▼ |  | The Typed Racket Reference

1 | Type Reference
2 | Special Form Reference
3 | Libraries Provided With Typed Racket
4 | Utilities
5 | Exploring Types
6 | Typed Racket Syntax Without Type Checking
7 | Typed Regions
8 | Optimization in Typed Racket
9 | Legacy Forms
10 | Compatibility Languages
11 | Experimental Features

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with-type

5.92
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### 7`` 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 first form, an expression, checks that body ...+ has the type type. If the last expression in body ...+ returns multiple values, type must be a type of the form (values t ...). Uses of the result values are appropriately checked by contracts generated from 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.

Examples:
> > (with-type #:result Number 3)
> 3
> > ((with-type #:result (Number -> Number)
> (lambda: ([x : Number]) (add1 x)))
> #f)
> contract violation:
> expected: Number
> given: #f
> in: the 1st argument of
> (-> Number Number)
> contract from: (region typed-region)
> blaming: top-level
> > (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 contract
> promised: String
> produced: 'hello
> in: String
> contract from: top-level
> blaming: top-level
> at: eval:5.0
> > (with-type ([fun (Number -> Number)]
> [val Number])
> (define (fun x) x)
> (define val 17))
> > (fun val)
> 17

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