=>

=> separates a type’s constraint context from the rest of the type. It also appears in define-class and define-instance heads.

Syntax

⟨class⟩ :a => ⟨type⟩
(⟨class1⟩ :a) (⟨class2⟩ :a) => ⟨type⟩

Semantics

• Everything to the left of => is a type class constraint (also called a predicate).
• Everything to the right is the main type or class head.
• => is purely part of type syntax. It is not a value-level operator.

Options

• A single predicate can appear without extra parentheses.
• Multiple predicates are written as separate parenthesized constraints.
• In class declarations, => introduces superclass-style requirements.

Example

(declare sort-pair ((Ord :a) => Tuple :a :a -> Tuple :a :a))

;; To be Ord, you must be Eq
(define-class (Eq :t => Ord :t)
  ...)