Hello,

Section 4.3.2 of the R language definition [1] states that argument matching to formal arguments is a 3-pass process to match arguments to a function. An error is generated if any (supplied) arguments are left unmatched. Interestingly the opposite is not true as any unmatched formals does not generate an error.

> f <- function(x,y,z) x
> f(2)
[1] 2
> f(2,3)
[1] 2

Since R is lazily evaluated, I understand that it is not an error for an unused argument to be unassigned. However, it is surprising to me that a function need not be called with all its required arguments. I guess in this situation technically "required arguments" means required and referenced arguments. 

> f()
Error in f() : argument "x" is missing, with no default

Can anyone shed light on the reasoning for this design choice?

Warm Regards,
Brian Rowe


[1] http://cran.r-project.org/doc/manuals/r-release/R-lang.html#Argument-matching


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