How to create "nothing" instead of an "empty thing"?

Questions
,
1

I have the following (specific) problem which has its cause in a more general problem: I would like to create a grid, where one or more rows are only printed depending on some condition.

E.g. in the following example we have a grid with four rows. The third row should only be printed if some condition is true. If the condition is false, a grid with only three rows should be printed:

#grid(
      columns: (auto, auto),
      row-gutter: 1em,
      column-gutter: 1em,
      align: (right, left),
      "hallo1", "world1",
      "hallo2", "world2",
      if (false) {
        "hallo3", "world2"
      },
      "hallo4", "world4",
)

For the sake of simplicity I’ve just used false as a condition within the if-statement.

Unfortunately the result is:
image

I.e. Typst produces a grid with four rows, where the content of the third row is sort of an empty element (presumably with height 0). But as row-gutter is 1em, we have a vertical gap of that size.

There is a work-around in this case: I can set row-gutter to 0em and use a box in each row to give it the desired vertical space as follows:

#grid(
      columns: (auto, auto),
      row-gutter: 0em,
      column-gutter: 1em,
      align: (right, left),
      box("hallo1", height: 16pt), "world1",
      box("hallo2", height: 16pt), "world2",
      if (false) {
        box("hallo3", height: 16pt)
      },
      if (false) {
        text("world3")
      },
      box("hallo4", height: 16pt), "world4",
)

This leads to the desired output, but the code necessary to produce it is not ideal (to put it mildly):
image

In my opinion, the basic problem is that the if-statement produces some sort of empty element, if the condition evaluates to false. In my opinion it should produce “nothing”, i.e. it should render a grid which consists of only three rows.

Is there any way to achieve this in Typst or is this a known behaviour? … and if so, why is Typst implemented this way?

2

No comment on the implementation reasons, but the way I’ve approached this in the past is to make an array of rows, do some higher-order functions on the array to manipulate and remove rows, then flatten and spread the array into the table arguments.

#grid(
  columns: (auto, auto, auto),
  row-gutter: 1em,
  column-gutter: 1em,
  align: (right, center, left),
  ..(
    ("apples", "and", "bananas"),
    ("this", "or", "that"),
    ("tea", "for", "two"),
    ("raining cats", "and", "dogs"),
    ("all", "or", "nothing"),
    ("up", "to", "you")
  )
  .filter(row => row.at(1) != "or")
  .flatten()
)

image

An if with no else evaluates to none if the condition is false. To remove elements of the array that evaluate to none, that’s also just a .filter:

#grid(
  columns: (auto, auto, auto),
  row-gutter: 1em,
  column-gutter: 1em,
  align: (right, center, left),
  ..(
    ("apples", "and", "bananas"),
    {if (false) {("this", "or", "that")}},
    ("tea", "for", "two"),
    {if (false) {("raining cats", "and", "dogs")}},
    ("all", "or", "nothing"),
    ("up", "to", "you")
  )
  .filter(row => row != none)
  .flatten()
)
1 Like
3

You don’t need to construct and filter an array, you can just spread the whole if statement, which contains the row as an array:

#grid(
    columns: (auto, auto),
    row-gutter: 1em,
    column-gutter: 1em,
    align: (right, left),

    "hallo1", "world1",
    "hallo2", "world2",
    ..if false { ("hallo3", "world2") },
    "hallo4", "world4",
)

This works because when spreading ..none, you don’t pass an “empty thing” to the grid function, but instead pass “nothing” as you intended.

3 Likes
4

Typst is expression-based, meaning that everything is an expression that gets evaluated to something. Other languages that do this include Scala, Rust, Kotlin, etc. (cf. Expression-oriented programming language - Wikipedia)
In such languages, an if statement also gets evaluated. In Typst’s case, it takes the value of the block which was chosen by the condition. For example:

#let a = if cond {3} else {4}

Here a would be 3 if cond is true, and 4 otherwise
In your case, you haven’t specified any else clause. As such, if the condition fails, the statement is evaluated to none

#let b = if cond {3}

Here b equals 3 if cond is true, and none otherwise

Consequently, if your condition is false, then the optional row’s cells are replaced by a single none value, which is interpreted as an empty cell by grid

4 Likes
5

Thanks to all for the really good answers!

1 Like