Structs and lists

Structs: your own types

A struct groups a few values into one thing. Declare it with its fields and their types:

struct Todo {
  title: string
  done: bool
}

Fields can go one per line (as above) or on one line with commas:

struct Point { x: float, y: float }

Make one by calling the struct’s name and giving every field by name:

let todo = Todo(title: "Buy milk", done: false)

Read and change fields with a dot:

label(todo.title)
todo.done = true

Lists

A list holds any number of values of the same type, in order.

state numbers = [1, 2, 3]          // a [int]
state names: [string] = []         // an empty list needs its type
state todos: [Todo] = []

What you can do with a list:

Asking for an item that isn’t there stops the app with a clear message, like there is no item 3; the list has 2.

Values are copied

When you assign a struct or list, you get a copy. Changing the copy leaves the original alone:

let a = Todo(title: "x", done: false)
let b = a
b.done = true      // a.done is still false

There’s one exception, and it’s the one you want: a for loop over state gives you the real items, so you can change them in place:

fn finish_all() {
  for todo in todos {
    todo.done = true     // changes the todos in state
  }
}

Lists in the UI

A for loop inside a window or view shows its elements once per item:

for todo in todos {
  checkbox(checked: $todo.done, title: todo.title)
}

When todos changes, the loop is rebuilt to match.

Positions and counting

Name a second variable before the item to get its position, counting from 0. A loop that finds an item can return its position, and a row can remove itself:

fn position_of(name: string) -> int {
  for index, item in names {
    if item == name {
      return index
    }
  }
  return -1
}

To repeat something a number of times, loop over a range. for i in 0..n counts i from 0 up to, but not including, n, so 0..items.len() is every position in items. The position and the count are set by the loop, so they can’t be assigned.

This app numbers its rows, removes the one you click, and draws a meter with a range:

state names = ["Ada", "Grace", "Linus"]
state draft = ""
state rating = 3

fn position_of(name: string) -> int {
  for index, item in names {
    if item == name {
      return index
    }
  }
  return -1
}

fn add_name() {
  if draft != "" && position_of(draft) < 0 {
    names.push(draft)
    draft = ""
  }
}

fn meter(filled: int) -> string {
  let bar = ""
  for step in 0..5 {
    if step < filled {
      bar = bar + "●"
    } else {
      bar = bar + "○"
    }
  }
  return bar
}

window("Lineup", width: 320, height: 420) {
  column(spacing: 12, padding: 20) {
    for index, name in names {
      row(spacing: 8) {
        label("{index + 1}. {name}")
        spacer()
        button("Remove {name}") { names.remove(index) }
      }
    }
    row(spacing: 8) {
      text_field($draft, placeholder: "Name")
      button("Add") { add_name() }
    }
    if position_of(draft) >= 0 {
      label("{draft} is already number {position_of(draft) + 1}", style: .caption)
    }
    divider()
    row(spacing: 4) {
      button("Fewer") { rating -= 1 }
      for star in 0..rating {
        label("★{star + 1}")
      }
      button("More") { rating += 1 }
    }
    label(meter(rating))
  }
}