[Rd] [External] Re: capture "->"

[Dmitri Popavenko]

Dear Barry,

In general, I believe users are already accustomed with the classical
arrows "->" and "<-" which are used as such in quoted expressions.
But I agree that "-.>" is a very neat trick, thanks a lot. A small dot,
what a difference.

All the best,
Dmitri

On Mon, Mar 4, 2024 at 11:40 AM Barry Rowlingson <
b.rowlingson using lancaster.ac.uk> wrote:

> It seems like you want to use -> and <- as arrows with different meanings
> to "A gets the value of B" in your package, as a means of writing
> expressions in your package language.
>
> Another possibility would be to use different symbols instead of the
> problematic -> and <-, for example you could use <.~ and ~.> which are not
> at all flipped or changed before you get a chance to parse your expression.
> It might make your language parser a bit trickier though. Let's see how
> these things turn into R's AST using `lobstr`:
>
>  > library(lobstr)
>  > ast(A ~.> B)
> █─`~`
> ├─A
> └─█─`>`
>   ├─.
>   └─B
>  > ast(A <.~ B)
> █─`~`
> ├─█─`<`
> │ ├─A
> │ └─.
> └─B
>
> You'd have to unpick that tree to figure out you've got A and B on either
> side of your expression, and that the direction of the expression is L-R or
> R-L.
>
> You could also use -.> and <.- symbols, leading to a different tree
>
>  > ast(A -.> B)
> █─`>`
> ├─█─`-`
> │ ├─A
> │ └─.
> └─B
>  > ast(A <.- B)
> █─`<`
> ├─A
> └─█─`-`
>   ├─.
>   └─B
>
> Without knowing the complexity of your language expressions (especially if
> it allows dots and minus signs with special meanings) I'm not sure if A)
> this will work or B) this will bend your brain in horrible directions in
> order to make it work... Although you don't need to parse the AST as above,
> you can always deparse to get the text version of it:
>
>  > textex = function(x){deparse(substitute(x))}
>  > textex(A <.~ B)
> [1] "A < . ~ B"
>
> The <.~ form has an advantage over the <.- form if you want to do complex
> expressions with more than one arrow, since the ~ form is syntactically
> correct but the - form isnt:
>
>  > textex(A <.~ B ~.> C)
> [1] "A < . ~ B ~ . > C"
>  > textex(A <.- B -.> C)
> Error: unexpected '>' in "textex(A <.- B -.>"
>
>
> Barry
>
>
> On Sun, Mar 3, 2024 at 12:25 PM Dmitri Popavenko <
> dmitri.popavenko using gmail.com> wrote:
>
>> This email originated outside the University. Check before clicking links
>> or attachments.
>>
>> On Sat, Mar 2, 2024 at 7:58 PM Gabor Grothendieck <
>> ggrothendieck using gmail.com>
>> wrote:
>>
>> > Would it be good enough to pass it as a formula?  Using your definition
>> of
>> > foo
>> >
>> >   foo(~ A -> result)
>> >   ## result <- ~A
>> >
>> >   foo(~ result <- A)
>> >   ## ~result <- A
>> >
>>
>> Yes, to pass as a formula would be the idea.
>> It's just that the parser inverses "~A -> result" into "result <- ~A".
>> We are seeking for any way possible to flag this inversion.
>>
>> Avi, thank you for your efforts too. Wrapping symbols into percent signs
>> is
>> an option, but as Duncan says it is much more intuitive to just quote the
>> expression.
>> The challenge is to somehow flag the parser inversion, otherwise a quoted
>> expression seems to be the only solution possible.
>>
>> Regards,
>> Dmitri
>>
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>>
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