Method chaining with R

Question

Is it possible to chain functions in R?

Sample data:

m <- matrix(c(1:10, 11:20), nrow = 10, ncol = 2)

For example, I would like to r

Answer 1

Sort of, but I think it's un-idiomatic and maybe fragile/not a good idea. (This is implied, I think, by @RichieCotton's comment above.)

From http://cran.r-project.org/doc/manuals/R-lang.html :

10.3.4 Special operators

R allows user-defined infix operators. These have the form of a string of characters delimited by the ‘%’ character. The string can contain any printable character except ‘%’. The escape sequences for strings do not apply here.

Note that the following operators are predefined

 %% %*% %/% %in% %o% %x%

"%@%" <- function(x,f) {
    f(x)
}

sqr <- function(x) x^2
x <- 1:4

x %@% mean  ## 2.5
x %@% mean %@% sqr  ## 6.25
x %@% (mean %@% sqr)  ## fails

Given m as defined above -- maybe what you had in mind?

 m %@% colMeans %@% sum  ## 21

Notes:

• your example is a bit funny, because mean(x) always returns a scalar (i.e. a length-1 vector), so sum(mean(x)) is always going to be the same as mean(x)
• the infix operators have to be surrounded by %, so you can't have anything as compact as a single symbol (and %% is taken already).
• this sort of chaining is non-associative, which worries me -- it seems that the examples above work, so R is (apparently) evaluating left-to-right, but I don't know that that's guaranteed ...

edit: the question now asks how additional arguments can be incorporated. I don't think the syntax suggested (x %@% fun1(arg1) %@% fun2(arg2)) will work without some serious magic. This is the closest I can get at the moment -- creating a wrapper function that creates a modified version of the original function.

F <- function(f,...) {
    function(x) {
        f(x,...)
    }
}

Testing:

pow <- function(x,b=2) { x^b }
sqr <- function(x) x^2
x <- 1:4

x %@% F(mean,na.rm=TRUE)  ## 2.5
x %@% F(mean,na.rm=TRUE) %@% F(pow,3)  ## 16.25

(Note that I have used F as a function here, which may be dicey in some situations because it overwrites the F==FALSE shortcut)