TR can't generate contracts for prefab structs?

It looks to me like TR can’t generate contracts for prefab structs in a way that I would expect it to. Specifically, here’s a TR file and a Racket file that uses a TR-defined prefab structure:

makes-foo.rkt:

#lang typed/racket

(provide (struct-out Foo)
         get-a-foo)

(struct Foo ([a : String]) #:prefab)

(define (get-a-foo)
  (Foo "oeuth”))


uses-foo.rkt:

#lang racket

(require "makes-foo.rkt")

(define a (get-a-foo))


This doesn’t typecheck (at least not in version  6.11.0.2--2017-11-05, tell me if I should just upgrade), with the message:

Type Checker: could not convert type to a contract;
 contract generation not supported for this type
  identifier: get-a-foo
  type: (-> Foo) in: (get-a-foo)

My first thought was that this was related to mutability, but the docs and my experiments suggest that prefab structures are default-immutable, and that this is reflected in a way that should be visible to the type-checker. Am I missing something, or is this just something that we haven’t yet had time to implement?


John





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