Evaluation in programming

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# Evaluation in programming

## `tidyeval`

.center[<img src="https://rlang.r-lib.org/reference/figures/rlang.png" width="100px"/>]

### A. Ginolhac | rworkshop | 2021-09-10

## 🪄 .bold[Advanced topic] <svg aria-hidden="true" role="img" viewBox="0 0 512 512" style="height:35px;width:35px;vertical-align:-0.125em;margin-left:auto;margin-right:auto;font-size:inherit;fill:currentColor;overflow:visible;position:relative;"><path d="M224 96l16-32 32-16-32-16-16-32-16 32-32 16 32 16 16 32zM80 160l26.66-53.33L160 80l-53.34-26.67L80 0 53.34 53.33 0 80l53.34 26.67L80 160zm352 128l-26.66 53.33L352 368l53.34 26.67L432 448l26.66-53.33L512 368l-53.34-26.67L432 288zm70.62-193.77L417.77 9.38C411.53 3.12 403.34 0 395.15 0c-8.19 0-16.38 3.12-22.63 9.38L9.38 372.52c-12.5 12.5-12.5 32.76 0 45.25l84.85 84.85c6.25 6.25 14.44 9.37 22.62 9.37 8.19 0 16.38-3.12 22.63-9.37l363.14-363.15c12.5-12.48 12.5-32.75 0-45.24zM359.45 203.46l-50.91-50.91 86.6-86.6 50.91 50.91-86.6 86.6z"/></svg>

---

# Learning objectives

.flex[
  .w-60.bg-washed-green.b--green.ba.bw2.br3.shadow-5.ph3.mt3.mr1.ml6[
  .large[.gbox[You will learn to:]

.right[<img src="img/00/kt.png" class="float-img"/>]

- Understand how quoted functions work
- Why we need **tidyeval**
- How to apply it for your programming
- Embrace the curly-curly operator and forget the above points

]]]

---
 
# Environments and promises

.pull-left[

### Functions enclose their variables

```r
x <- 1
plus_one <- function(x) {
  x <- x + 1
  x
}
plus_one(x)
```

```
[1] 2
```

```r
plus_one(x)
```

```
[1] 2
```

.Large[The `x` object in .bold[Global.Env] wasn't modified]

]

.pull-right[

### Separate assignment and evaluation

```r
msg <- "old"
delayedAssign("promise", msg)
msg <- "new!"
promise # new!
```

```
[1] "new!"
```

#### The <u>promise</u> was created when `msg` was .green[`"old"`]

---

# Standard vs Non-standard evaluation

.pull-left[

#### <svg aria-hidden="true" role="img" viewBox="0 0 581 512" style="height:20px;width:22.7px;vertical-align:-0.125em;margin-left:auto;margin-right:auto;font-size:inherit;fill:steelblue;overflow:visible;position:relative;"><path d="M581 226.6C581 119.1 450.9 32 290.5 32S0 119.1 0 226.6C0 322.4 103.3 402 239.4 418.1V480h99.1v-61.5c24.3-2.7 47.6-7.4 69.4-13.9L448 480h112l-67.4-113.7c54.5-35.4 88.4-84.9 88.4-139.7zm-466.8 14.5c0-73.5 98.9-133 220.8-133s211.9 40.7 211.9 133c0 50.1-26.5 85-70.3 106.4-2.4-1.6-4.7-2.9-6.4-3.7-10.2-5.2-27.8-10.5-27.8-10.5s86.6-6.4 86.6-92.7-90.6-87.9-90.6-87.9h-199V361c-74.1-21.5-125.2-67.1-125.2-119.9zm225.1 38.3v-55.6c57.8 0 87.8-6.8 87.8 27.3 0 36.5-38.2 28.3-87.8 28.3zm-.9 72.5H365c10.8 0 18.9 11.7 24 19.2-16.1 1.9-33 2.8-50.6 2.9v-22.1z"/></svg> .large[base], must refers to known objects

- But quoting (as no evaluation) is used for axis labels

```r
plot(swiss$Education, swiss$Examination)
```

<img src="lecture11_tidyeval_files/figure-html/unnamed-chunk-3-1.png" width="360" />
]

.pull-right[
#### `ggplot2` or widely in the tidyverse

- Evaluating columns in **data** context.

```r
ggplot(swiss, aes(x = Education, y = Examination)) +
  geom_point()
```

