Typed Racket actually has a rather marvelous type system. The following is fine therein and statically type checks: (let ([x (if (condition) "string" (lis

Typed Racket actually has a rather marvelous type system. The following is fine therein and statically type checks: (let ([x (if (condition) "string" (list "a list"))]) (if (string? x) (string-length? x) (length? x))) By process of elimination, because the type of the variable is `(U string? (List-of string?))`, the type checker can prove that if `(string? x)~ be false, the type of `x` must be `list?`, and that `(length x)` succeeds.