• |  | Option Contracts

Option Contracts
option/ c
exercise-option
transfer/ c
waive-option
tweak-option
has-option?
has-option-with-contract?
invariant/ c

6.8
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## Option Contracts

(require racket/contract/option)
| package: option-contract-lib

This module introduces option contracts, a flavor of behavioral software contracts. With option contracts developers control in a programmatic manner whether, when, and how often contracts are checked. Using this flavor of contracts, Racketeers can mimic any compiler flag system but also create run-time informed checking systems.

> > procedure
> > (option/c |  | c |  |  |  |
> > | [ | #:with-contract with |  |  |  |
> > |  | #:tester tester |  |  |  |
> > |  | #:invariant invariant |  |  |  |
> > |  | #:immutable immutable |  |  |  |
> > |  | #:flat? flat? |  |  |  |
> > |  | #:struct struct-id]) |  | → |  | contract?
> c : contract?
> with : boolean? = #f
> tester : (or/c (-> any boolean?) 'dont-care) = 'dont-care
> invariant : (or/c (-> any boolean?) 'dont-care) = 'dont-care
> immutable : (or/c #t #f 'dont-care) = 'dont-care
> flat? : boolean? = #f
> struct-id : (or/c identifier? 'none) = 'none

Returns a contract that recognizes vectors or hashes or instances of struct struct-id. The data structure must match c and pass the tester.

When an option/c contract is attached to a value, the value is checked against the tester, if tester is a predicate. After that, contract checking is disabled for the value, if with is #f. If with is #t contract checking for the value remains enabled for c.

If waive-option is applied to a value guarded by an option/c contract, then waive-option returns the value after removing the option/c guard. If exercise-option is applied to a value guarded by an option/c contract, then exercise-option returns the value with contract checking enabled for c. If the invariant argument is a predicate, then exercise-option returns the value with contract checking enabled for (invariant/c c invariant #:immutable immutable #:flat? flat? #:struct struct-id).

The arguments flat? and immutable should be provided only if invariant is a predicate. In any other case, the result is a contract error.

Examples:

> > (module server0 racket
> (require racket/contract/option)
> (provide
> (contract-out
> [vec (option/c (vectorof number?))]))
> (define vec (vector 1 2 3 4)))
> > (require 'server0)
> > (vector-set! vec 1 'foo)
> > (vector-ref vec 1)
> 'foo
> > (module server1 racket
> (require racket/contract/option)
> (provide
> (contract-out
> [vec (option/c (vectorof number?) #:with-contract #t)]))
> (define vec (vector 1 2 3 4)))
> > (require 'server1)
> > (vector-set! vec 1 'foo)
> vec: contract violation
> expected: number?
> given: 'foo
> in: an element of
> the option of
> (option/c
> (vectorof number?)
> #:with-contract
> #t)
> contract from: server1
> blaming: top-level
> (assuming the contract is correct)
> at: eval:6.0
> > (module server2 racket
> (require racket/contract/option)
> (provide
> (contract-out
> [vec (option/c (vectorof number?) #:tester sorted?)]))
> (define vec (vector 1 42 3 4))
> (define (sorted? vec)
> (for/and ([el vec]
> [cel (vector-drop vec 1)])
> (<= el cel))))
> > (require 'server2)
> vec: broke its own contract;
> in: option contract tester #<procedure:sorted?> of
> (option/c
> (vectorof number?)
> #:tester
> #<procedure:sorted?>)
> contract from: server2
> blaming: server2
> (assuming the contract is correct)
> at: eval:9.0

> procedure (exercise-option x) → any/c
> x : any/c

Returns x with contract checking enabled if an option/c guards x. In any other case it returns x. The result of exercise-option loses the guard related to option/c, if it has one to begin with, and thus its contract checking status cannot change further.

Examples:

> > (module server3 racket
> (require racket/contract/option)
> (provide (contract-out [foo (option/c (-> number? symbol?))]))
> (define foo (λ (x) x)))
> > (require 'server3 racket/contract/option)
> (define e-foo (exercise-option foo))
> > (foo 42)
> 42
> > (e-foo 'wrong)
> foo: contract violation
> expected: number?
> given: 'wrong
> in: the 1st argument of
> the option of
> (option/c (-> number? symbol?))
> contract from: server3
> blaming: top-level
> (assuming the contract is correct)
> at: eval:11.0
> > ((exercise-option e-foo) 'wrong)
> foo: contract violation
> expected: number?
> given: 'wrong
> in: the 1st argument of
> the option of
> (option/c (-> number? symbol?))
> contract from: server3
> blaming: top-level
> (assuming the contract is correct)
> at: eval:11.0

> value transfer/c : contract?

A contract that accepts any value. If the value is guarded with an option/c contract, transfer/c modifies the blame information for the option/c contract by adding the providing module and its client to the positive and negative blame parties respectively. If the value is not a value guarded with an option/c contract, then transfer/c is equivalent to any/c.

