On 12/1/2022 5:38 PM, Milan Zimmermann wrote:
> Would it be reasonable to suggest removing existing functionality as
> follows:
[snip]
> The currently allowed
>
> 3. {command-call}
> 4. (function-call)
>
> would represent syntax error.
>
> (3. is outright wrong as it brings silent invalid results)
Both 3 and 4 are valid forms, although I think (lisp) and $(lisp) are
equivalent. In particular, 3 is important for being able to check the
exit status of external programs:
~ $ if {sh -c 'exit 0'} {echo yes} {echo no}
yes
~ $ if {sh -c 'exit 1'} {echo yes} {echo no}
no
~ $ if ${sh -c 'exit 0'} {echo yes} {echo no}
no ;; Wrong!
~ $ if ${sh -c 'exit 1'} {echo yes} {echo no}
no
Ah, thanks for pointing this out. I kept searching for a practical semantics that justifies use of if {}. This is definitely important, non-replaceable syntax. My suggestion to ban it was definitely incorrect then.
I think there's an argument that {lisp-function} should work the same as
${lisp-function}, but only for "regular" Lisp functions (i.e. excluding
eshell/FOO ones; since those are designed to imitate external commands,
they have different semantics).
I agree. I did not find a situation where {lisp-function} and ${lisp-function} would behave differently, but I have not looked yet.
So I put the following to my notes, based on your suggestions. If anyone else is reading, please disregard my earlier suggestion to disallow the "if {..}" form. Justified by checking external programs status.
1. use ~if {function-call}~ (only?) to check EXIT STATUS of the EXTERNAL function (program) call.
2. use ~if ${function-call}~ for everything else Eshell, including internal functions passed exported variables (as they force Eshell syntax)
3. use ~if (elisp-function-call)~ for everything pure-elisp, including elisp functions passed setq-ed variables
4. use ~if $(elisp-function-call)~ seems equivalent and interchangeable with the above
Examples of 1. - check exit status of EXTERNAL program
- if {sh -c 'exit 0'} {echo "external succeeded"} {echo "external failed"} # external succeeded
- if {sh -c 'exit 1'} {echo "external succeeded"} {echo "external failed"} # external failed
- if ${sh -c 'exit 0'} {echo "external succeeded"} {echo "external failed"} # WRONG external failed
- if ${sh -c 'exit 1'} {echo "external succeeded"} {echo "external failed"} # external failed
- Other example, if we want to check for success/failure in sed before calling it
- # just to test: echo "aaa" | sed 's/aaa/bbb/' # bbb
- if {echo "aaa" | sed 's/aaa/bbb/'} {echo sed-success} {echo sed-failure} # sed-success
- # just to test: echo "aaa" | sed 's/aaa/bbb' # /usr/bin/sed: -e _expression_ #1, char 9: unterminated `s' command
- if {echo "aaa" | sed 's/aaa/bbb'} {echo sed-success} {echo sed-failure} # sed-failure
Examples
- export a="3"
- Bad use: if {equal $a "0"} { echo YES } { echo NO } # YES -- WRONG!!! (silently!!)
- Good use: if ${equal $a "0"} { echo YES } { echo NO } # NO -- correct