18.19 Pairlists

• Pairlists are a remnant of R’s past and have been replaced by lists almost everywhere.
• The only place you are likely to see pairlists in R is when working with calls to the function, as the formal arguments to a function are stored in a pairlist:

f <- expr(function(x, y = 10) x + y)

args <- f[[2]]
args
#> $x
#> 
#> 
#> $y
#> [1] 10

typeof(args)
#> [1] "pairlist"

• Fortunately, whenever you encounter a pairlist, you can treat it just like a regular list:

pl <- pairlist(x = 1, y = 2)

length(pl)
#> [1] 2

pl$x
#> [1] 1