Aqui está o código:

mf = function(..., expr) {
    expr = substitute(expr)
    print(class(expr))
    print(str(expr))
    expr
}
mf(a = 1, b = 2, expr = {matrix(NA, 4, 4)})

Resultado:

[1] "{"
length 2 {  matrix(NA, 4, 4) }
 - attr(*, "srcref")=List of 2
  ..$ :Class 'srcref'  atomic [1:8] 1 25 1 25 25 25 1 1
  .. .. ..- attr(*, "srcfile")=Classes 'srcfilecopy', 'srcfile' <environment: 0x7fbcdbce3860> 
  ..$ :Class 'srcref'  atomic [1:8] 1 26 1 41 26 41 1 1
  .. .. ..- attr(*, "srcfile")=Classes 'srcfilecopy', 'srcfile' <environment: 0x7fbcdbce3860> 
 - attr(*, "srcfile")=Classes 'srcfilecopy', 'srcfile' <environment: 0x7fbcdbce3860> 
 - attr(*, "wholeSrcref")=Class 'srcref'  atomic [1:8] 1 0 1 42 0 42 1 1
  .. ..- attr(*, "srcfile")=Classes 'srcfilecopy', 'srcfile' <environment: 0x7fbcdbce3860> 
NULL
{
matrix(NA, 4, 4)
}

Aparentemente, o resultado desubstitute(expr) produz algo da classe "{". O que é exatamente essa classe? Porque é{matrix(NA, 4, 4)} de comprimento 2? O que significam esses atributos estranhos?