsun wrote

You have answered the question in your preceding message:

runsun wrote

That is exactly what is happening when you write something like:

the filter /if/ is deciding "the fate of the conditional before it". In
fact, the conditional expression above would not act as an expression but as
a filter. And that is confusing!

I agree that is inappropriate to call the construct if-else a filter and
that it is just a conditional that accepts filtering. But it is not
equivalent to ?: because it is not an expression either.

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Ronaldo wrote

My message was to explain that your trivial example is illogical. How could
anyone invent an illogical usage of an idea in order to argue against that
idea ?

/

/

No! it is to decide the fate of the variable AFTER it ... when the condition
"a%2==0" is met, the previous variable of if's, a, is then decided without
invoking the if-filter. So it's not decided by the "if-filter". The 'if'
filter comes into play only when "a%2==0" is not met. Under that condition,
the 'if' filter is to decide the variable/code AFTER it, just like anywhere
'if' is used in this and other languages.

No, the condition expression (the ?:) is not a filter. The "if (a!=1) a*10"
is, working only when condition "a % 2 == 0" is not met.

Re-write it to the currently accepted form:

[ for (a=A) if(a % 2 == 0 ) a else if (a!=1) a*10 ]

Now you have two if's, the 2nd one is a filter but the 1st is not. That's
where the confusion comes.

Yes, "if-else" is a conditional that accepts filtering is the best way to
describe it. Better if added: "if" w/o "else" serves as a filter.

But, other than the filtering, show me an example that can only be expressed
by "if-else" but not "?:".

I don't think you could, because they serve the exact same role (other than
the filtering).

$  Runsun Pan, PhD $ libs: doctest , faces ( git ), offline doc ( git ), runscad.py ( 2 , git ), synwrite ( 2 );   $ tips: hash ( 2 ), matrix ( 2 , 3 ), sweep ( 2 , 3 ), var ( 2 ), lerp , animation ( gif , prodVid , animlib ), precision ( 2 ), xl-control , type , rounded polygon , chfont , tailRecur ( 2, 3 ), isosphere ( 2 ), area , vol/center , RGB , CurvedImg , tests ( 2 ); $ Apps: rollApp , blockscad , openjscad , on AWS ( pdf )

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This will be my third message trying to clarify my point about your proposed
syntax. And, I promise, it will be the last one.

The operator ?: is a ternary operator. As such, it requires 3 operand, no
more no less. In the expression you prefer:

when a=3 the operator ?: receives the following operands: true, 3 and
<nothing>. That is, it receives just 2 operands as the filter if returns
nothing at all. And you expect the operator ?: returns nothing at all in
this case. That is a twisted (and confusing) behavior of it.

The if-else construct on the other hand has the very special property
that it returns nothing if the followed branch (either true or false)
receives nothing. And that is the reason why it cannot be used (like filter*
if*) as a operand of any other operator except itself and the filter* if*.

If we allow the operator ?: receives less that 3 operands why not allow
the sum operator* +* receives less that two?

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Ronaldo, as you write it, ?: is a ternary operator. So it has to return a
result, as it is the outer expression. You seem to argue that this result
may also be nothing, which afaik currently is only an implicit feature (or
call it a hack) of "if".

Ronaldo wrote

I guess, you can always express a filter like this in the right order:
if (a!=1) (a % 2 == 0)? a : a*10

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Parkinbot wrote

I couldn't say it better.

Parkinbot wrote

Surely, we always can but after hard work and at the expenses of clarity in
less obvious cases. The snapshot version syntax solves this and we certainly
can replace all ?: by if-else without fear.

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Parkinbot wrote

This is exactly the same as "if-else" case. The example in the WIP:

could be written /in the right order/, as you wish, as :

and they return the same thing, which once again implies that "?:" behaves
exactly like "if-else", which is what I've been arguing all along.

Parkinbot wrote

I believe that this belief of what "if-else" is is the source of conflict
that causes conversations between Ronaldo and I went parallel.

Afaik, "if-else" by itself has to return something for every single item it
handles, therefor is a "catch-all" construct and shouldn't be considered a
filter. Ronaldo, on the other hand, thinks "if-else" is a filter, that it
"has the very special property that it returns nothing if the followed
branch (either true or false) receives nothing," therefor it can be used
with "if(w/o else)" filter, but "?:" cannot.

Ronaldo forgot one thing: the "special property" he mentioned is a result
of our design, NOT the reason for our design. One cannot design a behavior,
then turn around to argue that having that behavior is the reason we need to
design that behavior, or even go further to argue that we cannot add that
same behavior to others because they don't have it.

"if-else" by itself is never a filter. Even in OpenSCAD when we allow
"if(w/o else)" to be in "if-else" branch, it's the "if(w/o else)" that
executes the filtering, not the "if-else". The ability of  "if-else" to
accept "if(w/o else)" and turn its branch into a filter is NOT a nature of
"if-else", but an add-on feature we intentionally put it in.

In terms of language feature, having a filter in a conditional expression is
a novel and welcomed idea. But mixing "if-else" and "if(w/o else)" means
that users have to deal with that an "if" in OpenSCAD list comprehension is
sometimes a filter, sometimes not. That's where the confusion is.

$  Runsun Pan, PhD $ libs: doctest , faces ( git ), offline doc ( git ), runscad.py ( 2 , git ), synwrite ( 2 );   $ tips: hash ( 2 ), matrix ( 2 , 3 ), sweep ( 2 , 3 ), var ( 2 ), lerp , animation ( gif , prodVid , animlib ), precision ( 2 ), xl-control , type , rounded polygon , chfont , tailRecur ( 2, 3 ), isosphere ( 2 ), area , vol/center , RGB , CurvedImg , tests ( 2 ); $ Apps: rollApp , blockscad , openjscad , on AWS ( pdf )

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I must admit that the idea of having if/else, if within an expression
having this semantics is a bit awkward. Personally I wouldn't introduce it,
but implement the filter by introducing a new symbol like null that
explicates what happens:

in lc-context null could resolve to nothing and in expression context to 0
(or maybe undef).
In the second step I'd define a binary operator op1?op2 that defaults to
the ternary operator op1?op2:null. This new binary operator can then be
used anywhere.

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As I said earlier: in conjunction with each if / else can add different
numbers of elements to a list, which ? : can't as it is a single expression.

E.g.
if(a %2) a else each a, a * a

On 1 November 2016 at 19:17, Parkinbot rudolf@parkinbot.com wrote: