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Bindings: let and var

A binding associates a name with a value or computation. let creates an immutable reference; var creates a mutable one.

let fixedNumericBinding = 10
let fixedStringBinding = "the quick brown fox"
var value = 10
value = 20

Functions are bindings too​

When a let binding takes parameters, it becomes a function — re-evaluated every time it's called, rather than computed once:

let alwaysTen() = 10
let plusOne(value: int) = value + 1
let add(left, right) = left + right

Execution blocks​

Bindings can nest inside each other, and a block can compute a result with return:

let value = {
  let connection = resolveDatabaseConnection()
  let buildRequest = constructRequest()

  return connection.performQuery(buildRequest)
}

Parameter and return types can be inferred or written explicitly:

# The input and output are listed explicitly.
let square(input: i32) : i32 {
  return input * input
}

A gotcha: top-level bindings resolve lazily​

Global (top-level) bindings are resolved the first time they're used at runtime — not in the order they're written in the file:

# In program order, first comes before second.
let first = println("first")
let second = println("second")

# But global bindings are resolved in runtime order,
# so "second" prints before "first".
entrypoint main() {
  second
  first
}