Bounds

A bound says which traits a type parameter's type has to implement. Without one, nothing is known about the type except that it is one - not even == is available on it.

Example#

fn largest<Item: Compare>(values: List<Item>): Item? {
  values.get(0).map { first => values.fold first { a, b => a.max b } }
}

fn printSorted<Item>(items: List<Item>) where Item: Compare & Show {
  const ordered = items.sorted { _ }
  for item in ordered {
    print item
  }
}

printSorted([3, 1, 2])
print largest([3, 1, 2])

Syntax#

fn <name><Item: <Trait>>(...)                 inline, one parameter
fn <name><Item>(...) where Item: <Trait>      after where, same meaning
fn <name><A, B>(...) where A: <Trait>, B: <Trait> & <Trait>   several parameters, `&` combines traits
fn <name>(...): <Type> where Item: <Trait>                    a member's own bound

Rules#

1. A bound is written inline, right after the parameter's name (<Item: Compare>), or after where (where Item: Compare). The two forms mean the same thing; where is what several parameters or several traits on one parameter read best with.

2. Several traits on one parameter combine with &. where Item: Compare & Show is the same & that intersects traits in a type position; see Trait intersections.

3. where can carry the bounds of more than one parameter, separated by ,.

   fn zip<Left, Right>(lefts: List<Left>, rights: List<Right>): List<(Left, Right)>
     where Left: Show, Right: Show

4. A member of a trait can carry a where clause of its own, and then it is not a requirement on every implementor - it exists only where the clause holds. Iterate<Item> declares fn toSet(): Set<Item> where Item: Hash, so every Iterate has the method, but calling it is only legal where Item happens to be Hash:

   const numbers: List<Int> = [3, 1, 2]
   print numbers.toSet()

   Does not compile

   type Task {
     action: () => Void
   }
   
   const tasks: List<Task> = []
   const unique = tasks.toSet()
   // error: `Task` does not implement `Hash`

5. A bound is checked at the call, not inside the declaration that asks for it. largest<Item: Compare> type checks its own body against exactly the members Compare promises, and a call that cannot supply a Compare type is rejected at the call.

   fn largest<Item: Compare>(values: List<Item>): Item? {
     values.get(0).map { first => values.fold first { a, b => a.max b } }
   }
   
   print largest([3, 1, 2])

   Does not compile

   fn largest<Item: Compare>(values: List<Item>): Item? {
     values.get(0).map { first => values.fold first { a, b => a.max b } }
   }
   
   type Task {
     action: () => Void
   }
   
   const tasks: List<Task> = []
   print largest(tasks)
   // error: `Task` does not implement `Compare`

What this is not#

A bound is not a cast, and it does not narrow what a value already is. Inside largest<Item: Compare>, item stays an Item - the bound only adds the members of Compare to what can be called on it, it does not turn item into a Compare-typed value. Nothing about largest changes when it is called with an Int instead of some other Compare type, and nothing inside it can tell the two apart.

fn describe<Item: Show>(value: Item): String {
  "the value is {value}"
}

print describe(5)

trait Loud {
  fn shout(): String
}

type Silent {
  loud: Loud
}

type Robot with Loud {
  fn shout(): String {
    "BEEP"
  }
}

fn describe<Item: Show>(value: Item): String {
  "the value is {value}"
}

print describe(Silent(Robot()))
// error: `Silent` does not implement `Show`

Related#

• Type parameters - where a parameter is declared, before it is bounded.
• Trait intersections - the & a bound and a type position share.
• Traits as types - a bound compared to a trait used as a parameter's own type.
• Inference - what a bound does and does not help the compiler infer.