<img src="lecture11_tidyeval_files/figure-html/unnamed-chunk-4-1.png" width="360" />
]

---

# Quoting

.pull-left[
.rbox[<svg aria-hidden="true" role="img" viewBox="0 0 512 512" style="height:20px;width:20px;vertical-align:-0.125em;margin-left:auto;margin-right:auto;font-size:inherit;fill:currentColor;overflow:visible;position:relative;"><path d="M504 256c0 136.997-111.043 248-248 248S8 392.997 8 256C8 119.083 119.043 8 256 8s248 111.083 248 248zm-248 50c-25.405 0-46 20.595-46 46s20.595 46 46 46 46-20.595 46-46-20.595-46-46-46zm-43.673-165.346l7.418 136c.347 6.364 5.609 11.346 11.982 11.346h48.546c6.373 0 11.635-4.982 11.982-11.346l7.418-136c.375-6.874-5.098-12.654-11.982-12.654h-63.383c-6.884 0-12.356 5.78-11.981 12.654z"/></svg> Does not mean adding quotes!]

#### But capture an expression

```r
quote(Education)
```

```
Education
```

```r
quote(swiss$Education)
```

```
swiss$Education
```

#### Without quoting, we cannot evaluate

```r
eval(Education, envir = swiss)
```

```
Error in eval(Education, envir = swiss): object 'Education' not found
```

`envir` tells where to find the vector `Education`

]

.pull-right[
.bbox[Quoting allows to evaluate when needed]

```r
eval(quote(Education), envir = swiss)
```

```
 [1] 12  9  5  7 15  7  7  8  7 13  6 12  7 12  5  2  8 28 20  9 10  3 12  6  1
[26]  8  3 10 19  8  2  6  2  6  3  9  3 13 12 11 13 32  7  7 53 29 29
```

]

---
class: center, middle

### Non-standard evaluation (NSE)

### Unquoted variable names

.footnote[[technically WRONG, but useful approximation. - .bold[Jenny Bryan], rstudio::conf2019](https://www.youtube.com/watch?v=2BXPLnLMTYo)]

---

# Quotation also works for expression

.pull-left[

### Exist in <svg aria-hidden="true" role="img" viewBox="0 0 581 512" style="height:40px;width:45.39px;vertical-align:-0.125em;margin-left:auto;margin-right:auto;font-size:inherit;fill:steelblue;overflow:visible;position:relative;"><path d="M581 226.6C581 119.1 450.9 32 290.5 32S0 119.1 0 226.6C0 322.4 103.3 402 239.4 418.1V480h99.1v-61.5c24.3-2.7 47.6-7.4 69.4-13.9L448 480h112l-67.4-113.7c54.5-35.4 88.4-84.9 88.4-139.7zm-466.8 14.5c0-73.5 98.9-133 220.8-133s211.9 40.7 211.9 133c0 50.1-26.5 85-70.3 106.4-2.4-1.6-4.7-2.9-6.4-3.7-10.2-5.2-27.8-10.5-27.8-10.5s86.6-6.4 86.6-92.7-90.6-87.9-90.6-87.9h-199V361c-74.1-21.5-125.2-67.1-125.2-119.9zm225.1 38.3v-55.6c57.8 0 87.8-6.8 87.8 27.3 0 36.5-38.2 28.3-87.8 28.3zm-.9 72.5H365c10.8 0 18.9 11.7 24 19.2-16.1 1.9-33 2.8-50.6 2.9v-22.1z"/></svg> .bold[base]

```r
subset(swiss, (Education + Examination) > 60)[, 2:4]
```

```
             Agriculture Examination Education
Neuchatel           17.6          35        32
V. De Geneve         1.2          37        53
```
]

.pull-right[

### `dplyr`

```r
filter(swiss, (Education + Examination) > 60) %>%
  select(2:4)
```

```
             Agriculture Examination Education
Neuchatel           17.6          35        32
V. De Geneve         1.2          37        53
```
]

---

# Evaluating expression

.flex[
.w-70.bg-washed-yellow.b--gold.ba.bw2.br3.shadow-5.ph3.mt3.mr1[
.large.gbox[Standard evaluation of an expression]
  
.large[To evaluate an .red[expression], 
you search .red[environments] for name bindings-values and perform the evaluation immediately.
]]

