Extending suggestion for stopifnot

f(m) %::% matrix : matrix
f(m) %as% { m }

f(as.data.frame(matrix(rnorm(12),nrow=3)))

f(1)

f

On Tue, Aug 20, 2013 at 11:41 AM, ivo welch <ivo.welch at anderson.ucla.edu> wrote:

I am using a variant of stopifnot a lot.  can I suggest that base R
extends its functionality?  I know how to do this for myself.  this is
a suggestion for beginners and students.  I don't think it would break
anything.

first, I think it would be more useful if it had an optional character
string, so users could write

 stopifnot( is.matrix(m), "m is not a matrix" )

this would mean that stopifnot would have to detect whether the last
argument is a string.  (I think stopifnot should have had only one
condition, and one should have used all() to test multiple conditions,
but this is a bridge that was already crossed.)  upon failure,
stopifnot should print the character string.  that's it.


A second enhancement would be a "smart string", which knows that
everything inside {{...}} should be evaluated.

 stopifnot( is.matrix(m), "m is not a matrix, but a {{class(m)}}" )

I think using a function (in this case paste) is cleaner:

paste("m is not a matrix, but a", class(m))

It avoids adding a new convention ("evaluate everything between {{
}}") and has additional arguments.

my own programming variant looks even nicer,

  is.matrix(m) %or% "m is not a matrix but a {{class(m)}}"

In R you can write it as

is.matrix(m) || stop("m is not a matrix but a ", class(m))

Examples:

m = 1

is.matrix(m) || stop("m is not a matrix but a ", class(m))

Error: m is not a matrix but a numeric

m = matrix(0,2,2)
is.matrix(m) || stop("m is not a matrix but a ", class(m))

[1] TRUE

But the construct

if (!is.matrix(m)) stop("m is not a matrix but a ", class(m))

is more readable for people not used to Pearl.