Some problems related to `quasiquote`.

[NoahStoryM]

What version of Racket are you using?

v8.4 [cs]

What program did you run?

In untyped version:

Welcome to Racket v8.4 [cs].
> (define a '(x y z))
> (define b 'b)
> `(,@a . ,b)
'(x y z . b)
> 

In typed version:

Welcome to Racket v8.4 [cs].
> (define a '(x y z))
> (define b 'b)
> `(,@a . ,b)
string:1:4: Type Checker: type mismatch
  expected: (Listof a)
  given: (List 'x 'y 'z)
  in: a
 [,bt for context]
> ,bt
string:1:4: Type Checker: type mismatch
  expected: (Listof a)
  given: (List 'x 'y 'z)
  in: a
  location...:
   string:1:4
  context...:
   /usr/share/racket/pkgs/typed-racket-lib/typed-racket/typecheck/check-below.rkt:62:0: check-below
   /usr/share/racket/pkgs/typed-racket-lib/typed-racket/typecheck/tc-app-helper.rkt:32:0: tc/funapp1
   /usr/share/racket/pkgs/typed-racket-lib/typed-racket/typecheck/tc-app-helper.rkt:183:0: domain-mismatches
   /usr/share/racket/pkgs/typed-racket-lib/typed-racket/typecheck/tc-funapp.rkt:71:0: tc/funapp
   .../private/runtime.rkt:80:24: temp667
   .../private/runtime.rkt:89:23: fail-handler110
   /usr/share/racket/pkgs/typed-racket-lib/typed-racket/typecheck/tc-expr-unit.rkt:112:0: tc-expr/check
   /usr/share/racket/pkgs/typed-racket-lib/typed-racket/typecheck/tc-toplevel.rkt:694:0: tc-toplevel-form
   .../private/runtime.rkt:80:24: temp25
   /usr/share/racket/pkgs/xrepl-lib/xrepl/xrepl.rkt:1573:0
   /usr/share/racket/collects/racket/repl.rkt:11:26
> 

And in some cases, the error message will prompt that it is related to qq-append:

#lang typed/racket/base

(define-type S-Exp (U Symbol Null (Pair S-Exp S-Exp)))
(define-predicate s-exp? S-Exp)

(: a (Listof Symbol))
(define a '(x y z))

(: b (U Null Symbol))
(define b 'b)

(ann `(,@a . ,b) S-Exp)

What should have happened?

'(x y z . b)

If you got an error message, please include it here.

draft.rkt:12:5: Type Checker: Polymorphic function `qq-append' could not be applied to arguments:
Types: (Listof a) (Listof b)  -> (Listof (U a b))
Arguments: (Listof Symbol) (U Null Symbol)
Expected result: S-Exp

  in: (quasiquote ((unquote-splicing a) unquote b))
  location...:
   draft2.rkt:12:5
  context...:
   /usr/share/racket/pkgs/typed-racket-lib/typed-racket/typecheck/tc-toplevel.rkt:455:0: type-check
   /usr/share/racket/pkgs/typed-racket-lib/typed-racket/typecheck/tc-toplevel.rkt:682:0: tc-module
   /usr/share/racket/pkgs/typed-racket-lib/typed-racket/tc-setup.rkt:101:12
   /usr/share/racket/pkgs/typed-racket-lib/typed-racket/typed-racket.rkt:22:4