On this page:
non-empty-string?
non-empty-list?
non-empty-bytes?
non-empty-vector?
singleton-list?
port-number?
tcp-listen-port?
path-piece?
if/ c
failure-result/ c
rename-contract
option/ c
4.1 Flat Contracts
nat/ c
pos/ c
truth/ c
4.2 Syntax Object Contracts
syntax-datum/ c
syntax-listof/ c
syntax-list/ c
4.3 Higher-Order Contracts
thunk/ c
unary/ c
binary/ c
predicate/ c
predicate-like/ c
comparison/ c
comparison-like/ c
sequence/ c
dict/ c
Version: 5.2

4 Contracts

 (require unstable/contract)

This library is unstable; compatibility will not be maintained. See Unstable: May Change Without Warning for more information.

(non-empty-string? x)  boolean?
  x : any/c
(non-empty-list? x)  boolean?
  x : any/c
(non-empty-bytes? x)  boolean?
  x : any/c
(non-empty-vector? x)  boolean?
  x : any/c
Returns #t if x is of the appropriate data type (string, list, bytes, or vector, respectively) and is not empty; returns #f otherwise.

(singleton-list? x)  boolean?
  x : any/c
Returns #t if x is a list of one element; returns #f otherwise.

Equivalent to (between/c 1 65535).

Equivalent to (between/c 0 65535).

Equivalent to (or/c path-string? (symbols 'up 'same)).

The subsequent bindings were added by Ryan Culpepper.

(if/c predicate then-contract else-contract)  contract?
  predicate : (-> any/c any/c)
  then-contract : contract?
  else-contract : contract?
Produces a contract that, when applied to a value, first tests the value with predicate; if predicate returns true, the then-contract is applied; otherwise, the else-contract is applied. The resulting contract is a flat contract if both then-contract and else-contract are flat contracts.

For example, the following contract enforces that if a value is a procedure, it is a thunk; otherwise it can be any (non-procedure) value:
Note that the following contract is not equivalent:

(or/c (-> any) any/c) ; wrong!

The last contract is the same as any/c because or/c tries flat contracts before higher-order contracts.

A contract that describes the failure result arguments of procedures such as hash-ref.

Equivalent to (if/c procedure? (-> any) any/c).

(rename-contract contract name)  contract?
  contract : contract?
  name : any/c
Produces a contract that acts like contract but with the name name.

The resulting contract is a flat contract if contract is a flat contract.

The subsequent bindings were added by Asumu Takikawa.

(option/c contract)  contract?
  contract : contract?
Creates a contract that acts like contract but will also accept #f. Intended to describe situations where a failure or default value may be used.

The subsequent bindings were added by Carl Eastlund <cce@racket-lang.org>.

4.1 Flat Contracts

This contract recognizes natural numbers that satisfy exact-nonnegative-integer?.

This contract recognizes positive integers that satisfy exact-positive-integer?.

This contract recognizes Scheme truth values, i.e., any value, but with a more informative name and description. Use it in negative positions for arguments that accept arbitrary truth values that may not be booleans.

4.2 Syntax Object Contracts

(syntax-datum/c datum/c)  flat-contract?
  datum/c : any/c
Recognizes syntax objects stx such that (syntax->datum stx) satisfies datum/c.

(syntax-listof/c elem/c)  flat-contract?
  elem/c : any/c
Recognizes syntax objects stx such that (syntax->list stx) satisfies (listof elem/c).

(syntax-list/c elem/c ...)  flat-contract?
  elem/c : any/c
Recognizes syntax objects stx such that (syntax->list stx) satisfies (list/c elem/c ...).

4.3 Higher-Order Contracts

These contracts recognize functions that accept 0, 1, or 2 arguments, respectively, and produce a single result.

These contracts recognize predicates: functions of a single argument that produce a boolean result.

The first constrains its output to satisfy boolean?. Use predicate/c in positive position for predicates that guarantee a result of #t or #f.

The second constrains its output to satisfy truth/c. Use predicate-like/c in negative position for predicates passed as arguments that may return arbitrary values as truth values.

These contracts recognize comparisons: functions of two arguments that produce a boolean result.

The first constrains its output to satisfy boolean?. Use comparison/c in positive position for comparisons that guarantee a result of #t or #f.

The second constrains its output to satisfy truth/c. Use comparison-like/c in negative position for comparisons passed as arguments that may return arbitrary values as truth values.

(sequence/c elem/c ...)  contract?
  elem/c : contract?
Wraps a sequence, obligating it to produce as many values as there are elem/c contracts, and obligating each value to satisfy the corresponding elem/c. The result is not guaranteed to be the same kind of sequence as the original value; for instance, a wrapped list is not guaranteed to satisfy list?.

Examples:

> (define/contract predicates
    (sequence/c (-> any/c boolean?))
    (in-list (list integer?
                   string->symbol)))
> (for ([P predicates])
    (printf "~s\n" (P "cat")))

#f

predicates: self-contract violation, expected: boolean?,

given: 'cat

  contract from: (definition predicates), blaming:

(definition predicates)

  contract:

    (sequence/c (-> any/c boolean?))

  at: eval:2.0

> (define/contract numbers&strings
    (sequence/c number? string?)
    (in-dict (list (cons 1 "one")
                   (cons 2 "two")
                   (cons 3 'three))))
> (for ([(N S) numbers&strings])
    (printf "~s: ~a\n" N S))

1: one

2: two

numbers&strings: self-contract violation, expected:

string?, given: 'three

  contract from: (definition numbers&strings), blaming:

(definition numbers&strings)

  contract: (sequence/c number? string?)

        at: eval:4.0

(dict/c key/c value/c)  contract?
  key/c : contract?
  value/c : contract?
Wraps a dictionary, obligating its keys to satisfy key/c and their corresponding values to satisfy value/c. The result is not guaranteed to be the same kind of dictionary as the original value; for instance, a wrapped hash table is not guaranteed to satisfy hash?.

Examples:

> (define/contract table
    (dict/c symbol? string?)
    (make-immutable-hash (list (cons 'A "A") (cons 'B 2) (cons 3 "C"))))
> (dict-ref table 'A)

"A"

> (dict-ref table 'B)

table: self-contract violation, expected: string?, given: 2

  contract from: (definition table), blaming: (definition

table)

  contract: (dict/c symbol? string?)

        at: eval:6.0

> (dict-ref table 3)

table: contract violation, expected: symbol?, given: 3

  contract from: top-level, blaming: (definition table)

  contract: (dict/c symbol? string?)

        at: eval:6.0

Warning: Bear in mind that key and value contracts are re-wrapped on every dictionary operation, and dictionaries wrapped in dict/c multiple times will perform the checks as many times for each operation. Especially for immutable dictionaries (which may be passed through a constructor that involves dict/c on each update), contract-wrapped dictionaries may be much less efficient than the original dictionaries.