Re: [PATCH] Support the mksh's ${|func;} substitution

[Sebastian Gniazdowski <sgniazdowski@gmail.com>]

On Sat, 7 Sep 2019 at 17:07, Stephane Chazelas
<stephane.chazelas@gmail.com> wrote:
> Note that mksh's operator can do ${|REPLY=value}, it's not
> limited to functions.

Ok, true, it can also run binary commands.

> The ; is also not necessary

I think that this is undocumented feature, as the docs say:

"Another variant of substitution are the valsubs (value substitutions)
${|command;} which are also executed in the current environment, like
funsubs, but share their I/O with the parent; instead, they evaluate
to whatever the, initially empty, expression-local variable REPLY is
set to within the commands."

> Note that in zsh anything is a valid function name, just like
> just about anything is a valid command name. So your operator
> would be incompatible with mksh's if it accepted arbitrary
> function names unless you handled quoting in there:
>
> $ ./Src/zsh -c '"REPLY=value"() { REPLY=x; echo done; }; REPLY\=value; echo ${|REPLY=value;}'
> done
> zsh:1: no such function: REPLY=value

That's a valid and nice point, and it does yield some concerns,
however for any problems to appear the user would have to have at
least a function with "targetvariable=..." in name, i.e. a coincidence
of two places in code, the targetvar= in the ${|...;} and in the
function name, and this can be controlled and seems unlikely to occur
by accident.

> With those fixed, i.e. when it's really like mksh's ${|code},
> I'd agree the feature could be useful, but I suspect that would
> be harder to implement as it would mean changing the parsing.

The parsing would have to be changed to prevent the "=" in function names?

I think that I've chosen an initial wrong direction: to implement the
substitution "as-is", with it's form, treating it as a model. Instead,
I should have implemented the feature, not the substitution. Zsh has
its own ways to set-up complex substitutions and this is done via the
parens flags.

Would you consider such method, i.e. to not impose mksh's substitution
ways on Zsh, but instead assign a flag, like e.g.: ${(|)funct}, still
useful?

> Note that beside the math functions, zsh already has something
> similar with its "dynamic named directory" framework (a feature
> I always found quite obscure/far fetched myself).
>
> echo "${| REPLY=value}"
>
> could be done in zsh with:
>
> zsh_directory_name() {
>   [[ $1 = d ]] && [[ $2 = //* ]] || return
>   eval " ${2#//}"
>   reply=("$REPLY" ${#2})
> }
>
> echo ${${${(D):-//REPLY=value}#\~\[}%\]}
>
> (even more convoluted than your math function approach).

That's interesting, it actually allows to do:

arr=( val1 val2 abc1 abc3 )
funct() { REPLY="${(C)MATCH}"; }
zsh_directory_name() { ... }
print -rl ${arr[@]//(#m)*/${${${(D):-//funct}#\~\[}%\]}}

Output:
Val1
Val2
Abc1
Abc3

-- 
Sebastian Gniazdowski
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