Using a quasiquoted argument to a function within a purrr:map call

Question

I'm having a little trouble figuring out quasiquotation, specifically I have a function which takes an argument which specifies which variable should go into a model which is then run within a purrr::map call.

I've been working from: https://dplyr.tidyverse.org/articles/programming.html

# libs
library(tidyverse)
library(broom)
# dummy data
df <- data.frame(
    "a"=rep(c("alpha","beta"),50),
    "b"=rnorm(100),
    "value1"=rnorm(100),
    "value2"=rnorm(100)
)

---

model <- function(var) {
    var <- enquo(var)
    df %>%
        group_by(a) %>%
        nest() %>%
        mutate(model=map(data, ~ lm(b ~ (!! var),data=.)))
}

model(value1)

> Error in mutate_impl(.data, dots) : Evaluation error: invalid model formula.

putting the name in directly works as expected:

df %>%
    group_by(a) %>%
    nest() %>%
    mutate(model=map(data, ~ lm(b ~ value1,data=.))) %>%
    unnest(model %>% map(glance))

I can use !! var !! var within a function:

modelX <- function(var,df=df) {
    var <- enquo(var)
    df %>%
        select(!! var)
}

modelX(value1,df) 

I'm assuming that this has something to do with the fact the !! var !! var is referring to a value in the nested tibble data , I've been poking around with rlang::qq_show() but haven't been able to figure it out so far'

Answer 1

The enquo() will attempt to track the enviroment of the symbol you pass in, but you don't really want that included in the formula you are passing to lm . It would be better to capture that as a symbol rather than a quosure. Try this

model <- function(var) {
  var <- ensym(var)
  df %>%
    group_by(a) %>%
    nest() %>%
    mutate(model=map(data, ~ lm(b ~ !!var, data=.)))
}

Worked for me with dplyr_0.7.6 and purrr_0.2.5