Examples:

> > (module server4 racket
> (require racket/contract/option)
> (provide (contract-out [foo (option/c (-> number? symbol?))]))
> (define foo (λ (x) x)))
> > (module middleman racket
> (require racket/contract/option 'server4)
> (provide (contract-out [foo transfer/c])))
> > (require 'middleman racket/contract/option)
> (define e-foo (exercise-option foo))
> > (e-foo 1)
> foo: broke its own contract
> promised: symbol?
> produced: 1
> in: the range of
> the option of
> (option/c (-> number? symbol?))
> contract from: server4
> blaming multiple parties:
> middleman
> server4
> (assuming the contract is correct)
> at: eval:17.0
> > (module server5 racket
> (require racket/contract/option)
> (provide (contract-out [boo transfer/c]))
> (define (boo x) x))
> > (require 'server5)
> > (boo 42)
> 42

> procedure (waive-option x) → any/c
> x : any/c

If an option/c guards x, then waive-option returns x without the option/c guard. In any other case it returns x. The result of waive-option loses the guard related to option/c, if it had one to begin with, and thus its contract checking status cannot change further.

Examples:

> > (module server6 racket
> (require racket/contract/option)
> (provide (contract-out [bar (option/c (-> number? symbol?))]))
> (define bar (λ (x) x)))
> > (require 'server6 racket/contract/option)
> (define e-bar (waive-option bar))
> > (e-bar 'wrong)
> 'wrong
> > ((waive-option e-bar) 'wrong)
> 'wrong

> procedure (tweak-option x) → any/c
> x : any/c

If an option/c guards x and contract checking for x is enabled, then tweak-option returns x with contract checking for x disabled. If an option/c guards x and contract checking for x is disabled, then tweak-option returns x with contract checking for x enabled. In any other case it returns x. The result of tweak-option retains the guard related to option/c if it has one to begin with and thus its contract checking status can change further using tweak-option, exercise-option or waive-option.

Examples:

> > (module server7 racket
> (require racket/contract/option)
> (provide (contract-out [bar (option/c (-> number? symbol?))]))
> (define bar (λ (x) x)))
> > (require 'server7 racket/contract/option)
> (define t-bar (tweak-option bar))
> > (t-bar 'wrong)
> bar: contract violation
> expected: number?
> given: 'wrong
> in: the 1st argument of
> the option of
> (option/c (-> number? symbol?))
> contract from: server7
> blaming: top-level
> (assuming the contract is correct)
> at: eval:30.0
> > ((tweak-option t-bar) 'wrong)
> 'wrong
> > ((waive-option t-bar) 'wrong)
> 'wrong
> > ((exercise-option t-bar) 'wrong)
> bar: contract violation
> expected: number?
> given: 'wrong
> in: the 1st argument of
> the option of
> (option/c (-> number? symbol?))
> contract from: server7
> blaming: top-level
> (assuming the contract is correct)
> at: eval:30.0

> procedure (has-option? v) → boolean?
> v : any/c

Returns #t if v has an option contract.

> procedure (has-option-with-contract? v) → boolean?
> v : any/c

Returns #t if v has an option contract with contract checking enabled.

> > procedure
> > (invariant/c |  | c |  |  |  |
> > |  | invariant |  |  |  |
> > | [ | #:immutable immutable |  |  |  |
> > |  | #:flat? flat? |  |  |  |
> > |  | #:struct struct-id]) |  | → |  | contract?
> c : contract?
> invariant : (-> any boolean?)
> immutable : (or/c #t #f 'dont-care) = 'dont-care
> flat? : boolean? = #f
> struct-id : (or/c identifier? 'none) = 'none

Returns a contract that recognizes vectors or hashes or instances of struct struct-id. The data structure must match c and satisfy the invariant argument.

If the flat? argument is #t, then the resulting contract is a flat contract, and the c arguments must also be flat contracts. Such flat contracts will be unsound if applied to a mutable data structure, as they will not check future operations on the vector.

If the immutable argument is #t and the c arguments are flat contracts, the result will be a flat contract. If the c arguments are chaperone contracts, then the result will be a chaperone contract.

Examples:

> > (module server8 racket
> (require racket/contract/option)
> (provide
> change
> (contract-out
> [vec (invariant/c
> any/c
> sorted?)]))
> (define vec (vector 1 2 3 4 5))
> (define (change) (vector-set! vec 2 42))
> (define (sorted? vec)
> (for/and ([el vec]
> [cel (vector-drop vec 1)])
> (<= el cel))))
> > (require 'server8)
> > (vector-set! vec 2 42)
> vec: contract violation
> expected vector that satisfies #<procedure:sorted?> given:
> '#(1 2 42 4 5)
> in: (invariant/c any/c #<procedure:sorted?>)
> contract from: server8
> blaming: top-level
> (assuming the contract is correct)
> at: eval:37.0
> > (change)
> > (vector-ref vec 2)
> vec: broke its own contract
> expected vector that satisfies #<procedure:sorted?> given:
> '#(1 2 42 4 5)
> in: (invariant/c any/c #<procedure:sorted?>)
> contract from: server8
> blaming: server8
> (assuming the contract is correct)
> at: eval:37.0

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