paste()ing columns whose names are stored in a variable

Question

How do I create a new column from columns whose names are contained in a character vector?

Given these two variables:

data <- tibble(numbers = 1:10, letters = letters[1:10])
columns <- c("numbers","letters")

What command would produce this output?

# A tibble: 10 x 3
   numbers letters combined
     <int> <chr>   <chr>   
 1       1 a       1-a     
 2       2 b       2-b     
 3       3 c       3-c     
 4       4 d       4-d     
 5       5 e       5-e     
 6       6 f       6-f     
 7       7 g       7-g     
 8       8 h       8-h     
 9       9 i       9-i     
10      10 j       10-j  

My first thought was mutate(data, combined = paste(!!columns, sep = "-")) but this does not work.

Error: Problem with `mutate()` input `combined`.
x Input `combined` can't be recycled to size 10.
ℹ Input `combined` is `paste(c("numbers", "letters"))`.
ℹ Input `combined` must be size 10 or 1, not 2.

Answer 1

not the prettiest but this should work

do.call(
  paste,
  c(data[, columns], list(sep = '-'))
)

Answer 2

 cbind( data , combined = paste( data[[ columns[1] ]], data[[ columns[2] ]], sep=“-“)