Example code for https://aksela.wordpress.com/2018/07/17/append-element-to-function-body/

Append element to function body 1.r

ll <- list(A=1, B=2, C=3)

append(ll, list(X=5:6), after=1) # works
append(ll, list(X=5:6), after="A") # does not work

Append element to function body 2.r

#EX1
f1 <- function(x) {
    x + 1
}

f1(3)
# 4

body(f1)[[2]][[3]] <- 10

f1(3)
# 13

#EX2
f2 <- function(x) {
    summary(x)
}

f2(mtcars[1:3])
#      mpg             cyl             disp
# Min.   :10.40   Min.   :4.000   Min.   : 71.1
# 1st Qu.:15.43   1st Qu.:4.000   1st Qu.:120.8
# Median :19.20   Median :6.000   Median :196.3
# Mean   :20.09   Mean   :6.188   Mean   :230.7
# 3rd Qu.:22.80   3rd Qu.:8.000   3rd Qu.:326.0
# Max.   :33.90   Max.   :8.000   Max.   :472.0  

body(f2)[[2]]$digits <- 2

f2(mtcars[1:3])
#      mpg          cyl           disp
# Min.   :10   Min.   :4.0   Min.   : 71
# 1st Qu.:15   1st Qu.:4.0   1st Qu.:121
# Median :19   Median :6.0   Median :196
# Mean   :20   Mean   :6.2   Mean   :231
# 3rd Qu.:23   3rd Qu.:8.0   3rd Qu.:326
# Max.   :34   Max.   :8.0   Max.   :472

Append element to function body 3.r

# from
append <- function (x, values, after = length(x)) {
    lengx <- length(x)
    if (!after)
        c(values, x)
    else if (after >= lengx) 
        c(x, values)
    else c(x[1L:after], values, x[(after + 1L):lengx])
}

# to
adjoin <- function (x, values, after = length(x)) {
    lengx <- length(x)
    if (is.character(after)) 
        after <- which(names(x) == after)
    if (!after) 
        c(values, x)
    else if (after >= lengx) 
        c(x, values)
    else c(x[1L:after], values, x[(after + 1L):lengx])
}

Append element to function body 4.r

adjoin <- append

adjoin.body <- append(as.list(body(append)), 
  quote(
    if (is.character(after))
        after <- which(names(x) == after)
  ), 
  after=2)

body(adjoin) <- as.call(adjoin.body)

Append element to function body 5.r

ll <- list(A=1, B=2, C=3)

adjoin(ll, list(X=5:6), after=1)
adjoin(ll, list(X=5:6), after="A")
# both works