A lambda is a function without a public name. Use it when a small rule is needed in one place.

let twice := \(x : Int) : Int => x + x;
twice(21);

Lambda shape

let twice := \(x : Int) : Int => x + x;
twice(21);

The parameter list is on the left. The result expression is on the right of =>.

Use lambdas for small local behavior

Lambdas are good for map, filter, fold, and small callbacks.

let option := import "@std/option";

option.someOf[Int](41)
|> option.map[Int, Int](\(value : Int) : Int => value + 1);

If the lambda grows beyond one idea, give it a name with let.

Use a named function when the rule is reused.

let twice (x : Int) : Int := x + x;
twice(21);

Use a lambda when the rule belongs to one call site.

\(value : Int) : Int => value + 1;

Both forms have parameters and a result expression. The lambda has no public name.