.w-70.bg-washed-yellow.b--gold.ba.bw2.br3.shadow-5.ph3.mt3.mr1[
.large[   
.bbox[Non-standard evaluation]
 means you might

* Modify the .red[expression] or 
 * Modify the chain of searched .red[environments]

before evaluation. 
]
]
]
--
.flex[
.w-70.bg-washed-yellow.b--gold.ba.bw2.br3.shadow-5.ph3.mt3.ml6[
.center.Large[Why `swiss` is found while being absent in `Global_Env`?]
]]
.footnote[[Jenny Bryan, rstudio::conf2019](https://www.youtube.com/watch?v=2BXPLnLMTYo)]

---
class: nvs1

# Values in environments are searched in a precise order

.pull-left[
![](https://d33wubrfki0l68.cloudfront.net/038b2da4f5db1d2a8acaf4ee1e7d08d04ab36ebc/ac22a/diagrams/environments/search-path.png)

```r
# callr allows to run a clean R session
callr::r(function() rlang::search_envs())
```

```
 [[1]] $ <env: global>
 [[2]] $ <env: package:stats>
 [[3]] $ <env: package:graphics>
 [[4]] $ <env: package:grDevices>
 [[5]] $ <env: package:utils>
 [[6]] $ <env: package:datasets>
 [[7]] $ <env: package:methods>
 [[8]] $ <env: Autoloads>
 [[9]] $ <env: tools:callr>
[[10]] $ <env: package:base>
```
- `swiss` lies in `datasets`, so found after 5 fails
]

.pull-right[
![](https://d33wubrfki0l68.cloudfront.net/7c87a5711e92f0269cead3e59fc1e1e45f3667e9/0290f/diagrams/environments/search-path-2.png)

```r
callr::r(function() {
* library(forcats)
  rlang::search_envs()
})
```

```
 [[1]] $ <env: global>
 [[2]] $ <env: package:forcats>
 [[3]] $ <env: package:stats>
 [[4]] $ <env: package:graphics>
 [[5]] $ <env: package:grDevices>
 [[6]] $ <env: package:utils>
 [[7]] $ <env: package:datasets>
 [[8]] $ <env: package:methods>
 [[9]] $ <env: Autoloads>
[[10]] $ <env: tools:callr>
[[11]] $ <env: package:base>
```
]

.footnote[[Advanced R](https://adv-r.hadley.nz/environments.html) by Hadley Wickham]

---

# Quoting prevents evaluation, nice, but how does it work?

.pull-left[
.large[
`Education` is unknown in the .bold[Global Environment]

```r
Education
```

```
Error in eval(expr, envir, enclos): object 'Education' not found
```

Quoting .bold[prevents] evaluation
]

```r
quote(Education)
```

```
Education
```
.large[But how to .bold[force] evaluation then?]

]

.pull-right[
### <svg aria-hidden="true" role="img" viewBox="0 0 581 512" style="height:30px;width:34.04px;vertical-align:-0.125em;margin-left:auto;margin-right:auto;font-size:inherit;fill:steelblue;overflow:visible;position:relative;"><path d="M581 226.6C581 119.1 450.9 32 290.5 32S0 119.1 0 226.6C0 322.4 103.3 402 239.4 418.1V480h99.1v-61.5c24.3-2.7 47.6-7.4 69.4-13.9L448 480h112l-67.4-113.7c54.5-35.4 88.4-84.9 88.4-139.7zm-466.8 14.5c0-73.5 98.9-133 220.8-133s211.9 40.7 211.9 133c0 50.1-26.5 85-70.3 106.4-2.4-1.6-4.7-2.9-6.4-3.7-10.2-5.2-27.8-10.5-27.8-10.5s86.6-6.4 86.6-92.7-90.6-87.9-90.6-87.9h-199V361c-74.1-21.5-125.2-67.1-125.2-119.9zm225.1 38.3v-55.6c57.8 0 87.8-6.8 87.8 27.3 0 36.5-38.2 28.3-87.8 28.3zm-.9 72.5H365c10.8 0 18.9 11.7 24 19.2-16.1 1.9-33 2.8-50.6 2.9v-22.1z"/></svg> uses `eval()` in _data_ context with `envir`

```r
Education <- "tic"
# the Education in the GlobalEnv won't clash 
eval(expr = quote(Education), envir = swiss)
```

```
 [1] 12  9  5  7 15  7  7  8  7 13  6 12  7 12  5  2  8 28 20  9 10  3 12  6  1
[26]  8  3 10 19  8  2  6  2  6  3  9  3 13 12 11 13 32  7  7 53 29 29
```
]

