How use an expression object as input in sum function in R - r

Question

I want to use the content of my "Input" Object inside my min.rss function but i have the following error :
Error in sum((input[[1]])) : invalid 'type' (language) of argument
I don't understand why sum is not able to read the content of input[[1]] while when i copy/paste the content of input[[1]] directly in sum it's working
Here is my code :
var = mtcars
var$mpg2 = var$mpg
var$mpg = NULL
mDegree = 10
mList = c()
for (i in 1:mDegree){
mList = c(mList,paste0(paste0(paste0('var[[',i),']]'),'*'))
}
mListBis = c()
for (j in 1:mDegree){
mListBis = c(mListBis,paste0(paste0(paste0('par[',j+1),']'),'+'))
}
mChar = "par[1]+"
for (k in 1:mDegree){
mChar = paste0(mChar,paste0(mList[k],mListBis[k]))
}
mChar = strtrim(mChar, nchar(mChar)-1)
mChar = paste0(mChar,'-',paste0('var[[',i+1),']]')
mChar = paste0('(',mChar,')^2')
input = parse(text = mChar)
min.RSS <- function(data, par) {
with(data,
sum((input[[1]])))
}
#########################################################
#########################################################
(result <- optim(par = c(0, 1, 0, 1, 1, 0, 1, 0, 1, 0), min.RSS, data = var))
My principal objective is to make the following code more responsive :
mtcars <- data.frame(mtcars)
dat=data.frame(y=mtcars$mpg, x=mtcars$wt)
min.RSS <- function(data, par) {
with(data,
sum((par[1] + par[2] * x - y)^2))
}
(result <- optim(par = c(0, 1), min.RSS, data = dat))