#### 7.1Data-structure Contracts

A flat contract can be fully checked immediately for a given value.

 (flat-contract predicate) → flat-contract? predicate : (any/c . -> . any/c)
Constructs a flat contract from predicate. A value satisfies the contract if the predicate returns a true value.

 (flat-named-contract type-name predicate) → flat-contract? type-name : any/c predicate : (or/c flat-contract? (any/c . -> . any))
On predicates like flat-contract, but the first argument must be the (quoted) name of a contract used for error reporting. For example,
 (flat-named-contract 'odd-integer (lambda (x) (and (integer? x) (odd? x))))
turns the predicate into a contract with the name odd-integer.

On flat contracts, the new flat contract is the same as the old except for the name.

 any/c : flat-contract?
A flat contract that accepts any value.

When using this contract as the result portion of a function contract, consider using any instead; using any leads to better memory performance, but it also allows multiple results.

 none/c : flat-contract?
A flat contract that accepts no values.

 (or/c contract ...) → contract? contract : contract?
Takes any number of contracts and returns a contract that accepts any value that any one of the contracts accepts, individually.

The or/c result tests any value by applying the contracts in order, from left to right, with the exception that it always moves the non-flat contracts (if any) to the end, checking them last. Thus, a contract such as (or/c (not/c real?) positive?) is guaranteed to only invoke the positive? predicate on real numbers.

If all of the arguments are procedures or flat contracts, the result is a flat contract. If only one of the arguments is a higher-order contract, the result is a contract that just checks the flat contracts and, if they don’t pass, applies the higher-order contract.

If there are multiple higher-order contracts, or/c uses contract-first-order-passes? to distinguish between them. More precisely, when an or/c is checked, it first checks all of the flat contracts. If none of them pass, it calls contract-first-order-passes? with each of the higher-order contracts. If only one returns true, or/c uses that contract. If none of them return true, it signals a contract violation. If more than one returns true, it also signals a contract violation. For example, this contract
 (or/c (-> number? number?) (-> string? string? string?))
does not accept a function like this one: (lambda args ...) since it cannot tell which of the two arrow contracts should be used with the function.

 (and/c contract ...) → contract? contract : (or/c contract? (any/c . -> . any/c))
Takes any number of contracts and returns a contract that checks that accepts any value that satisfies all of the contracts, simultaneously.

If all of the arguments are procedures or flat contracts, the result is a flat contract.

The contract produced by and/c tests any value by applying the contracts in order, from left to right.

 (not/c flat-contract) → flat-contract? flat-contract : (or/c flat-contract? (any/c . -> . any/c))
Accepts a flat contracts or a predicate and returns a flat contract that checks the inverse of the argument.

 (=/c z) → flat-contract? z : real?
Returns a flat contract that requires the input to be a number and = to z.

 (
Returns a flat contract that requires the input to be a number and < to n.

 (>/c n) → flat-contract? n : real?
Like </c, but for >.

 (<=/c n) → flat-contract? n : real?
Like </c, but for <=.

 (>=/c n) → flat-contract? n : real?
Like </c, but for >=.

 (between/c n m) → flat-contract? n : real? m : real?
Returns a flat contract that requires the input to be a between n and m or equal to one of them.

 (real-in n m) → flat-contract? n : real? m : real?
Returns a flat contract that requires the input to be a real number between n and m, inclusive.

 (integer-in j k) → flat-contract? j : exact-integer? k : exact-integer?
Returns a flat contract that requires the input to be an exact integer between j and k, inclusive.

 natural-number/c : flat-contract?
A flat contract that requires the input to be an exact non-negative integer.

 (string-len/c len) → flat-contract? len : exact-nonnegative-integer?
Returns a flat contract that recognizes strings that have fewer than len characters.

 false/c : flat-contract?
This is just #f. It is here for backwards compatibility.

 printable/c : flat-contract?
A flat contract that recognizes values that can be written out and read back in with write and read.

 (one-of/c v ...+) → flat-contract? v : any/c
Accepts any number of atomic values and returns a flat contract that recognizes those values, using eqv? as the comparison predicate. For the purposes of one-of/c, atomic values are defined to be: characters, symbols, booleans, null keywords, numbers, void, and undefined.

 (symbols sym ...+) → flat-contract? sym : symbol?
Accepts any number of symbols and returns a flat contract that recognizes those symbols.

 (vectorof c [ #:immutable immutable #:flat? flat?]) → contract?
c : contract?
immutable : (or/c #t #f 'dont-care) = 'dont-care
flat? : boolean? = #f
Returns a contract that recognizes vectors. The elements of the vector must match c.

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

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

When a higher-order vectorof contract is applied to a vector, the result is not eq? to the input. The result will be a copy for immutable vectors and a chaperone or proxy of the input for mutable vectors.

 (vector-immutableof c) → contract? c : contract?
Returns the same contract as (vectorof c #:immutable #t). This exists for reasons of backwards compatibility, and may be removed in the future.

 (vector/c c ... [ #:immutable immutable #:flat? flat?]) → contract?
c : contract?
immutable : (or/c #t #f 'dont-care) = 'dont-care
flat? : boolean? = #f
Returns a contract that recognizes vectors whose length match the number of contracts given. Each element of the vector must match its corresponding contract.

