9 Typed Regions
The with-type form allows for localized Typed Racket regions in otherwise untyped code.
syntax
(with-type result-spec fv-clause body ...+)
(with-type export-spec fv-clause body ...+)
fv-clause =
| #:freevars ([id fv-type] ...) result-spec = #:result type export-spec = ([export-id export-type] ...)
The second form, which can be used as a definition, checks that each of the export-ids has the specified type. These types are also enforced in the surrounding code with contracts.
The ids are assumed to have the types ascribed to them; these types are converted to contracts and checked dynamically.
> (with-type #:result Number 3) 3
> ((with-type #:result (Number -> Number) (lambda: ([x : Number]) (add1 x))) #f) .../contract/region.rkt:764:62: contract violation
expected: number?
given: #f
in: the 1st argument of
(-> number? any)
contract from: (region typed-region)
blaming: top-level
(assuming the contract is correct)
> (let ([x "hello"]) (with-type #:result String #:freevars ([x String]) (string-append x ", world"))) "hello, world"
> (let ([x 'hello]) (with-type #:result String #:freevars ([x String]) (string-append x ", world"))) x: broke its own contract
promised: string?
produced: 'hello
in: string?
contract from: top-level
blaming: top-level
(assuming the contract is correct)
at: eval:5:0
> (with-type ([fun (Number -> Number)] [val Number]) (define (fun x) x) (define val 17)) > (fun val) 17