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(is this) A bug? echo seems to accept children

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int_i

(is this) A bug? echo seems to accept children

trying to understand why my things weren't rendering... luckily I noticed I forgot a ";" after the echo statement. Simple example:

echo("hello world")
cylinder(h=3, r=2);

No errors, but the cylinder won't render.
using version 2019.05

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MichaelAtOz

Re: (is this) A bug? echo seems to accept children

Administrator

int_i wrote

I noticed I forgot a ";" after the echo statement. Simple example:

echo("hello world")
cylinder(h=3, r=2);

No errors, but the cylinder won't render.
using version 2019.05

In 2019.05 echo() is basically a null operation, it does not propagate the geometry of its children (being children because of the missing ';').

In newer versions (new release 2021.01), echo() has been enhanced, it does the echo output, but also passes any child geometry upward. And echo() can also be used in an expression, e.g. to help debug a recursive function.

OpenSCAD Admin - email* me if you need anything, or if I've done something stupid...
* on the Forum, click on my MichaelAtOz label, there is a link to email me.

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Obviously inclusion of works of previous authors is not included in the above.

nophead

Re: (is this) A bug? echo seems to accept children

In reply to this post by int_i

In the latest version it renders the cylinder.

On Sun, 7 Mar 2021 at 12:45, int_i <[hidden email]> wrote:

trying to understand why my things weren't rendering... luckily I noticed I forgot a ";" after the echo statement. Simple example:

echo("hello world")
cylinder(h=3, r=2);

No errors, but the cylinder won't render.
using version 2019.05
Sent from the OpenSCAD mailing list archive at Nabble.com.
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Parkinbot

Re: (is this) A bug? echo seems to accept children

echo() can appear almost everywhere. However it is not "invisible" in code, because it can appear at the end of an expression.

a = 20; 
cube(echo(a) a);  // OK
cube([a, a, echo(a) a] ); // OK
cube(a echo(a)); // Error
cube([a, a, a echo(a)] ); // Error

adrianv

Re: (is this) A bug? echo seems to accept children

I think it's a little better to understand that echo in an expression context returns its child. That is "echo(...) expr" is just equal to expr. It's not just something you can insert anywhere.

So
"echo(a) b" is equal to the child b
"echo(a)echo(b)echo(c) d" is equal to the child d
"echo(a)" is equal to undef because the child is missing

"a echo(a)" doesn't make sense because you can't put one expression after another. It would be like writing "a b", which is also an error. If you change it to "a+echo(a)" it is a syntax error, apparently because the + has higher priority than children (odd). If you add parentheses and get "a+(echo(a))" then it parses to a+undef and gives you a warning about a sum containing an undef.

Parkinbot wrote

echo() can appear almost everywhere. However it is not "invisible" in code,
because it can appear at the end of an expression.

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