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 mutable vectors, 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.

When a higher-order vector/c contract is applied to a vector, the result is not eq? to the input. The result will be a copy for immutable vectors and a chaperone or proxy of the input for mutable vectors.

 (vector-immutable/c c ...) → contract? c : contract?
Returns the same contract as (vector/c c ... #:immutable #t). This exists for reasons of backwards compatibility, and may be removed in the future.

 (box/c c [ #:immutable immutable #:flat? flat?]) → contract?
c : contract?
immutable : (or/c #t #f 'dont-care) = 'dont-care
flat? : boolean? = #f
Returns a contract that recognizes boxes. The content of the box must match c.

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

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

When a higher-order box/c contract is applied to a box, the result is not eq? to the input. The result will be a copy for immutable boxes and either a chaperone or proxy of the input for mutable boxes.

 (box-immutable/c c) → contract? c : contract?
Returns the same contract as (box/c c #:immutable #t). This exists for reasons of backwards compatibility, and may be removed in the future.

 (listof c) → contract? c : (or/c contract? (any/c . -> . any/c))
Returns a contract that recognizes a list whose every element matches the contract c. Beware that when this contract is applied to a value, the result is not necessarily eq? to the input.

 (non-empty-listof c) → contract? c : (or/c contract? (any/c . -> . any/c))
Returns a contract that recognizes non-empty lists whose elements match the contract c. Beware that when this contract is applied to a value, the result is not necessarily eq? to the input.

 (cons/c car-c cdr-c) → contract? car-c : contract? cdr-c : contract?
Produces a contract the recognizes pairs first and second elements match car-c and cdr-c, respectively. Beware that when this contract is applied to a value, the result is not necessarily eq? to the input.

 (list/c c ...) → contract? c : (or/c contract? (any/c . -> . any/c))
Produces a contract for a list. The number of elements in the list must match the number of arguments supplied to list/c, and each element of the list must match the corresponding contract. Beware that when this contract is applied to a value, the result is not necessarily eq? to the input.

 (syntax/c c) → flat-contract? c : flat-contract?
Produces a flat contract that recognizes syntax objects whose syntax-e content matches c.

 (struct/c struct-id flat-contract-expr ...)
Produces a flat contract that recognizes instances of the structure type named by struct-id, and whose field values match the flat contracts produced by the flat-contract-exprs.

 (parameter/c c) → contract? c : contract?
Produces a contract on parameters whose values must match contract.

 (hash/c key val [ #:immutable immutable #:flat? flat?]) → contract?
key : chaperone-contract?
val : contract?
immutable : (or/c #t #f 'dont-care) = 'dont-care
flat? : boolean? = #f
Produces a contract that recognizes hash tables with keys and values as specified by the key and val arguments.

If the flat? argument is #t, then the resulting contract is a flat contract, and the key and val arguments must also be flat contracts. Such flat contracts will be unsound if applied to mutable hash tables, as they will not check future operations on the hash table.

If the immutable argument is #t and the key and val arguments are flat contracts, the result will be a flat contract. If either the domain or the range is a chaperone contract, then the result will be a chaperone contract.

If the key argument is a chaperone contract, then the resulting contract can only be applied to equal?-based hash tables. When a higher-order hash/c contract is applied to a hash table, the result is not eq? to the input. The result will be a copy for immutable hash tables, and either a chaperone or proxy of the input for mutable hash tables.

 (flat-rec-contract id flat-contract-expr ...)

Constructs a recursive flat contract. A flat-contract-expr can refer to id to refer recursively to the generated contract.

For example, the contract

 (flat-rec-contract sexp (cons/c sexp sexp) number? symbol?)

is a flat contract that checks for (a limited form of) S-expressions. It says that an sexp is either two sexp combined with cons, or a number, or a symbol.

Note that if the contract is applied to a circular value, contract checking will not terminate.}

 (flat-murec-contract ([id flat-contract-expr ...] ...) body ...+)
A generalization of flat-rec-contract for defining several mutually recursive flat contracts simultaneously. Each id is visible in the entire flat-murec-contract form, and the result of the final body is the result of the entire form.

 any
Represents a contract that is always satisfied. In particular, it can accept multiple values. It can only be used in a result position of contracts like ->. Using any elsewhere is a syntax error.

 (promise/c expr)
Constructs a contract on a promise. The contract does not force the promise, but when the promise is forced, the contract checks that the result value meets the contract produced by expr.

 (new-∃/c name) → contract? name : symbol?
Constructs a new existential contract.

Existential contracts accept all values when in positive positions (e.g., function returns) and wraps the value in an opaque struct, hiding the precise value. In negative positions (e.g. function inputs), it accepts only values that were previously accepted in negative positions (by checking for the wrappers).

For example, this contract:
 (let ([a (new-∃/c 'a)]) (-> (-> a a) any/c))
describes a function that accepts the identity function (or a non-terminating function) and returns an arbitrary value. That is, the first use of the a appears in a positive position and thus inputs to that function are wrapped with an opaque struct. Then, when the function returns, it is checked to see if the result is wrapped, since the second a appears in a negative position.