R How to Pass a function as a String Inside another Function

Question

Any assistance on this little conundrum would be mightily appreciated thanks.

I am trying to pass an argument to the tq_transmute function from the tidyquant package; the value for the argument is a function, however I would like to pass it as a string (out with the scope of the example below I’ll be passing it via a Shiny selectInput).

I have tried every way I can think of to turn the string 'apply.quarterly' into the object apply.quarterly accepted by the mutate_fun argument. The commented lines are my failed attempts.

Ultimately, I would like to extend this concept to the other arguments also i.e. FUN = max to FUN = ‘max’.

library(tidyverse)
library(tidyquant)
library(rlang)

FANG %>%
  group_by(symbol) %>%
  tq_transmute(select     = adjusted, 
               mutate_fun = apply.quarterly,
               # mutate_fun = sym('apply.quarterly'),
               # mutate_fun = syms('apply.quarterly'),
               # mutate_fun = !!sym('apply.quarterly'),
               # mutate_fun = !!!sym('apply.quarterly'),
               # mutate_fun = eval(parse('apply.quarterly')),
               # mutate_fun = eval(substitute('apply.quarterly')),
               # mutate_fun = enquo('apply.quarterly'),
               # mutate_fun = expr(!!enquo('apply.quarterly')),
               # mutate_fun = quo('apply.quarterly'),
               # mutate_fun = enquos('apply.quarterly'),
               # mutate_fun = enquote('apply.quarterly'),
               # mutate_fun = quote('apply.quarterly'),
               # mutate_fun = substitute('apply.quarterly'),
               # mutate_fun = parse('apply.quarterly'),
               # mutate_fun = parse('apply.quarterly'),
               # mutate_fun = ensym('apply.quarterly'),
               # mutate_fun = rlang::as_function('apply.quarterly'),
               # mutate_fun = rlang::as_closure('apply.quarterly'),
               # mutate_fun = rlang::as_quosure('apply.quarterly'),
               # mutate_fun = rlang::as_quosure('apply.quarterly'),
               # mutate_fun = enexpr('apply.quarterly'),
               # mutate_fun = enexprs('apply.quarterly'),
               # mutate_fun = ensym('apply.quarterly'),
               # mutate_fun = ensyms('apply.quarterly'),
               # mutate_fun = eval_tidy('apply.quarterly'),
               # mutate_fun = exprs('apply.quarterly'),
               # mutate_fun = expr_deparse('apply.quarterly'),
               # mutate_fun = expr_label('apply.quarterly'),
               # mutate_fun = expr_label(substitute('apply.quarterly')),
               # mutate_fun = expr_label(quote('apply.quarterly')),
               # mutate_fun = parse_expr('apply.quarterly'),
               # mutate_fun = quasiquotation('apply.quarterly'),
               # mutate_fun = quotation('apply.quarterly'),
               # mutate_fun = quotation('apply.quarterly'),
               FUN        = max, 
               col_rename = "max.close")

Answer 1

It seems that function is a bit finicky for some reason. One way would be to change the call and then evaulate that. For example

myfun <- "apply.quarterly"
bquote(FANG %>%
  group_by(symbol) %>%
  tq_transmute(select     = adjusted, 
               mutate_fun = .(as.name(myfun)),
               FUN        = max, 
               col_rename = "max.close")) %>% 
  eval()

or if you prefer rlang syntax

myfun <- "apply.quarterly"
quo(FANG %>%
         group_by(symbol) %>%
         tq_transmute(select     = adjusted, 
                      mutate_fun = !!sym(myfun),
                      FUN        = max, 
                      col_rename = "max.close")) %>% 
  eval_tidy()

Note that we have to treat the entire expression as rlang quosure. Unless the tq_transmute function was specifically written to handle rlang features like !! then they won't work by default.

Answer 2

You can use blast() for quasiquotation with immediate evaluation

blast <- function(expr, env = caller_env()) {
  eval_bare(enexpr(expr), env)
}

blast(list(!!!1:3))
#> [[1]]
#> [1] 1
#>
#> [[2]]
#> [1] 2
#>
#> [[3]]
#> [1] 3

Then

myfun <- "apply.quarterly"

blast(
  FANG %>%
    group_by(symbol) %>%
    tq_transmute(
      select = adjusted,
      mutate_fun = !!sym(myfun),
      FUN = max,
      col_rename = "max.close"
    )
)

Answer 3

You can obtain the function from the string carrying the name of the function with get. The following works, for example:

s <- get("sum")
s(c(1,2,3))