14 Feb 2009

New Contract-Related Features

posted by Stevie

In SVN I’ve added three new major features that involve contracts. One allows for more fine-grained control of contracts, and the other two allow for the use of contracts with signatures and units.

Contract Regions

Contract regions allow the programmer to protect a region of code with a contract boundary. In addition to the wrapped code, the programmer also provides a name for the region which is used in blame situations and a list of exported variables which can either be protected with contracts or unprotected. The region provides a true contract boundary, in that uses of contracted exports within the region are unprotected. Contract regions are specified with the with-contract form. The following contract region defines two mutually recursive functions:

(with-contract region1
 ([f (-> number? boolean?)]
  [g (-> number? boolean?)])
 (define (f n) (if (zero? n) #f (g (sub1 n))))
 (define (g n) (if (zero? n) #t (f (sub1 n)))))
 ```

The internal calls to `f` and `g` are uncontracted, but calls to `f`and `g` outside this region would be appropriately contracted.  First-order checks are performed at the region, so the
following region:

```racket
(with-contract region2
 ([n number?])
 (define n #t))
 ```

results in the following error:

```racket
(region region2) broke the contract number? on n; expected <number?>, given: #t

Notice that the blame not only gives the name of the region, but describes what type of contract boundary was involved.

For contracting a single definition, there is the define/contract form which has a similar syntax to define, except that it takes a contract before the body of the definition.

To compare the two forms, the following two definitions are equivalent:

(with-contract fact
 ([fact (-> number? number?)])
 (define (fact n)
   (if (zero? n) 1 (* n (fact (sub1 n))))))

(define/contract (fact n)
 (-> number? number?)
 (if (zero? n) 1 (* n (fact (sub1 n)))))

First order checks are similarly performed at the definition for define/contract, so

(define/contract (fact n)
 (-> number?)
 (if (zero? n) 1 (* n (fact (sub1 n)))))

results in

(function fact) broke the contract (-> number?) on fact; expected a procedure that accepts no arguments without any keywords, given: #<procedure:fact>

Signature Contracts

In addition to contract regions, units are also now contract boundaries. One way to use contracts with units is to add contracts to unit signatures via the contracted signature form.

(define-signature toy-factory^
 ((contracted
   [build-toys (-> integer? (listof toy?))]
   [repaint    (-> toy? symbol? toy?)]
   [toy?       (-> any/c boolean?)]
   [toy-color  (-> toy? symbol?)])))

Notice that contracts in a signature can use variables listed in the signature.Now if we take the following implementation of that signature:

(define-unit [email protected]
 (import)
 (export toy-factory^)

 (define-struct toy (color) #:transparent)

 (define (build-toys n)
   (for/list ([i (in-range n)])
     (make-toy 'blue)))

 (define (repaint t col)
   (make-toy col)))

We get the appropriate contract checks on those exports:

> (define-values/invoke-unit/infer [email protected])
> (build-toys 3)
(#(struct:toy blue) #(struct:toy blue) #(struct:toy blue))
> (build-toys #f)
top-level broke the contract (-> integer? (listof toy?))
 on build-toys; expected >, given: #f

As before, uses of contracted exports inside the unit are not checked.

Since units are contract boundaries, they can be blamed appropriately. Take the following definitions:

(define-unit [email protected]
 (import toy-factory^)
 (export)
 (let ([toys (build-toys 3)])
   (repaint 3 'blue)))

(define-compound-unit/infer [email protected]
 (import) (export)
 (link [email protected] [email protected]))
When we invoke the combined unit:> (define-values/invoke-unit/infer [email protected])
(unit [email protected]) broke the contract
 (-> toy? symbol? toy?)
on repaint; expected >, given: 3

Unit Contracts

However, we may not always be able to add contracts to signatures. For example, there are many already-existing signatures in PLT Scheme that one may want to implement, or a programmer may want to take a unit value and add contracts to it after the fact.

To do this, there is the unit/c contract combinator. It takes a list of imports and exports, where each signature is paired with a list of variables and their contracts for each signature. So if we had the uncontracted version of the toy-factory^ signature:

(define-signature toy-factory^
 (build-toys repaint toy? toy-color))

the following contracts would be appropriate for a unit that imports nothing and exports that signature:

(unit/c (import) (export))
(unit/c (import) (export toy-factory^))
(unit/c
 (import)
 (export (toy-factory^
          [toy-color (-> toy? symbol?)])))
(unit/c
 (import)
 (export (toy-factory^
          [build-toys (-> integer? (listof toy?))]
          [repaint    (-> toy? symbol? toy?)]
          [toy?       (-> any/c boolean?)]
          [toy-color  (-> toy? symbol?)])))

Unit contracts can contain a superset of the import signatures and a subset of the export signatures for a given unit value. Also, variables that are not listed for a given signature are left alone when the contracts are being added.

Since the results of applying unit/c is a new unit, then adding a contract can cause link inference to fail. For example, if we change the definition of [email protected] above to

(define/contract [email protected]
 (unit/c
  (import)
  (export (toy-factory^
           [build-toys (-> integer? (listof toy?))]
           [repaint    (-> toy? symbol? toy?)]
           [toy?       (-> any/c boolean?)]
           [toy-color  (-> toy? symbol?)])))
 (unit
   (import)
   (export toy-factory^)

   (define-struct toy (color) #:transparent)

   (define (build-toys n)
     (for/list ([i (in-range n)])
       (make-toy 'blue)))

   (define (repaint t col)
     (make-toy col))))

Then when we try to combine it with the [email protected] unit, we get:

define-compound-unit/infer: not a unit definition in: [email protected]

One way to solve this is to use define-unit-binding to set up the static information for the new contracted value. Another possibility for unit definitions is to use define-unit/contract:

(define-unit/contract [email protected]
 (import)
 (export (toy-factory^
          [build-toys (-> integer? (listof toy?))]
          [repaint    (-> toy? symbol? toy?)]
          [toy?       (-> any/c boolean?)]
          [toy-color  (-> toy? symbol?)]))

 (define-struct toy (color) #:transparent)

 (define (build-toys n)
   (for/list ([i (in-range n)])
     (make-toy 'blue)))

 (define (repaint t col)
   (make-toy col)))

More about these features can be found in the Reference, and a short section about signature and unit contracts has been added to the Guide.

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