---

# Evaluation in the tidyverse

.pull-left[
.large[
Works even if an object name is colliding with the .bold[Global Environment]
- `Education` is evaluated in the `swiss` context .bold[only]
]

```r
Education <- 20
filter(swiss, Education > 40)
```

```
             Fertility Agriculture Examination Education Catholic
V. De Geneve        35         1.2          37        53    42.34
             Infant.Mortality
V. De Geneve               18
```
]

.pull-right[

.large[
- `.data` and `.env` pronouns exist if you need to precise .bold[who] is .bold[where]

```r
filter(swiss, .data$Education > .env$Education)
```

```
             Fertility Agriculture Examination Education Catholic
Lausanne          55.7        19.4          26        28    12.11
Neuchatel         64.4        17.6          35        32    16.92
V. De Geneve      35.0         1.2          37        53    42.34
Rive Droite       44.7        46.6          16        29    50.43
Rive Gauche       42.8        27.7          22        29    58.33
             Infant.Mortality
Lausanne                 20.2
Neuchatel                23.0
V. De Geneve             18.0
Rive Droite              18.2
Rive Gauche              19.3
```
]
#### But you better avoid names collision
]

---

# Quotations of expressions

.pull-left[

### Doing it by hand

```r
expr <- quote((Education + Examination) > 60)
expr
```

```
(Education + Examination) > 60
```

```r
swiss[eval(expr, envir = swiss), 1:2]
```

```
             Fertility Agriculture
Neuchatel         64.4        17.6
V. De Geneve      35.0         1.2
```
]

.pull-right[

### In a function it .red[fail!] <svg aria-hidden="true" role="img" viewBox="0 0 512 512" style="height:1em;width:1em;vertical-align:-0.125em;margin-left:auto;margin-right:auto;font-size:inherit;fill:currentColor;overflow:visible;position:relative;"><path d="M440.5 88.5l-52 52L415 167c9.4 9.4 9.4 24.6 0 33.9l-17.4 17.4c11.8 26.1 18.4 55.1 18.4 85.6 0 114.9-93.1 208-208 208S0 418.9 0 304 93.1 96 208 96c30.5 0 59.5 6.6 85.6 18.4L311 97c9.4-9.4 24.6-9.4 33.9 0l26.5 26.5 52-52 17.1 17zM500 60h-24c-6.6 0-12 5.4-12 12s5.4 12 12 12h24c6.6 0 12-5.4 12-12s-5.4-12-12-12zM440 0c-6.6 0-12 5.4-12 12v24c0 6.6 5.4 12 12 12s12-5.4 12-12V12c0-6.6-5.4-12-12-12zm33.9 55l17-17c4.7-4.7 4.7-12.3 0-17-4.7-4.7-12.3-4.7-17 0l-17 17c-4.7 4.7-4.7 12.3 0 17 4.8 4.7 12.4 4.7 17 0zm-67.8 0c4.7 4.7 12.3 4.7 17 0 4.7-4.7 4.7-12.3 0-17l-17-17c-4.7-4.7-12.3-4.7-17 0-4.7 4.7-4.7 12.3 0 17l17 17zm67.8 34c-4.7-4.7-12.3-4.7-17 0-4.7 4.7-4.7 12.3 0 17l17 17c4.7 4.7 12.3 4.7 17 0 4.7-4.7 4.7-12.3 0-17l-17-17zM112 272c0-35.3 28.7-64 64-64 8.8 0 16-7.2 16-16s-7.2-16-16-16c-52.9 0-96 43.1-96 96 0 8.8 7.2 16 16 16s16-7.2 16-16z"/></svg>

```r
quoted_filter <- function(data, expr) {
  q_expr <- quote(expr)
  data[eval(q_expr, envir = data), 1:2]
}
quoted_filter(swiss, (Education + Examination) > 60)
```

```
Error in eval(q_expr, envir = data): object 'Examination' not found
```
]

