▼ |  | The Typed Racket Reference

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

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with-type

7.2
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### 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 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/region.rkt:700:62: 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

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