Splicing syntax parameterize disables Typed Racket type annotations

Question

When I run the following, I get a type error as expected:

#lang typed/racket
(require racket/stxparam)
(define-syntax-parameter x #f)
(syntax-parameterize ([x #'foo])
  (: n Number)
  (define n "string")
  'foo)

But when I use splicing-syntax-parameterize instead, suddenly I get no type errors

#lang typed/racket
(require racket/stxparam
         racket/splicing)
(define-syntax-parameter x #f)
(splicing-syntax-parameterize ([x #'foo])
  (: n Number)
  (define n "string"))

Examining at the REPL shows that n has type String , despite the annotation. The documentation for splicing-syntax-parameterize claims that it treats definitions in the same manner as begin :

Like syntax-parameterize, except that in a definition context, the body forms are spliced into the enclosing definition context (in the same way as for begin).

However, if I use begin instead of splicing-syntax-parameterize :

#lang typed/racket
(begin
  (: n Number)
  (define n "string"))

Then I do get a type error. Is there some reason splicing-syntax-parameterize strips away type annotations from definitions, but begin does not? I would hazard a guess it's because the splicing-syntax-parameterize is basing its behavior on the untyped begin form instead of the begin form from Typed Racket, and thus knows absolutely nothing about the : form. If that's the case, how can I work around this?

Answer 1

Yes, this is a bug. It turns out that splicing-syntax-parameterize has a somewhat strange expansion that doesn't play nicely with the way Typed Racket detects and installs type annotations.

I have written a fix for this bug, which you can find here . As expected, your original program now fails to typecheck. This fix should be available in Racket 6.1.1.8 or greater, so if you need this behavior, use a snapshot build or use the newest version of Typed Racket.

Otherwise, this should be included in the next Racket release.