---

# Expressions in functions

#### `substitute()` is needed

.pull-left[

```r
quoted_filter_sub <- function(data, expr) {
  q_expr <- substitute(expr)
  # returns both the results and substituted expression
  list(data[eval(q_expr, envir = data), 1:2],
       q_expr
  )
}
quoted_filter_sub(swiss, (Education + Examination) > 60)
```

```
[[1]]
             Fertility Agriculture
Neuchatel         64.4        17.6
V. De Geneve      35.0         1.2

[[2]]
(Education + Examination) > 60
```
]

.pull-right[
### _Substituting and Quoting Expressions_ help page

.large[
> `substitute` returns the parse tree for the (unevaluated) expression expr, substituting any variables bound in env. `quote` simply returns its argument. The argument is not evaluated and can be any R expression.

- Complex but works, so .bold[why] do we need `tidyeval`?
]
]

---

# Why do we need `tidyeval`, _i. e_ quasiquotation?

.pull-left[

### Add one variable in GlobalEnv

```r
threshold <- 60
quoted_filter_sub(swiss, (Education + Examination) > threshold)
```

```
[[1]]
             Fertility Agriculture
Neuchatel         64.4        17.6
V. De Geneve      35.0         1.2

[[2]]
(Education + Examination) > threshold
```

]

.pull-right[

### .red[Error], names clash <svg aria-hidden="true" role="img" viewBox="0 0 512 512" style="height:30px;width:30px;vertical-align:-0.125em;margin-left:auto;margin-right:auto;font-size:inherit;fill:red;overflow:visible;position:relative;"><path d="M440.5 88.5l-52 52L415 167c9.4 9.4 9.4 24.6 0 33.9l-17.4 17.4c11.8 26.1 18.4 55.1 18.4 85.6 0 114.9-93.1 208-208 208S0 418.9 0 304 93.1 96 208 96c30.5 0 59.5 6.6 85.6 18.4L311 97c9.4-9.4 24.6-9.4 33.9 0l26.5 26.5 52-52 17.1 17zM500 60h-24c-6.6 0-12 5.4-12 12s5.4 12 12 12h24c6.6 0 12-5.4 12-12s-5.4-12-12-12zM440 0c-6.6 0-12 5.4-12 12v24c0 6.6 5.4 12 12 12s12-5.4 12-12V12c0-6.6-5.4-12-12-12zm33.9 55l17-17c4.7-4.7 4.7-12.3 0-17-4.7-4.7-12.3-4.7-17 0l-17 17c-4.7 4.7-4.7 12.3 0 17 4.8 4.7 12.4 4.7 17 0zm-67.8 0c4.7 4.7 12.3 4.7 17 0 4.7-4.7 4.7-12.3 0-17l-17-17c-4.7-4.7-12.3-4.7-17 0-4.7 4.7-4.7 12.3 0 17l17 17zm67.8 34c-4.7-4.7-12.3-4.7-17 0-4.7 4.7-4.7 12.3 0 17l17 17c4.7 4.7 12.3 4.7 17 0 4.7-4.7 4.7-12.3 0-17l-17-17zM112 272c0-35.3 28.7-64 64-64 8.8 0 16-7.2 16-16s-7.2-16-16-16c-52.9 0-96 43.1-96 96 0 8.8 7.2 16 16 16s16-7.2 16-16z"/></svg>
.Large[
`Fertility` is .bold[also] a data column
]

```r
Fertility <- 60
quoted_filter_sub(swiss, (Education + Examination) > Fertility)
```

```
[[1]]
             Fertility Agriculture
Neuchatel         64.4        17.6
V. De Geneve      35.0         1.2
Rive Droite       44.7        46.6
Rive Gauche       42.8        27.7

[[2]]
(Education + Examination) > Fertility
```
]

---

# Quasiquotation, bang bang ‼️ operator

.large[
- When we need to unquote **part** of the expression: .green.large[!!]
- `rlang::qq_show()` helper to check
]

.flex[
.w-60.b--blue.ba.bw2.br3.shadow-5.ph4.mt5.ml6[
.bbox[Demonstration]

```r
quo((Education + Examination) > Fertility)
```

```
<quosure>
expr: ^(Education + Examination) > Fertility
env:  0x5641f6598958
```

```r
rlang::qq_show(quo((Education + Examination) > !! Fertility))
```

```
quo((Education + Examination) > 60)
```
]
]
---

