*Software for data analysis programming with R:*

He was referring to prime directive from Star Trek. One of the practice in this direction is to have a proper checks in place for the types we use. We can trust that if we pass for example a wrong type to our function, it will fail gracefully. So a type system of a programming language is quite important in mission critical numerical computations. Since R language is weakly typed language, or dynamically typed similar to Perl, Python or Matlab/Octave, most of R users omit to place type checks in their functions if not rarely. For example take the following function that takes arguments of a matrix, a vector and a function name. It applies the named function to each columns of the matrix listed in the given vector. Assuming named function is returning a single number our function will return a vector of numbers....This places an obligation on all creators of software to program in such a

way that the computations can be understood and trusted. This obligation

I label the Prime Directive.

myMatrixOperation <- function(A, v, fName) { sliceA <- A[, v]; apply(sliceA, 2, fName); }

*if*statements for each argument in our function. So, function may look like:

myMatrixOperation <- function(A, v, fName) { if(!is.matrix(A)) { stop("A is not a matrix"); } if(!is.vector(v)) { stop("v is not a vector"); } if(!is.funcion(fName)) { stop("fName is not a function"); } sliceA <- A[, v]; apply(sliceA, 2, fName); }

setClass("mySlice", representation(A="matrix", v="vector", fName="function")) myS <- new("mySlice",, A=matrix(rnorm(9),3,3),v=c(1,2), fName=mean) str(myS) Formal class 'mySlice' [package ".GlobalEnv"] with 3 slots ..@ A : num [1:3, 1:3] 0.356 -0.34 -0.642 -0.466 2.915 ... ..@ v : num [1:2] 1 2 ..@ fName:function (x, ...)

Now if we re-write the function that uses our S4 class with type checking only to passing object once.

is.mySlice <- function(obj) { l <- FALSE if(class(obj)[1] == "mySlice") { l <- TRUE } l } myMatrixOperation <- function(mySliceObject) { if(!is.mySlice(mySliceObject)) { stop("argument is not class of mySlice") } sliceA <- mySliceObject@A[, mySliceObject@v]; apply(sliceA, 2, mySliceObject@fName); }