If one is to create their own infix function, it needs to have the form %fun_name%. Why isn't that the case with := of tidyverse? Also, the same with the operator !!, also form tidyverse.
Why doesn't the infix function := from tidyverse need the % around it?
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1 Answers
These operators are not functions. They are only meaningful when supplied as part of arguments to functions that use non-standard evaluation and know how to handle them. The R interpreter does not treat these operators as functions and you cannot call them directly. If you call !!something outside of a quasiquoted argument the interpreter treats it as:
!(!(something))
If you call := outside of a quasiquoted argument you get an exception.
As an aside, you can always overload base infix functions or define new S3 methods for them without needing %. The interpreter always treats these symbols as infix functions and they are not reserved. e.g:
> `+` <- function(x,y) x - y
> 1 + 2
[1] -1
`/.foo` <- function(x, y) x * y
> my_object <- structure(1, class = "foo")
> 1 / 2
[1] 0.5
> my_object / 2
[1] 2
attr(,"class")
[1] "foo"
For some reason R does allows the definition of a function := and will evaluate the resulting function as an infix function. However, this is not how rlang or data.table actually implement :=. Other symbols that begin with :, e.g. :+, or end with =, e.g. !=, are not interpreted as infix functions; I am not sure why this should be the case, as the symbol := is not used in base R. The interpreter's unique ability to evaluate it as an infix function is unexpected, at least to me.
> `:=` <- function(x,y) x - y
> 5 := 2
[1] 3
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