Advanced R

# Advanced R
## Chapter 18: Expressions
### Maya Gans

---

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# A note on vocabulary

- Expression
- Parsing
- Deparsing
- Evaluating

```r
ggplot(x = "other_color")
```

### *Here's a stab:*

An **expression** is a list of calls, which can be created through **parsing** text. These expressions can become text again via **deparsing**

---

# Parsing vs Parser ...?

> Parsing is the process of analyzing text made of a sequence of tokens to determine its grammatical structure with respect to a given (more or less) formal grammar. The parser then builds a data structure based on the tokens.

```r
x2 <- parse(text = "2 * a")
class(x2)
```

```
## [1] "expression"
```
]

.footnote[
We can `parse` expressions, simulating how the *parser* creates expressions before compiling code? Is that right? 
]

---

# The Shiny App

![](shiny.png)

---

# Abstract Syntax Trees

```r
lobstr::ast(
  round(
    mean(beer_reviews$review_overall)
  , 2)
)
```

```
## o-round 
## +-o-mean 
## | \-o-`$` 
## |   +-beer_reviews 
## |   \-review_overall 
## \-2
```
]

```r
lobstr::ast(
if (x == TRUE) {
  y <- yes
} else {
  y <- no
}
)
```

```
## o-`if` 
## +-o-`==` 
## | +-x 
## | \-TRUE 
## +-o-`{` 
## | \-o-`<-` 
## |   +-y 
## |   \-yes 
## \-o-`{` 
##   \-o-`<-` 
##     +-y 
##     \-no
```
]

---

* Constants
* Symbols
* Calls

---

# Constants

```r
rlang::is_syntactic_literal("beer_review")
```

```
## [1] TRUE
```

---

# Symbols

Represent the name of an object. This is not the same as a string!

### Using a String

```r
beer_reviews %>%
  dplyr::filter(!!"review_overall" >= 5) %>%
  dplyr::pull(review_overall) %>%
  head()
```

```
## [1] 1.5 3.0 3.0 3.0 4.0 3.0
```
]

### Using a sym

```r
beer_reviews %>%
  dplyr::filter(!!sym("review_overall") >= 5) %>%
  dplyr::pull(review_overall) %>%
  head()
```

```
## [1] 5 5 5 5 5 5
```
]

---

# Calls

```r
x <- expr(dplyr::filter(beer_reviews, review_overall >= 5))

typeof(x)
```

```
## [1] "language"
```

```r
is.call(x)
```

```
## [1] TRUE
```
]

```r
as.list(x[-1])
```

```
## [[1]]
## beer_reviews
## 
## [[2]]
## review_overall >= 5
```
]

```r
ast(
  dplyr::filter(beer_reviews, review_overall >= 5)
)
```

```
## o-o-`::` 
## | +-dplyr 
## | \-filter 
## +-beer_reviews 
## \-o-`>=` 
##   +-review_overall 
##   \-5
```
]

---

---

# Operator precedence

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</style>
<div id="nuzfauutoy" style="overflow-x:auto;overflow-y:auto;width:auto;height:auto;"><table class="gt_table">
  
