Programming with dplyr and lazyeval

Question

I am having issues refactoring dplyr in a way that preserves non-standard evaluation. Lets say I want to create a function that always selects and renames.

library(lazyeval)
library(dplyr)

df <- data.frame(a = c(1,2,3), f = c(4,5,6), lm = c(7, 8 , 9))

select_happy<- function(df, col){
    col <- lazy(col)
    fo <- interp(~x, x=col)
    select_(df, happy=fo)
}

f <- function(){
    print('foo')
}

select_happy() is written according to the answer to this post Refactor R code when library functions use non-standard evaluation. select_happy() works on column names that are either undefined or defined in the global environment. However, it runs into issues when a column name is also the name of a function in another namespace.

select_happy(df, a)
#   happy
# 1     1
# 2     2
# 3     3

select_happy(df, f)
#   happy
# 1     4
# 2     5
# 3     6

select_happy(df, lm)
# Error in eval(expr, envir, enclos) (from #4) : object 'datafile' not found

environment(f)
# <environment: R_GlobalEnv>

environment(lm)
# <environment: namespace:stats>

Calling lazy() on f and lm shows a difference in the lazy object, where the function definition for lm is appearing in the lazy object, and for f it is just the name of the function.

lazy(f)
# <lazy>
#   expr: f
#   env:  <environment: R_GlobalEnv>

lazy(lm)
# <lazy>
#   expr: function (formula, data, subset, weights, na.action, method = "qr",  ...
#   env:  <environment: R_GlobalEnv>

substitute appears to work with lm.

 select_happy<- function(df, col){
     col <- substitute(col) # <- substitute() instead of lazy()
     fo <- interp(~x, x=col)
     select_(df, happy=fo)
}

select_happy(df, lm)
#   happy
# 1     7 
# 2     8
# 3     9

However, after reading the vignette on lazyeval it seems that lazy should serve as a superior substitute for substitute. Additionally, the regular select function works just fine.

select(df, happy=lm)
#   happy
# 1     7
# 2     8
# 3     9

My question is how can I write select_happy() so that it works in all the ways that select() does? I'm having a hard time wrapping my head around the scoping and non-standard evaluation. More generally, what would be a solid strategy for programming with dplyr that could avoid these and other issues?

Edit

I tested out docendo discimus's solution and it worked great, but I would like to know if there is a way to use arguments, rather than dots, for the function. I think it is also important to be able to use interp() because you might want to feed input into a more complicated formula, like in the post I linked to earlier. I think the core of the issue come down to the fact that lazy_dots() is capturing the expression differently from lazy(). I would like to understand why they are behaving differently, and how to use lazy() to get the same functionality as lazy_dots().

g <- function(...){
    lazy_dots(...)
}

h <-  function(x){
    lazy(x)
}

g(lm)[[1]]
# <lazy>
#   expr: lm
#   env:  <environment: R_GlobalEnv>
h(lm)
# <lazy>
#   expr: function (formula, data, subset, weights, na.action, method = "qr",  ...
#   env:  <environment: R_GlobalEnv> 

Even changing .follow__symbols to FALSE for lazy() so that it is the same as lazy_dots() does not work.

lazy
# function (expr, env = parent.frame(), .follow_symbols = TRUE) 
# {
#     .Call(make_lazy, quote(expr), environment(), .follow_symbols)
# }
# <environment: namespace:lazyeval>

lazy_dots
# function (..., .follow_symbols = FALSE) 
# {
#     if (nargs() == 0) 
#         return(structure(list(), class = "lazy_dots"))
#     .Call(make_lazy_dots, environment(), .follow_symbols)
# }
# <environment: namespace:lazyeval>


h2 <-  function(x){
    lazy(x, .follow_symbols=FALSE)
}

h2(lm)
# <lazy>
#  expr: x
#  env:  <environment: 0xe4a42a8>

I just feel really kind of stuck as to what to do.

Answer 1

One option may be to make write select_happy almost the same way as the standard select function:

select_happy<- function(df, ...){
  select_(df, .dots = setNames(lazy_dots(...), "happy"))
}

f <- function(){
  print('foo')
}

> select_happy(df, a)
  happy
1     1
2     2
3     3
> 
> select_happy(df, f)
  happy
1     4
2     5
3     6
> 
> select_happy(df, lm)
  happy
1     7
2     8
3     9

Note that the function definition of the standard select function is:

> select
function (.data, ...) 
{
    select_(.data, .dots = lazyeval::lazy_dots(...))
}
<environment: namespace:dplyr>

Also note that by this definition, select_happy accepts multiple columns to be selected, but will name any additional columns "NA":

> select_happy(df, lm, a)
  happy NA
1     7  1
2     8  2
3     9  3

Of course you could make some modifications for such cases, for example:

select_happy<- function(df, ...){
  dots <- lazy_dots(...)
  n <- length(dots)
  if(n == 1) newnames <- "happy" else newnames <- paste0("happy", seq_len(n))
  select_(df, .dots = setNames(dots, newnames))
}

> select_happy(df, f)
  happy
1     4
2     5
3     6

> select_happy(df, lm, a)
  happy1 happy2
1      7      1
2      8      2
3      9      3