[racket] Weird typed racket error

I think you don't want those #{} annotations in the function definition:

> (define (sphere3d #:posn #{ctr.posn : (Vectorof Real)} #:r #{r :
> Nonnegative-Real} #:color [#{color : Plot-Color} "black"])

The following typechecks OK for me:

(define (sphere3d #:posn ctr.posn #:r r #:color [color "black"])

In other words, when you annotate the function using `(: sphere3d
___)`, you've told Typed Racket all it needs to know about the
function type. Then you can write the function definition just as you
would in untyped Racket.