Re: st: Detecting a macro expansion error

[Nick Cox <njcoxstata@gmail.com>]

Sorry, but you have lost me.

Local macros are Stata entities. Reading them into Mata to compare
them and writing back results to Stata looks unnecessarily roundabout.
I haven't felt a need for that hitherto.

I can't see the logic of your first block of code. Local macro -temp-
is defined as a copy of local macro -str-. This can't fail as it is
legal whether -str- is empty or not, so I can't see the point of
-capture-. I don't see why you think that -capture- suppresses macro
expansion.

As the two macros are now identical, there is no point to testing
whether they are not.

I have a prejudice that delaying macro expansion is almost always
unnecessary, developed at length on Gabi Huiber's blog.

http://enoriver.net/index.php/2011/02/10/delayed-macro-substitution/#comments

The "almost always" is because I don't want to appear dogmatic, but I
can't recall a convincing example.  I have seen several very
unconvincing examples, however.

Nick

On Sun, Jan 1, 2012 at 11:29 PM, Matthew White
<mwhite@poverty-action.org> wrote:
> Hi Nick,
>
> Thanks for your response; that's a good idea. An empty string actually
> is acceptable, but I realized that I probably just don't want to allow
> the user to pass a macro expansion to the program. Of course, I want
> the user to be able to use macros, but I won't allow delayed macro
> expansions (e.g., using -\-). Supposing the string I'm parsing is
> stored in local `str', my code is:
>
> capture {
>    local temp "`str'"
> }
> mata: if (st_local("temp") != st_local("str")) st_local("syntaxerr", "1");;
> if `syntaxerr' {
> * error and exit
> }
>
> The -capture- block is to suppress the macro expansion error message;
> -capture- by itself doesn't seem to do this.
>
> Before I got to this point, I tried writing a Mata function that
> searches for embedded macro expansions in a string and tests them in
> Stata. If the expansion errors, the function errors. Unlike the
> expansion error, the function error can be picked up by -capture-. I
> haven't done much testing of the function, but here it is with an
> example in case there's interest:
>
> mata:
> void function checkstring(string scalar str)
> {
>        real scalar   match
>        string scalar expansion
>
>        do {
>                match = regexm(str, "(`" + `"[^'"][^']*')"')
>                if (match) {
>                        printf("{txt}Evaluating: {res}%s\n", regexs(0))
>                        expansion = strrtrim(substr(regexs(0), 2, strlen(regexs(0)) - 2))
>                        if (substr(expansion, 1, 1) == "=" | substr(expansion, 1, 1) == ":") {
>                                stata("local test " + expansion)
>                        }
>                        else if (substr(expansion, 1, 2) == "++") {
>                                stata("local ++" + substr(expansion, 3, .))
>                                stata("local --" + substr(expansion, 3, .))
>                        }
>                        else if (substr(expansion, -2, 2) == "++") {
>                                stata("local ++" + substr(expansion, 1, strlen(expansion) - 2))
>                                stata("local --" + substr(expansion, 1, strlen(expansion) - 2))
>                        }
>                        else if (substr(expansion, 1, 2) == "--") {
>                                stata("local --" + substr(expansion, 3, .))
>                                stata("local ++" + substr(expansion, 3, .))
>                        }
>                        else if (substr(strrtrim(expansion), -2, 2) == "--") {
>                                stata("local ++" + substr(expansion, 1, strlen(expansion) - 2))
>                                stata("local --" + substr(expansion, 1, strlen(expansion) - 2))
>                        }
>
>                        str = regexr(str, "`" + `"[^'"][^']*'"', "")
>                }
>        } while(match)
> }
> end
>
> capture noi mata: checkstring("\`='Hello world")
> di _rc
>
> Best,
> Matt
>
> On Sun, Jan 1, 2012 at 2:26 PM, Nick Cox <njcoxstata@gmail.com> wrote:
>> You could try testing whether the expression evaluates to an empty
>> string. The usefulness of that approach depends on whether an empty
>> string is acceptable on other grounds.
>>
>> Nick
>>
>> On Sun, Jan 1, 2012 at 7:04 PM, Matthew White <mwhite@poverty-action.org> wrote:
>>
>>> Using Stata 11.2 SE, I'm writing a program that requires me to parse a
>>> string passed to the program. I want the program to exit if the string
>>> contains a macro expansion that results in an error. Currently, Stata
>>> displays an error about the macro expansion without exiting, and then
>>> continues to parse the string and run the program. Generally, I want
>>> to know how I can detect whether a command results in a macro
>>> expansion error. Here's an example of what I mean:
>>>
>>> . display "`='Hello world"
>>> invalid syntax
>>> Hello world
>>>
>>> Here, the macro expansion `=' results in an invalid syntax error, but
>>> -display- is still executed (after `=' evaluates to ""), and there's
>>> no return code (r(198) is not displayed). -capture- doesn't help:
>>>
>>> . capture display "`='Hello world"
>>> invalid syntax
>>>
>>> . display _rc
>>> 0
>>>
>>> Essentially what I'd like is something like -capture- that I can use
>>> to determine whether a command results in a macro expansion error. Any
>>> ideas?
>>>
>> *
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>
>
>
> --
> Matthew White
> Data Coordinator
> Innovations for Poverty Action
> 101 Whitney Avenue, New Haven, CT 06510 USA
> +1 434-305-9861
> www.poverty-action.org
>
> *
> *   For searches and help try:
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> *   http://www.ats.ucla.edu/stat/stata/

*
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