[9fans] rc: null list in concatenation

[9fans] rc: null list in concatenation

[trebol]

Hello everyone.

I'm curious about the behavior of rc concatenating null strings (brakes
execution on error...).  It's a feature, rationally thought-out, or a bug?

If anyone can tell me the story behind, I'll be grateful.

trebol.

P.D.
Sorry for previous mail without subject.

Re: [9fans] rc: null list in concatenation

[arisawa]

example is required.

> 2014/11/25 13:24、trebol@india.com のメール：
> 
> Hello everyone.
> 
> I'm curious about the behavior of rc concatenating null strings (brakes
> execution on error...).  It's a feature, rationally thought-out, or a bug?
> 
> If anyone can tell me the story behind, I'll be grateful.
> 
> trebol.
> 
> P.D.
> Sorry for previous mail without subject.
> 
> 

Re: [9fans] rc: null list in concatenation

[trebol]

a = `{ls file}^test; echo 'this part is not executed if file doesn''t exist'


> example is required.
> 
>> 2014/11/25 13:24、trebol@india.com のメール：
>> 
>> Hello everyone.
>> 
>> I'm curious about the behavior of rc concatenating null strings (brakes
>> execution on error...).  It's a feature, rationally thought-out, or a bug?
>> 
>> If anyone can tell me the story behind, I'll be grateful.
>> 
>> trebol.
>> 
>> P.D.
>> Sorry for previous mail without subject.
>> 
>> 

Re: [9fans] rc: null list in concatenation

[arisawa]

> 2014/11/25 15:56、trebol@india.com のメール：
> 
> a = `{ls file}^test; echo 'this part is not executed if file doesn''t exist'

probably your question is as follows:

maia% a = `{ls file}^test; echo 'this part is not executed if file doesn''t exist'
ls: file: 'file' directory entry not found
rc: null list in concatenation
maia% 

on the other hand,

maia% ls file;  echo 'this part is not executed if file doesn''t exist'
ls: file: 'file' directory entry not found
this part is not executed if file doesn't exist
maia% 

a similar  thing is:
maia% (aa bb)=aaaa; echo bbbb
rc: variable name not singleton!
maia% 

perhaps this is a feature.
rc stops execution if there is an error in grammar.

Re: [9fans] rc: null list in concatenation

[Charles Forsyth]

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It's a "null list" not a "null string". You can use a null string:

% s=''
% ls $s^/x

but not a null list (like using null in many lisp list operations, or the
difference between strcat of nil and strcat of ""):

% s=()
% ls $s^/x
rc (-/bin/rc): null list in concatenation

A completely unset variable is a null list. Also, `{...} returns  a list
(of words) not a string,
hence the diagnostic in that case.

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Re: [9fans] rc: null list in concatenation

[trebol]

> maia% (aa bb)=aaaa; echo bbbb
> rc: variable name not singleton!

Interesting, I could expect something like
'(aa bb) = aaaa --> aa=aaaa bb=aaaa'
but of course there is not a concatenation, but an assigment.

> It's a "null list" not a "null string". You can use a null string:
>
> % s=''
> % ls $s^/x
>
> but not a null list (like using null in many lisp list operations, or the
> difference between strcat of nil and strcat of ""):
>
> % s=()
> % ls $s^/x
> rc (-/bin/rc): null list in concatenation
>
> A completely unset variable is a null list. Also, `{...} returns  a list
> (of words) not a string,
> hence the diagnostic in that case.

Yeah... thanks a lot!
So the correct way must bem something like:

a = ('' `{ls file})^test; echo 'this part _is_ executed if file doesn''t exist'

I feel ashamed... The man page is beautifully written, except may be the
'\t\n' notation. Now this is not confusing for me because I've in mind that
Plan9 ignore those escape sequence, but at first with my unix reflexes
that was a problem, and I think it's important to remember those first
steps to improve documentation.

I remember the first time I type.
$ man bash

I had to power off the computer and go to the beach.

trebol.

Re: [9fans] rc: null list in concatenation

[trebol]

Sorry for all that noise.  The problem was simply confusion about strings,
list of strings, list of strings and|or other list of strings... and
too much coffee.

What I needed was simply the use of $" to work around possible empty
lists in concatenations (and of course read the man page carefully
again and again), my fault, sorry again.

trebol.

Re: [9fans] rc: null list in concatenation

[trebol]

> My be my poor English

It's the proof itself!

Re: [9fans] rc: null list in concatenation

[trebol]

> Also, when the '^' is omitted, like in
>
> 	$dir(1)^$dir(2)

 Also, when the '^' is omitted, like in

 	$dir(1)$dir(2)

Sorry for the mess...