R - How to create a function that accepts a code block as parameter?

Question

I would like to create a function that accepts a code block as a parameter. I am not sure how to do this in R. I would like to create a function that has similar behavior to the built-in within function.

data <- within(data, {
  var4 <- var2 + var3
  var5 <- var1 / var2
})

I am having trouble finding the within function definition within the R source code. Otherwise, I would probably have my answer.

Thanks

Answer 1

It’s very simple:

test <- function(block) {
    cat('before\n')
    rv <- block
    cat('after\n')
    rv
}

and to see how it works:

test_called <- test({
    cat('bar\n')
    'baz'
})

running the latter outputs:

before
bar
after

and test_called has the value 'baz'

this shows that the block only gets evaluated if you do something with it, instead of immediately when being passed.

Answer 2

This is what I was looking for. You just pass the code block (aka expression) to the eval() function. See ?eval for more details and variants.

foo <- function(expr) {
   result <- eval(expr)
   return(result)
}

Thanks!

UPDATE: To provide some context around this, I was trying to create a function that would execute a long-running expression and then cache (aka save) the result so that it didn't need to be re-run. Here is a simplified version of what I ended up creating. I am open to suggestions on how to do this better.

cache <- function (cache.name, expr) {

    result <- NULL
    cache.file <- sprintf ("%s/%s.rds", cache.dir, cache.name)

    # has the result already been cached?
    if (file.exists (cache.file)) {
        result <- readRDS(cache.file)

    } else {
        # eval the expression 
        result <- eval(expr)

        # cache the result
        saveRDS (result, cache.file, compress = cache.compress)
    }

    return(result)
}