Struct type properties for Typed/Racket

8.17

Struct type properties for Typed/Racket🔗ℹ

syntax
(struct/props maybe-type-vars name maybe-parent ([field : type] ...)
              options ...)

maybe-type-vars =
| (v ...)

maybe-parent =
| parent-id

options = #:transparent
| #:property prop:custom-write custom-write
| #:property prop:equal+hash equal+hash

   custom-write :
(→ name                    ; non-polymorphic case
   Output-Port
   (U #t #f 0 1)
   Any)
   custom-write :
(∀ (v ...)                 ; polymorphic case
   (→ (name v ...)
      Output-Port
      (U #t #f 0 1)
      Any))
   equal+hash :
(List (→ name                ; non-polymorphic case
         name
         (→ Any Any Boolean)
         Any)
      (→ v
         (→ Any Integer)
         Integer)
      (→ v
         (→ Any Integer)
         Integer))
   equal+hash :
(List (∀ (v ... v2 ...)      ; polymorphic case
         (→ (name v ...)
            (name v2 ...)
            (→ Any Any Boolean)
            Any))
      (∀ (v ...)
         (→ (name v ...)
            (→ Any Integer)
            Integer))
      (∀ (v ...)
         (→ (name v ...)
            (→ Any Integer)
            Integer)))

This form defines a typed/racket struct type, and accepts a small subset of racket’s struct type properties.

For prop:custom-write and prop:equal+hash, the function types are polymorphic only if the struct is polymorphic (i.e. maybe-type-vars is present) and has at least one type variable (i.e. maybe-type-vars is not just ()).

It implements these struct type properties in a type-safe manner: the current implementation in typed/racket does not properly type-check functions and values used as struct type properties. This library declares the user-provided functions outside of the struct definition, with the type given above (e.g. (∀ (v ...) (→ (name v ...) Output-Port (U #t #f 0 1) Any)) for the argument of the prop:custom-write property), to ensure that these functions and values are properly checked.

The API should (hopefully) stay backward-compatible when Typed/Racket officially supports (or rejects) structure type properties. In other words:

If typed/racket eventually implements the same interface as the one provided by this library, then we will update this library so that it simply re-provide struct renamed as struct/props.
If typed/racket eventually implements some type-safe struct type properties, then we will update this library will so that it translates back to typed/racket’s implementation, as much as possible.
If typed/racket eventually disallows struct type properties, then we will update this library so that it uses some typed/racket/unsafe tricks to still make them available, if it can be done.