# Do it With Quasi-Quotation

This post is one of a series of posts that I am doing about expressions, quasi-quotation, and tidy evaluation. This post shows some useful things you can actually do with these tools without attempting to explain what’s going on. The companion post provides a way of thinking about quasi-quotation through an analogy to a meta-recipe for cake.

So you’ve already made an initial attempt to learn quasi-quotation/tidy evaluation, and the hill to understanding these tools still looks high. You are asking “why bother?”, and tempted to just stick with what you know until forced, kicking-and-screaming to do it the “new way.” In the following I will try to provide some quick wins through exploration of a few different use cases of quasi-quotation.

## 1) Recode LOTS of values, with Quasi-Quotation

My first practical use of these tools was to apply a large number of replacements with dplyr::recode. If you are only replacing a couple of values, it is easy to type the replacements into the recode call:

# Recode values with named arguments
x <- sample(c("a", "b", "c"), 10, replace = TRUE)
dplyr::recode(x, a = "Apple")
##  [1] "c"     "c"     "Apple" "b"     "Apple" "c"     "c"     "Apple"
##  [9] "c"     "c"

But wait – do you really have to manually type in EVERY value that you want to replace?!?! Nope. You need to create a named list, and then use the rlang::UQS()/!!! operator (pronounced ‘bang bang bang’ or unquote-splice).

fruits <- list("Apple", "Banana", "Carrot")
fruit_lookup <- set_names(fruits, c("a", "b", "c"))

recode(x, !!!fruit_lookup)
##  [1] "Carrot" "Carrot" "Apple"  "Banana" "Apple"  "Carrot" "Carrot"
##  [8] "Apple"  "Carrot" "Carrot"

It needs to be a named list (not a character vector) because UQS()/!!! just works on lists. For the purpose of the example I wrote out the lookup, but its easy to make it from a dataframe, for example like:

fruit_df <- tribble(
~letter, ~fruit,
"a", "Apple",
"b", "Banana",
"c", "Carrot"
)

fruit_lookup2 <- set_names(fruit_df$fruit, fruit_df$letter) %>% as.list()
recode(x, !!!fruit_lookup2)
##  [1] "Carrot" "Carrot" "Apple"  "Banana" "Apple"  "Carrot" "Carrot"
##  [8] "Apple"  "Carrot" "Carrot"

Many tidyverse functions that take dots can utilize !!! in this way.

## 2) Don’t Repeat Yourself, with Quasi-Quotation

In the course of a data analysis script, I frequently find myself doing repeated group-by operations by the same or a similar set of variables several times. If I add a variable to the main dataset early on, I then have to update the list of group-by variables in many places.

suppressPackageStartupMessages(library("tidyverse"))

mpg2 <- mpg %>%
group_by(class, manufacturer, model) %>%
mutate(max_cty = max(cty),
max_hwy = max(hwy)) %>%
ungroup()

# ....

mpg3 <- mpg2 %>%
group_by(class, manufacturer, model) %>%
mutate(mpg_ratio = hwy / cty) %>%
ungroup()

# ...

mpg4 <- mpg3 %>%
group_by(class, manufacturer, model) %>%
# some other calculation
ungroup()

You know when you repeat something three times your supposed to write a function. But you haven’t totally figured out how to program with dplyr yet, and in any case its not obvious that you really have a function worth factoring out. Instead of writing a function, you can instead replace just the repeated set of group_by variables.

suppressPackageStartupMessages(library("rlang"))

group_vars <- syms(c("class", "manufacturer", "model"))

mpg2 <- mpg %>%
group_by(!!!group_vars) %>%
mutate(max_cty = max(cty),
max_hwy = max(hwy)) %>%
ungroup()

# ....

mpg3 <- mpg2 %>%
group_by(!!!group_vars) %>%
mutate(mpg_ratio = hwy / cty) %>%
ungroup()

# ...

mpg4 <- mpg3 %>%
group_by(!!!group_vars) %>%
# some other calculation
ungroup()

Now if you need to change the grouping variables, you only have to do it in one place. Also, this is a big step towards writing that function.

## 3) Switch the Analysis Variables in a Shiny App, with Quasi-Quotation

Shiny apps used for exploratory plotting commonly want to take the name of variables from the user, and be able to make plots cut by that inputs. Shiny inputs will give a string, which can be turned into a symbol with sym and then used in your dplyr analysis with !!.

# app.R - a simple one-file app
library("tidyverse")

ui <- bootstrapPage(
selectInput("group_var", "Group variable", choices = c("manufacturer", "model", "displ", "class") ),
plotOutput('plot')
)