# The bang bang operator `!!`

.pull-left[
### In `dplyr`, unquote only `Fertility`

- Keep the rest of the expression as before

```r
filter(swiss, (Education + Examination) > !! Fertility)
```

```
             Fertility Agriculture Examination Education Catholic
Neuchatel         64.4        17.6          35        32    16.92
V. De Geneve      35.0         1.2          37        53    42.34
             Infant.Mortality
Neuchatel                  23
V. De Geneve               18
```
]

.pull-right[

### .green[New], the `curly-curly` operator

```r
filter(swiss, (Education + Examination) > {{Fertility}})
```

---
class: nvs2, hide_logo

.flex[
.w-50.mr1[
<blockquote class="twitter-tweet"><p lang="en" dir="ltr">New in-depth blogpost on NSE with <a href="https://twitter.com/hashtag/rlang?src=hash&amp;ref_src=twsrc%5Etfw">#rlang</a>&#39;s quosures:<a href="https://t.co/yJQ81IdhYp">https://t.co/yJQ81IdhYp</a>Jump down the rabbit hole with me to see how they work, and of course, how to implement them in base <a href="https://twitter.com/hashtag/rstats?src=hash&amp;ref_src=twsrc%5Etfw">#rstats</a>. <a href="https://t.co/wo4GebqONt">pic.twitter.com/wo4GebqONt</a></p>&mdash; BrodieG (@BrodieGaslam) <a href="https://twitter.com/BrodieGaslam/status/1293166631741521922?ref_src=twsrc%5Etfw">August 11, 2020</a></blockquote> <script async src="https://platform.twitter.com/widgets.js" charset="utf-8"></script>

]

.w-50.bg-washed-blue.b--blue.ba.bw2.br3.shadow-5.ph3.ml1[
.large[.ybox[`rlang` is providing the toolkit]

- Really advanced
- Most the time, you don't need it
- Already exposed in main tidyverse packages
- Animation done in <svg aria-hidden="true" role="img" viewBox="0 0 581 512" style="height:20px;width:22.7px;vertical-align:-0.125em;margin-left:auto;margin-right:auto;font-size:inherit;fill:steelblue;overflow:visible;position:relative;"><path d="M581 226.6C581 119.1 450.9 32 290.5 32S0 119.1 0 226.6C0 322.4 103.3 402 239.4 418.1V480h99.1v-61.5c24.3-2.7 47.6-7.4 69.4-13.9L448 480h112l-67.4-113.7c54.5-35.4 88.4-84.9 88.4-139.7zm-466.8 14.5c0-73.5 98.9-133 220.8-133s211.9 40.7 211.9 133c0 50.1-26.5 85-70.3 106.4-2.4-1.6-4.7-2.9-6.4-3.7-10.2-5.2-27.8-10.5-27.8-10.5s86.6-6.4 86.6-92.7-90.6-87.9-90.6-87.9h-199V361c-74.1-21.5-125.2-67.1-125.2-119.9zm225.1 38.3v-55.6c57.8 0 87.8-6.8 87.8 27.3 0 36.5-38.2 28.3-87.8 28.3zm-.9 72.5H365c10.8 0 18.9 11.7 24 19.2-16.1 1.9-33 2.8-50.6 2.9v-22.1z"/></svg> with [`rayshader`](https://github.com/tylermorganwall/rayshader) by .bold[Brodie Gaslam]
]]
]

---

# `tidyeval`, what to remember
 
.pull-left[

### Use unquoted names in your own functions

- `Fertility` as a .bold[promise] (unknown in **Global Env**)
- Evaluation is delayed by `enquo()` 
- User decide the evaluation with `!!`
- Both can be abstracted with `{{}}`

```r
select_head <- function(.data, column, n = 5) {
  .data %>%
    select({{column}}) %>% 
    slice_head(n = n)
}
select_head(swiss, column = Fertility, n = 2)
```

```
           Fertility
Courtelary      80.2
Delemont        83.1
```
]

.pull-right[

```r
select_head(swiss, column = Fertility, n = 2)
```

```
           Fertility
Courtelary      80.2
Delemont        83.1
```

```r
select_head(swiss, column = c(Fertility, Catholic), n = 2)
```

```
           Fertility Catholic
Courtelary      80.2     9.96
Delemont        83.1    84.84
```

```r
select_head(swiss, column = Fertility:Examination, n = 3)
```

```
             Fertility Agriculture Examination
Courtelary        80.2        17.0          15
Delemont          83.1        45.1           6
Franches-Mnt      92.5        39.7           5
```

