What is “{” class in R?

Question

Here is the code:

mf = function(..., expr) {
    expr = substitute(expr)
    print(class(expr))
    print(str(expr))
    expr
}
mf(a = 1, b = 2, expr = {matr         


        

Answer 1

The { is the class for a block of code. Just looking at the classes, note the difference between these

mf(a = 1, b = 2, expr = {matrix(NA, 4, 4)})
# [1] "{"
mf(a = 1, b = 2, expr = matrix(NA, 4, 4))
# [1] "call"

A class of { can hold multiple statements. The length() indicates how many statements are in the block (including the start of the block). For example

length(quote({matrix(NA, 4, 4)}))
# [1] 2
length(quote({matrix(NA, 4, 4); matrix(NA,3,3)}))
# [1] 3
length(quote({}))
# [1] 1

The attributes "srcref" and "srcfile" are how R tracks where functions are defined for trying to give informative error messages. You can see the ?srcfile help page for more information about that.