# Define the server code
server <- function(input, output) {

output$plot <- renderPlot({ group_var <- input$group_var %>% sym()

df <- mpg %>%
group_by(!!group_var) %>%
summarize(cty = mean(cty),
hwy = mean(hwy))

ggplot(df, aes_string(x = "cty", y = "hwy", label = input$group_var)) + geom_text() }) } # Return a Shiny app object shinyApp(ui = ui, server = server) Right now you need to use !! for dplyr/tidyr whereas ggplot2 uses aes_ and aes_string. My understanding is that ggplot2 will also be switchinig over to the rlang quasi-quotation system at some point as well. As a caution, anytime you are turning shiny user input into code which is executed (such as here with unquoting), it is probably a good idea to do some input checking to make sure what you have received is within allowed values. Don’t let this be you. ## 4) Reprex Higher-Order Functions, With Quasi-Quotation It can be confusing to debug functions like map and lapply that take other functions as arguments (functions that operate on functions are called higher-order functions). Does the error have to do with the way the map call is being constructed, or does it have to do with the function you are passing in? Quasi-quotation makes it easy to construct dummy functions that help see the calls resulting from map or apply functions. Say you are trying to run a suite of different models on your data using different subsets of data (and all combinations of the two). But your code isn’t doing what you want: models <- c("JB2", "JBB", "CL") #list of models to apply data_grps <- c("A", "B", "C") result_data <- map2_dfr(models, data_grps, ~mynewmodel(.x, .y)) The instinct of every frustrated programmer is to post on Stackoverflow. But how can you make a reprex (reproducible example) given that you don’t want to share your proprietary data or modeling function? Quasi-quotation can be handy for isolating parts of your program by simulating other bits. Here we will simulate mynewmodel with a function that stores its parameters in a one-row dataframe. mynewmodel <- function(model, grp) { thecall <- expr(mynewmodel(UQ(model), UQ(grp))) df <- tibble( model = model, grp = grp, call = list(thecall) ) invisible(df) } models <- c("JB2", "JBB", "CL") #list of models to apply data_grps <- c("A", "B", "C") result_data <- map2_dfr(models, data_grps, ~mynewmodel(.x, .y)) result_data$call
## [[1]]
## mynewmodel("JB2", "A")
##
## [[2]]
## mynewmodel("JBB", "B")
##
## [[3]]
## mynewmodel("CL", "C")

This is now a full reprex, and the resulting dataframe gives a lot of information about what is going on so you can try different map variations without the possibility that the problem is something to do with your modeling function implementation.

## 5) Write New Higher-Order Functions (!?!), With Quasi-Quotation

Quasi-quotation tools have enticing possibilities for writing custom higher-order functions because they are all about manipulating expressions and code. I want to admit that this section is more ideas than practical applications, but I thought it would be interesting to see how you can use rlang functions to manipulate expressions in a way similar to purrr::map.

Let’s write a basic version of the map function that evaluates an expression. When I tried to read the source code for purrr::map I got pretty intimidated, but the pattern that map emulates – a loop of function calls – is possible to implement in a simple manner with rlang tools.

library(rlang)

expr_map <- function(.x, .expr, ...) {
out <- list()

for (i in seq_along(.x)) {
out[i] <- eval_tidy(.expr, data = dots_list(..., .x = .x[[i]]) )
}

out
}

Instead of an anonymous function (as is often used with map) we pass in an expression. We will iterate through a for loop for each element of .x and evaluate the expression. Really? We can write a version of map in 4 lines of code? There are many reasons why the real one is better, but this one really does accomplish the core pattern, but using expressions instead of functions. Like the real map, you can use .x as a pronoun

expr_map(c(1, 2, 3), expr(.x^2))
## [[1]]
## [1] 1
##
## [[2]]
## [1] 4
##
## [[3]]
## [1] 9

We can also refer to variables in the environment…

y=5

expr_map(c(1,2,3), expr(.x^2 + y))
## [[1]]
## [1] 6
##
## [[2]]
## [1] 9
##
## [[3]]
## [1] 14

And add variables to the overscope with dots, which will be seen ahead of the environment.

y=5

expr_map(c(1, 2, 3), expr(.x^2 + y), y=-1)
## [[1]]
## [1] 0
##
## [[2]]
## [1] 3
##
## [[3]]
## [1] 8

The possibilities are sort of endless. For example, it has always annoyed me that when you use a dataframe with pmap, you need to manually match up the columns with ..1, ..2, etc.

mydf <- tibble(
name = c("John", "Jay", "James"),
hgt = c(72, 68, 81),
wgt = c(171, 140, 190)
)

pmap(mydf, ~paste0("The BMI of ", ..1, " is ", round(..3^2 / ..2, 1) ))
## [[1]]
## [1] "The BMI of John is 406.1"
##
## [[2]]
## [1] "The BMI of Jay is 288.2"
##
## [[3]]
## [1] "The BMI of James is 445.7"

Our custom version will allow referencing the column names directly in the anonymous expression.

expr_pmap <- function(.data, .expr) {
out <- list()

for (i in 1:nrow(.data)) {
out[i] <- eval_tidy(.expr, data = .data[i, ] )
}

out
}

expr_pmap(mydf, expr(paste0("The BMI of ", name, " is ", round(wgt^2 / hgt, 2))) )
## [[1]]
## [1] "The BMI of John is 406.12"
##
## [[2]]
## [1] "The BMI of Jay is 288.24"
##
## [[3]]
## [1] "The BMI of James is 445.68"

This implementation is very slow and limited, but I think it is compelling to see how quickly we can write different types of higher-order functions akin to map functions to manipulate expressions instead of functions. I can’t tell yet in what ways higher-order functions based on expressions or quosures might have different capabilities than the ones we are used to based on functions.