]

.center.Large[All you need to remember, .bold[unquoted] arguments in functions: use `{{arg}}`]

---

# New column name, the operator `:=`

.large[
- To turn a **quosure** into a **name** that could be pasted
- Must uses a specific assignment `:=` (.bold[walrus], from `data.table`)
]
.pull-left[

```r
my_summarise <- function(.data, out_name, expr) {

summarise(.data, 
    {{out_name}} := mean({{expr}})
  )
}
```
]

.pull-right[

```r
my_summarise(swiss, m_fer, Fertility)
```

```
     m_fer
1 70.14255
```

```r
my_summarise(swiss, mean_exam, Examination)
```

```
  mean_exam
1  16.48936
```

]

---
class: hide_logo

# Before we stop

.flex[

.w-60.bg-washed-green.b--green.ba.bw2.br3.shadow-5.ph3.mt1.ml1[
.large[.gbox[You learned to:]]

- grasp non-standard evaluation
- use it with `tidyeval`
- Pass arguments as promises, not strings

> I can categorically say if you're pasting strings to program with dplyr, there is always better way.

.tr[[— _Hadley Wickham_](https://github.com/tidyverse/dplyr/issues/2663#issuecomment-294206288)]
]
.w-40.bg-washed-green.b--green.ba.bw2.br3.shadow-5.ph3.mt1.ml2[
.large[.bbox[Acknowledgments  🙏 👏]
* Lionel Henry
* Jenny Bryan
* Clause O. Wilke
* Hadley Wickham
]]]

.flex[
.w-60.bg-washed-green.b--green.ba.bw2.br3.shadow-5.ph3.mt2.ml1[
.large[.ybox[Further reading <svg aria-hidden="true" role="img" viewBox="0 0 448 512" style="height:1em;width:0.88em;vertical-align:-0.125em;margin-left:auto;margin-right:auto;font-size:inherit;fill:currentColor;overflow:visible;position:relative;"><path d="M448 360V24c0-13.3-10.7-24-24-24H96C43 0 0 43 0 96v320c0 53 43 96 96 96h328c13.3 0 24-10.7 24-24v-16c0-7.5-3.5-14.3-8.9-18.7-4.2-15.4-4.2-59.3 0-74.7 5.4-4.3 8.9-11.1 8.9-18.6zM128 134c0-3.3 2.7-6 6-6h212c3.3 0 6 2.7 6 6v20c0 3.3-2.7 6-6 6H134c-3.3 0-6-2.7-6-6v-20zm0 64c0-3.3 2.7-6 6-6h212c3.3 0 6 2.7 6 6v20c0 3.3-2.7 6-6 6H134c-3.3 0-6-2.7-6-6v-20zm253.4 250H96c-17.7 0-32-14.3-32-32 0-17.6 14.4-32 32-32h285.4c-1.9 17.1-1.9 46.9 0 64z"/></svg>]]

- [5min talk](https://www.youtube.com/watch?v=nERXS3ssntw) by Hadley Wickham
- [RStudio::conf 2019, Lazy evaluation](https://www.youtube.com/watch?v=2BXPLnLMTYo) by Jenny Bryan
- [1h Rstudio webinar](https://www.rstudio.com/resources/webinars/?wvideo=zt9kl921rh) by Lionel Henry
- [RStudio::conf 2017 slides](https://www.r-project.org/dsc/2017/slides/tidyeval-hygienic-fexprs.pdf) by Lionel Henry and Hadley Wickham
- [quasiquotation](https://adv-r.hadley.nz/quasiquotation.html) adv in R by Hadley Wickham
- [reduce, map to generate code, lm example](https://adv-r.hadley.nz/quasiquotation.html#map-reduce-to-generate-code) adv in R by Hadley Wickham
- [tidyeval ressources clustered](http://maraaverick.rbind.io/2017/08/tidyeval-resource-roundup/)  by Mara Averick
- [do it by quasiquotation](http://blog.jalsalam.com/posts/2017/quasi-quotation-applications/) by Jamel Alsalam
]
.w-40.pv2.ph3.mt1.ml1[
.huge[.bbox[Thank you for your attention!]]
]
]