[Rd] Support for user defined unary functions

[Jim Hester]

This works the same way as `?` is defined in R code, and `-`, `+`
(defined in C) do now, you define one function that handles calls with
both unary and binary arguments.

    quote(a %f% %f% b)
    #> a %f% (`%f%`(b))

    `%f%` <- function(a, b) if (missing(b)) { force(a); cat("unary\n")
} else { force(a);force(b);cat("binary\n") }
    a <- 1
    b <- 2

    a %f% %f% b
    #> unary
    #> binary


This also brings up the point about what happens to existing user
defined functions such as `%in%` when they are used as unary functions
(likely by mistake). Happily this provides a useful error when run
assuming no default value of the second argument.

    %in% a
    #> Error in match(x, table, nomatch = 0L) :
    #>   argument "table" is missing, with no default


On Thu, Mar 16, 2017 at 7:13 PM, Duncan Murdoch
<murdoch.duncan at gmail.com> wrote:
> I don't have a positive or negative opinion on this yet, but I do have a
> question.  If I define both unary and binary operators with the same name
> (in different frames, presumably), what would happen?
>
> Is "a %chr% b" a syntax error if unary %chr% is found first?  If both might
> be found, does "a %chr% %chr% b" mean "%chr%(a, %chr% b)", or is it a syntax
> error (like typing "a %chr%(%chr%(b))" would be)?
>
> Duncan Murdoch
>
>
>
>
>
> On 16/03/2017 10:24 AM, Jim Hester wrote:
>>
>> R has long supported user defined binary (infix) functions, defined
>> with `%fun%`. A one line change [1] to R's grammar allows users to
>> define unary (prefix) functions in the same manner.
>>
>>     `%chr%` <- function(x) as.character(x)
>>     `%identical%` <- function(x, y) identical(x, y)
>>
>>     %chr% 100
>>     #> [1] "100"
>>
>>     %chr% 100 %identical% "100"
>>     #> [1] TRUE
>>
>> This seems a natural extension of the existing functionality and
>> requires only a minor change to the grammar. If this change seems
>> acceptable I am happy to provide a complete patch with suitable tests
>> and documentation.
>>
>> [1]:
>> Index: src/main/gram.y
>> ===================================================================
>> --- src/main/gram.y     (revision 72358)
>> +++ src/main/gram.y     (working copy)
>> @@ -357,6 +357,7 @@
>>         |       '+' expr %prec UMINUS           { $$ = xxunary($1,$2);
>>  setId( $$, @$); }
>>         |       '!' expr %prec UNOT             { $$ = xxunary($1,$2);
>>  setId( $$, @$); }
>>         |       '~' expr %prec TILDE            { $$ = xxunary($1,$2);
>>  setId( $$, @$); }
>> +       |       SPECIAL expr                    { $$ = xxunary($1,$2);
>>  setId( $$, @$); }
>>         |       '?' expr                        { $$ = xxunary($1,$2);
>>  setId( $$, @$); }
>>
>>         |       expr ':'  expr                  { $$ =
>> xxbinary($2,$1,$3);      setId( $$, @$); }
>>
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>