Passing data-variables to R formulas

Question

Let's say I'd like to write anscombe %>% lm_tidy("x1", "y1") (Actually, I'd like to write anscombe %>% lm_tidy(x1, y1), where x1 and y1 are part of the data frame). So, as the following function seems working:

plot_gg <- function(df, x, y) {
  x <- enquo(x)
  y <- enquo(y)
  ggplot(df, aes(x = !!x, y = !!y)) + geom_point() +
    geom_smooth(formula = y ~ x, method="lm", se = FALSE)
}

I started writing the following function:

lm_tidy_1 <- function(df, x, y) {
  x <- enquo(x)
  y <- enquo(y)
  fm <- y ~ x            ##### I tried many stuff here!
  lm(fm, data=df)
}
## Error in model.frame.default(formula = fm, data = df, drop.unused.levels = TRUE) : 
##   object is not a matrix

One comment in passing in column name as argument states that embrace {{...}} is a shorthand notation for the quote-unquote pattern. Unfortunately, error messages were different in both situations:

lm_tidy_2 <- function(df, x, y) {
  fm <- !!enquo(y) ~ !!enquo(x) # alternative: {{y}} ~ {{x}} with different errors!!
  lm(fm, data=df)
}
## Error:
## ! Quosures can only be unquoted within a quasiquotation context.

This seems working (based on @jubas's answer but we're stuck with string handling and paste):

lm_tidy_str <- function(df, x, y) {
  fm <- formula(paste({{y}}, "~", {{x}}))
  lm(fm, data=df)
}

Yet again, {{y}} != !!enquo(y). But it's worse: the following function breaks down with the same Quosure error as earlier:

lm_tidy_str_1 <- function(df, x, y) {
  x <- enquo(x)
  y <- enquo(y)
  fm <- formula(paste(!!y, "~", !!x))
  lm(fm, data=df)
}

1. Is {{y}} != !!enquo(y)?
2. How to pass data-variables to lm?

EDIT: Sorry, there were left-overs from my many trials. I want to directly pass the data-variables (say x1 and y1) to the function that is going to use them as formula components (such as lm) and not their string versions ("x1" and "y1"): I try to avoid strings as long as possible and it's more streamlined from the user perspective.

Answer 1

Wrap the formula in "expr," then evaluate it.

library(dplyr)
lm_tidy <- function(df, x, y) {
  x <- sym(x)
  y <- sym(y)
  fm <- expr(!!y ~ !!x)
  lm(fm, data = df)
}

This function is equivalent:

lm_tidy <- function(df, x, y) {
  fm <- expr(!!sym(y) ~ !!sym(x))
  lm(fm, data = df)
}

Then

lm_tidy(mtcars, "cyl", "mpg")

gives

Call:
lm(formula = fm, data = .)

Coefficients:
(Intercept)          cyl  
     37.885       -2.876  

EDIT per comment below:

library(rlang)
lm_tidy_quo <- function(df, x, y){
    y <- enquo(y)
    x <- enquo(x)
    fm <- paste(quo_text(y), "~", quo_text(x))
    lm(fm, data = df)
}

You can then pass symbols as arguments

lm_tidy_quo(mtcars, cyl, mpg)