[External] Re: capture "->"

Maybe someone has already suggested this, but if your functions accepted
strings you could use sub or gsub to replace the -> with a symbol that
parsed at the same precedence as <-,
say <<-.  Then parse it and deal with it.  When it is time to display the
parsed and perhaps manipulated formulae to the user, deparse it and do the
reverse replacement.

encode <- function(string)gsub(perl=TRUE, "->", "<<-", x=string)
decode <- function(string)gsub(perl=TRUE, "<<-", "->", x=string)
rightArrow <- as.name("<<-")
leftArrow <- as.name("<-")
ast1 <- parse(text=encode("x1 + x2 -> a3"))[[1]]
ast2 <- parse(text=encode("y4 <- b5 + (b6 / b7)"))[[1]]
identical(ast1[[1]], rightArrow)

[1] TRUE

identical(ast2[[1]], leftArrow)

[1] TRUE

ast1[[3]] <- as.name("new_a3")
decode(deparse(ast1))

[1] "x1 + x2 -> new_a3"

-Bill

On Mon, Mar 4, 2024 at 1:59?AM Dmitri Popavenko <dmitri.popavenko at gmail.com>
wrote:

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 at 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 at 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 at 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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