Infix attr accessor #69

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1. lionel- commented on Apr 28, 2015

   

   lionel-

   on Apr 28, 2015

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   I've always been frustrated that @ does not work as an attribute accessor... Especially since Kevin Ushey mentioned on twitter that S4 slots are actually attributes.

   I suppose it would be highly inappropriate to force the matter and export a redefinition?

   `@` <- function(x, slot) {
     slot <- as.character(substitute(slot))
   
     if (isS4(x)) {
       env <- new.env(parent = parent.frame())
       env$`@` <- base::`@`
       eval(call("@", x, slot), envir = env)
     } else {
       attr(x, slot)
     }
   }
   
   mtcars@names
2. hadley commented on Apr 28, 2015

   

   hadley

   on Apr 28, 2015

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   Yeah, we can't override existing functions, so I think %@% is the best we can do. It might be worth trying to push upstream to R core though.
3. npjc commented on May 10, 2015

   

   npjc

   on May 10, 2015

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   would this follow then?

   x %@% "foo" <- "bar"    # attr(x,"foo") <- "bar"     // x.attr("foo") = "bar";

   OR use %<@% analogous to the %<>% relationship %>%

   if so then what of

   attributes(x) <- list(mycomment = "really special", foo = 3:2,bar = paste(1:6))

   x %<@% list(mycomment = "really special", foo = 3:2,bar = paste(1:6))

   then you need an .append = TRUE or such...
4. lionel- commented on May 10, 2015

   

   lionel-

   on May 10, 2015

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   The first one works:

   `%@%` <- attr
   `%@%<-` <- `attr<-`
   
   mtcars %@% "names"
   mtcars %@% "names" <- 1:11

   The idea of overloading these operators to work with lists seems interesting.

   `%@%` <- function(x, slots) {
     if (length(slots) > 1) {
       attributes(x)[slots]
     } else {
       attr(x, slots)
     }
   }
   
   `%@%<-` <- function(x, slots, value) {
     if (length(slots) > 1) {
       attributes(x)[slots] <- value
       x
     } else {
       attr(x, slots) <- value
     }
   }
   
   mat <- as.matrix(mtcars)
   mat %@% c("dimnames", "dim")
   mat %@% c("dimnames", "dim") <- list(NULL, c(2, 16, 11))

   But it's not good to have the output type change as a function of the number of slots requested. So we'd have to choose one of those two behaviours. The most useful is the list of attributes accessor probably, and we can still hope that R core will enable single attribute lookup with @ in the future :)
5. lionel- commented on May 10, 2015

   

   lionel-

   on May 10, 2015

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   it's not good to have the output type change as a function of the number of slots requested.

   One possibility is to make the behaviour depend on whether the slot is a vector or a list. But the syntax becomes kind of heavy then

   mat %@% list("dimnames", "dim") <- list(NULL, c(2, 16, 11))
6. hadley commented on Sep 24, 2015

   

   hadley

   on Sep 24, 2015

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   I'd rather keep it simple and stick with just getting of single values.
7. hadley commented on Nov 11, 2015

   

   hadley

   on Nov 11, 2015

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   Closed in 97ddec7
8. hadley

   closed this as completedon Nov 11, 2015