Std.Monad

Monad interface — structures that support sequencing with a failure arm.

A monad is a type constructor m equipped with pure (embed a value) and and_then (sequence, threading the unwrapped payload and short-circuiting on the failure arm). and_then is the operation the binder pipe |x|> desugars to: a |x|> f(x) lowers to and_then(a, fn(x) -> f(x)) (see docs/BIND_PIPE.md § 7).

Monad is genuinely higher-kinded: its head m has kind Type -> Type, so pure lifts an a to m(a) and and_then threads an a -> m(b) through an m(a). The built-in instances cover Result and Option; additional instances can be declared for user-defined container types.

A binary constructor such as Result(T, E) is admitted by applying the head to its payload only — m(a) — with the remaining parameters (E) filled by a fresh, universally-quantified variable on each instance method (Result(a, e)). This is what lets one payload-only interface cover both the unary Option and the binary Result.

Examples

cure

use Std.Monad

and_then(Ok(1), fn(x: Int) -> Result(Int, Atom) = Ok(x + 1))   # => Ok(2)
and_then(Error(:e), fn(x: Int) -> Result(Int, Atom) = Ok(x))   # => Error(:e)
pure(42)                                                        # => Ok(42)