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From: Janis Papanagnou <janis_papanagnou+ng@hotmail.com>
Newsgroups: comp.unix.shell
Subject: Re: Create functional processing pipe (without eval)?
Date: Wed, 7 May 2025 10:16:39 +0200
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On 07.05.2025 04:25, Kaz Kylheku wrote:
> On 2025-05-07, Janis Papanagnou <janis_papanagnou+ng@hotmail.com> wrote:
>> I'm pondering about creating a functional processing pipe, depending
>> on program parameters, and whether that's possible to achieve without
>> using 'eval'.
>>
>> Say, the program is called "filter" and may accept 0..N parameters and
>> depending on the set of parameters the respective pipe functionality
>> shall be defined like
>>
>>   filter               =>  cat
>>   filter p1            =>  cat | func p1
>>   filter p1 p2         =>  cat | func p1 | func p2
>>   filter p1 p2 ... pN  =>  cat | func p1 | func p2 | ... | func pN
>>
>> where "func" is working as filter and accepts exactly one parameter.
> 
> The problem is that the number of pipes on the right hand side
> depends on the number of arguments.
> 
> The number of pipes in a command pipe is a property of syntax;

Yes, this is the key observation!

> it is something that needs eval.
> 
> If you're interested in being able to make this kind of code
> transformation, perhaps you should start a project to add macros to a
> shell. (An educated guess informs me that you'd likely choose Korn).

Yes, Ksh is my standard shell.

But if there'd be (beyond 'eval') some sort of "meta-feature" in Bash
(or in other shell) I'd also be interested. Something safer and less
troublesome (quoting!) than 'eval'.

> 
> Macros avoid extra eval by taking advantage of the result of the
> substitution being "naturally thrown into the path of the evaluator",
> just like any other code that is not expanded by a macro.

Hmm.., yes, macros is a possible way to go. (Not sure I'd like to
take that path.) I'll think about that.

Thanks for the confirmation and for the suggestion!

Janis