R devel - Aug 2019 - ?Syntax wrong about `?`'s precedence ?

Dear all,

`?Syntax` documents that `?` has the lowest precedence, right under `=`.

Indeed it reads:

*The following unary and binary operators are defined. They are listed in
precedence groups, from highest to lowest.  *

and ends the list with

*<- <<-* *assignment (right to left)*
*=* *assignment (right to left)*
*?* *help (unary and binary)*
I believe it to be wrong, `=` has lower precedence than `?`.

See the following example :

    `?` <- `+`
    x = 2 ? 3
    x
    #> [1] 5

We see that `2 ? 3` is evaluated first, then the result is assigned to x,
showing
higher precedence for `?`.

Compare it to the similar code using `<-` :

    x <- 2 ? 3
    #> [1] 5
    x
    #> [1] 2

Here first `x <- 2` is evaluated, then its output is added to 3, and the
result
`5` is printed. and we verify that `x` is still `2`. Showing lower
precedence
for `?` consistent with the doc.

Hadley Wickham's package `lobstr` makes it easy to compare the parse trees:

    lobstr::ast({x = 2 ? 3})
    #> o-`{`
    #> \-o-`=`
    #>   +-x
    #>   \-o-`?`
    #>     +-2
    #>     \-3

    lobstr::ast({x <- 2 ? 3})
    #> o-`{`
    #> \-o-`?`
    #>   +-o-`<-`
    #>   | +-x
    #>   | \-2
    #>   \-3

Best regards,

Antoine

	[[alternative HTML version deleted]]

> From: R-devel [mailto:r-devel-bounces at r-project.org] On Behalf Of Ant F
> Sent: 29 August 2019 12:06
> To: r-devel at r-project.org
> Subject: [Rd] ?Syntax wrong about `?`'s precedence ?
> ...
> See the following example :
> 
>     `?` <- `+`
I'm curious; What did you expect to happen if you replace the function
'?' with the operator '+' ?
? is surely now being evaluated as a user-defined function and not as an
operator.
Would you expect the results of doing that to be the same as evaluation without
replacement?

S Ellison




*******************************************************************
This email and any attachments are confidential. Any use...{{dropped:8}}

Precedence is a property of the parser and has nothing to do with the
semantics assigned to various symbols.  Using just core R functions you can
see the precedence of '?' is between those of '=' and
'<-'.
> # '=' has lower precedence than '?'
> str(as.list(parse(text="a ? b = c")[[1]]))
List of 3
 $ : symbol  $ : language `?`(a, b)
 $ : symbol c
> str(as.list(parse(text="a = b ? c")[[1]]))
List of 3
 $ : symbol  $ : symbol a
 $ : language `?`(b, c)
> # '<-' has higher precedence than '?'
> str(as.list(parse(text="a ? b <- c")[[1]]))
List of 3
 $ : symbol ?
 $ : symbol a
 $ : language b <- c
> str(as.list(parse(text="a <- b ? c")[[1]]))
List of 3
 $ : symbol ?
 $ : language a <- b
 $ : symbol c

Bill Dunlap
TIBCO Software
wdunlap tibco.com


On Fri, Aug 30, 2019 at 4:41 AM Stephen Ellison <S.Ellison at
lgcgroup.com>
wrote:
> > From: R-devel [mailto:r-devel-bounces at r-project.org] On Behalf Of
Ant F
> > Sent: 29 August 2019 12:06
> > To: r-devel at r-project.org
> > Subject: [Rd] ?Syntax wrong about `?`'s precedence ?
> > ...
> > See the following example :
> >
> >     `?` <- `+`
>
> I'm curious; What did you expect to happen if you replace the function
'?'
> with the operator '+' ?
> ? is surely now being evaluated as a user-defined function and not as an
> operator.
> Would you expect the results of doing that to be the same as evaluation
> without replacement?
>
> S Ellison
>
>
>
>
> *******************************************************************
> This email and any attachments are confidential. Any u...{{dropped:10}}