  <thead class="gt_col_headings">
    <tr>
      <th class="gt_col_heading gt_columns_bottom_border gt_left" rowspan="1" colspan="1">operator</th>
      <th class="gt_col_heading gt_columns_bottom_border gt_left" rowspan="1" colspan="1">meaning</th>
      <th class="gt_col_heading gt_columns_bottom_border gt_left" rowspan="1" colspan="1">associativity</th>
    </tr>
  </thead>
  <tbody class="gt_table_body">
    <tr>
      <td class="gt_row gt_left">:: :::</td>
      <td class="gt_row gt_left">access variables in a namespace</td>
      <td class="gt_row gt_left">left-associative</td>
    </tr>
    <tr>
      <td class="gt_row gt_left">$ @</td>
      <td class="gt_row gt_left">component / slot extraction</td>
      <td class="gt_row gt_left">left-associative</td>
    </tr>
    <tr>
      <td class="gt_row gt_left">[ [[</td>
      <td class="gt_row gt_left">indexing</td>
      <td class="gt_row gt_left">left-associative</td>
    </tr>
    <tr>
      <td class="gt_row gt_left">^</td>
      <td class="gt_row gt_left">exponentiation (right to left)</td>
      <td class="gt_row gt_left" style="font-weight: bold;">right-associative</td>
    </tr>
    <tr>
      <td class="gt_row gt_left">- +</td>
      <td class="gt_row gt_left">unary minus and plus</td>
      <td class="gt_row gt_left">left-associative</td>
    </tr>
    <tr>
      <td class="gt_row gt_left">:</td>
      <td class="gt_row gt_left">sequence operator</td>
      <td class="gt_row gt_left">left-associative</td>
    </tr>
    <tr>
      <td class="gt_row gt_left" style="background-color: #FFD700;">%any%</td>
      <td class="gt_row gt_left" style="background-color: #FFD700;">special operators (including %% and %/%</td>
      <td class="gt_row gt_left" style="background-color: #FFD700;">left-associative</td>
    </tr>
    <tr>
      <td class="gt_row gt_left">* /</td>
      <td class="gt_row gt_left">multiply, divide</td>
      <td class="gt_row gt_left">left-associative</td>
    </tr>
    <tr>
      <td class="gt_row gt_left">+ -</td>
      <td class="gt_row gt_left">(binary) add, subtract</td>
      <td class="gt_row gt_left">left-associative</td>
    </tr>
    <tr>
      <td class="gt_row gt_left">< > <= >= == !=	</td>
      <td class="gt_row gt_left">ordering and comparison</td>
      <td class="gt_row gt_left">left-associative</td>
    </tr>
    <tr>
      <td class="gt_row gt_left">!</td>
      <td class="gt_row gt_left">negation</td>
      <td class="gt_row gt_left">left-associative</td>
    </tr>
    <tr>
      <td class="gt_row gt_left">& &&</td>
      <td class="gt_row gt_left">and</td>
      <td class="gt_row gt_left">left-associative</td>
    </tr>
    <tr>
      <td class="gt_row gt_left">| ||</td>
      <td class="gt_row gt_left">or</td>
      <td class="gt_row gt_left">left-associative</td>
    </tr>
    <tr>
      <td class="gt_row gt_left">~</td>
      <td class="gt_row gt_left">as in formulae</td>
      <td class="gt_row gt_left">left-associative</td>
    </tr>
    <tr>
      <td class="gt_row gt_left">-> ->></td>
      <td class="gt_row gt_left">rightwards assignment</td>
      <td class="gt_row gt_left" style="font-weight: bold;">right-associative</td>
    </tr>
    <tr>
      <td class="gt_row gt_left"><- <<-</td>
      <td class="gt_row gt_left">assignment (right to left)</td>
      <td class="gt_row gt_left" style="font-weight: bold;">right-associative</td>
    </tr>
    <tr>
      <td class="gt_row gt_left">=</td>
      <td class="gt_row gt_left">assignment (right to left)</td>
      <td class="gt_row gt_left" style="font-weight: bold;">right-associative</td>
    </tr>
    <tr>
      <td class="gt_row gt_left">?</td>
      <td class="gt_row gt_left">help (unary and binary)</td>
      <td class="gt_row gt_left">left-associative</td>
    </tr>
  </tbody>
  
  
</table></div>

---

# Parsing

_turn inputs from text into expressions_

```r
inputs <- list(
  arg_1 = "x",
  arg2 = "na.rm = TRUE"
)

map(inputs, parse_exprs)
```

```
## $arg_1
## $arg_1[[1]]
## x
## 
## 
## $arg2
## $arg2[[1]]
## na.rm = TRUE
```

---

# Deparsing

Maybe use this for..... warning messages to alert the user their code is trash?

```r
mayas_mean_function <- function(x, y, z) {
  stop(paste("lol you seriously tried to ", 
             expr_label(quote(x + y + z))))
}

mayas_mean_function(1, 3, 2)
```

```
Error in mayas_mean_function(1, 3, 2) : lol you seriously tried to `x + y + z`
```

How would we build on this to actually return the users specified values?

---

# Walking AST with Recursion

...But can we use recursion to BUILD an AST?

Once the application is working I figured that THIS is theoretically the way we could create and allow the user to nest functions, creating more complicated trees than `mean(x, na.rm = TRUE)`?

```r
func_1 <- "mean"
arg1_1 <- expr("x")
arg1_2 <- expr("na.rm = TRUE")

func_2 <- "round"
arg2_1 <- expr(call(func_1, arg1_1, arg1_2))
```

Then somehow put those arguments together to get:

```r
ast(round(mean(x, na.rm = TRUE)))
```

```
## o-round 
## \-o-mean 
##   +-x 
##   \-na.rm = TRUE
```

---

- Pairlist
- Missing arguments
- Expression vectors

---

## Pairlist and missing arguments

> Behind the scenes pairlists are implemented using a different data structure, a linked list instead of an array.

```r
f <- expr(function(z, y = 10) z + y)
str(f[[2]])
```

```
## Dotted pair list of 2
##  $ z: symbol 
##  $ y: num 10
```
]

```r
f_list <- list(
  z = missing_arg(),
  y = 10
)
str(f_list)
```

```
## List of 2
##  $ z: symbol 
##  $ y: num 10
```
]

Maybe we use `missing_arg()` in our shiny application where we create a function with `x` empty spaces, then use subsetting on our call to replace the missing arguments with the user supplied arguments?

---

## Expression vectors

Maybe we want to use `parse` instead of `parse_exps`? Not sure why this would be preferable, but hey, here's another way

```r
parse(text = c(inputs))
```

```
## expression(x, na.rm = TRUE)
```

---