R tricks with lazyeval::interp():

interp.r

# taken from 
# http://cran.rstudio.com/web/packages/dplyr/vignettes/nse.html
# What if you need to mingle constants and variables? Use the handy lazyeval::interp():

library(lazyeval)
# Interp works with formulas, quoted calls and strings (but formulas are best)
interp(~ x + y, x = 10)
#> ~10 + y
interp(quote(x + y), x = 10)
#> 10 + y
interp("x + y", x = 10)
#> [1] "10 + y"

# Use as.name if you have a character string that gives a variable name
interp(~ mean(var), var = as.name("mpg"))
#> ~mean(mpg)
# or supply the quoted name directly
interp(~ mean(var), var = quote(mpg))
#> ~mean(mpg)
Because every action in R is a function call you can use this same idea to modify functions:

interp(~ f(a, b), f = quote(mean))
#> ~mean(a, b)
interp(~ f(a, b), f = as.name("+"))
#> ~a + b
interp(~ f(a, b), f = quote(`if`))
#> ~if (a) b
If you already have a list of values, use .values:

interp(~ x + y, .values = list(x = 10))
#> ~10 + y

# You can also interpolate variables defined in the current
# environment, but this is a little risky becuase it's easy
# for this to change without you realising
y <- 10
interp(~ x + y, .values = environment())
#> ~x + 10