repr is a toplevel attribute that changes the runtime representation of a type or struct.

Syntax

(repr :enum)
(repr :transparent)
(repr :lisp)
(repr :native ⟨lisp-ty⟩)

Semantics

  • repr must appear immediately before a compatible define-type or define-struct.
  • :enum chooses a special representation of a non-parametric type based off of symbols. This allows efficient storage and fast matching.
  • :transparent creates a no-overhead type when the type definition is just a union of one branch. The representation of a transparent type is the same as the type being wrapped.
  • :lisp ensures the data type is fully compatible with Lisp: it provides a distinguished type definition and ensures the value is wrapped so it can be specialized in CLOS.
  • :native allows the programmer to tell Coalton what the representation of the type is on the Lisp side. It is used for making interfaces to Lisp code.
  • define-struct only supports transparent representation.
  • Representation choice affects generated Lisp layout, interop, and some optimizations.

Example

(repr :transparent)
(define-type Gray (Gray U8))

(repr :native cl:cons)
(define-type (Pair :car :cdr))

(declare make-pair (forall (:car :cdr) (:car * :cdr -> (Pair :car :cdr))))
(define (make-pair x y)
  (lisp (-> (Pair :car :cdr))
    (cl:cons x y)))