8.7 Building New Contract Combinators

The name argument is any value to be rendered using display to describe the contract when a violation occurs. The default name for simple higher-order contracts is anonymous-contract, for chaperone contracts is anonymous-chaperone-contract, and for flat contracts is anonymous-flat-contract.

The first-order predicate test is used to determine which values the contract applies to. This test is used by contract-first-order-passes?, and indirectly by or/c and from-or/c to determine which higher-order contract to wrap a value with when there are multiple higher-order contracts to choose from. The default test accepts any value. The predicate should be influenced by the value of (contract-first-order-okay-to-give-up?) (see it’s documentation for more explanation).

The late-neg-proj argument defines the behavior of applying the contract via a late neg projection. If it is supplied, this argument accepts a blame object that is missing one party (see also blame-missing-party?). Then it must return a function that accepts both the value that is getting the contract and the name of the missing blame party, in that order. The result must either be the value (perhaps suitably wrapped with a chaperone or impersonator to enforce the contract), or signal a contract violation using raise-blame-error. The default is #f.

The projection proj and val-first-proj are older mechanisms for defining the behavior of applying the contract. The proj argument is a curried function of two arguments: the first application accepts a blame object, and the second accepts a value to protect with the contract. The projection must either produce the value, suitably wrapped to enforce any higher-order aspects of the contract, or signal a contract violation using raise-blame-error. The default projection produces an error when the first-order test fails, and produces the value unchanged otherwise. The val-first-proj is like late-neg-proj, except with an extra layer of currying.

At least one of the late-neg-proj, proj, val-first-proj, or test must be non-#f.

The projection arguments (late-neg-proj, proj, and val-first-proj) must be in sync with the test argument. In particular, if the test argument returns #f for some value, then the projections must raise a blame error for that value and if the test argument returns #t for some value, then the projection must not signal any blame for this value, unless there are higher-order interactions later. In other words, for flat contracts, the test and projection arguments must check the same predicate (which is why thee default projection uses the test argument directly).

Projections for chaperone contracts must produce a value that passes chaperone-of? when compared with the original, uncontracted value. Projections for flat contracts must fail precisely when the first-order test does, and must produce the input value unchanged otherwise. Applying a flat contract may result in either an application of the predicate, or the projection, or both; therefore, the two must be consistent. The existence of a separate projection only serves to provide more specific error messages. Most flat contracts do not need to supply an explicit projection.

The stronger argument is used to implement contract-stronger?. The first argument is always the contract itself and the second argument is whatever was passed as the second argument to contract-stronger?. If no stronger argument is supplied, then a default that compares its arguments with equal? is used for flat contracts and chaperone contracts. For impersonator contracts constructed with make-contract that do not supply the stronger argument, contract-stronger? returns #f.

Similarly, the equivalent argument is used to implement contract-equivalent?. If it isn’t supplied or #false is supplied, then equal? is used for chaperone and flat contracts, and (λ (x y) #f) is used otherwise.

The is-list-contract? argument is used by the list-contract? predicate to determine if this is a contract that accepts only list? values.

Changed in version 6.0.1.13 of package base: Added the #:list-contract? argument.
Changed in version 6.90.0.30: Added the #:equivalent argument.

If v is not one of the coercible values, coerce-contract signals an error, using the first argument in the error message.

See make-contract for more details.

Added in version 6.1.1.5 of package base.

If c does not have a late-neg contract, then this function uses the original projection for it and logs a warning to the 'racket/contract logger.

See make-contract for more details.

Added in version 6.2.900.11 of package base.

If your combinator’s projections operate on complete blame objects (i.e., no missing blame parties), the blame object should be the first argument to this form. Otherwise (e.g., in the case of late-neg projections), a pair of the blame object and the missing party should be used instead.

Added in version 6.4.0.4 of package base.

To avoid that explosion, wrap each of the calls to the blame-accepting portion of the combinator in contract-pos/neg-doubling. It returns three values. The first is a boolean, indicating how to interpret the other two results. If the boolean is #t, then the other two results are the values of e1 and e2 and we are not too deep in the nesting. If the boolean is #f, then we have passed a threshold and it is not safe to evaluate e1 and e2 yet, as we are in danger of running into the exponential slowdown. In that case, the last two results are thunks that, when invoked, compute the values of e1 and e2.

As an example, vectorof uses contract-pos/neg-doubling wrapping its two calls to the blame-accepting part of the projection for its subcontract. When it receives a #f as that first boolean, it does not invoke the thunks right away, but waits until the interposition procedure that it attaches to the chaperoned vector is called. Then it invokes them (and caches the result). This delays the construction of the projections until they are actually needed, avoiding the exponential blowup.

Added in version 6.90.0.27 of package base.

8.7.1 Blame Objects

This section describes blame objects and operations on them.

The second positional argument, v, is the value that failed to satisfy the contract.

The remaining arguments are a format string, fmt, and its arguments, v-fmt ..., specifying an error message specific to the precise violation.

If fmt is a list, then the elements are concatenated together (with spaces added, unless there are already spaces at the ends of the strings), after first replacing symbols with either their string counterparts, or replacing 'given with "produced" and 'expected with "promised", depending on whether or not the b argument has been swapped or not (see blame-swap).

If fmt contains the symbols 'given: or 'expected:, they are replaced like 'given and 'expected are, but the replacements are prefixed with the string "\n  " to conform to the error message guidelines in Error Message Conventions.

The context argument describes one layer of the portion of the contract, typically of the form "the 1st argument of" (in the case of a function contract) or "a conjunct of" (in the case of an and/c contract).

The important argument is used to build the beginning part of the contract violation. The last important argument that gets added to a blame object is used. The class/c contract adds an important argument, as does the -> contract (when -> knows the name of the function getting the contract).

The swap? argument has the effect of calling blame-swap while adding the layer of context, but without creating an extra blame object.

Passing #f as the context string argument is no longer relevant. For backwards compatibility, blame-add-context returns b when context is #f.

Changed in version 6.90.0.29 of package base: The context argument being #f is no longer relevant.

8.7.2 Contracts as structs

The property prop:contract allows arbitrary structures to act as contracts. The property prop:chaperone-contract allows arbitrary structures to act as chaperone contracts; prop:chaperone-contract inherits prop:contract, so chaperone contract structures may also act as general contracts. The property prop:flat-contract allows arbitrary structures to act as flat contracts; prop:flat-contract inherits both prop:chaperone-contract and prop:procedure, so flat contract structures may also act as chaperone contracts, as general contracts, and as predicate procedures.

The value is expected to be the blame record for the contract on the value or a cons-pair of a blame record with a missing party and the missing party. The value-blame function reassembles the arguments of the pair into a complete blame record using blame-add-missing-party. If the value has one of the properties, but the value is not a blame object or a pair whose car position is a blame object, then has-blame? returns #f but value-blame returns #f.

At least one of the late-neg-proj, get-projection, val-first-proj, or get-first-order must be non-#f.

These accessors are passed as (optional) keyword arguments to build-contract-property, and are applied to instances of the appropriate structure type by the contract system. Their results are used analogously to the arguments of make-contract.

A chaperone contract property specifies the behavior of a structure when used as a chaperone contract. It is specified using build-chaperone-contract-property, and accepts exactly the same set of arguments as build-contract-property. The only difference is that the projection accessor must return a value that passes chaperone-of? when compared with the original, uncontracted value.

Changed in version 6.0.1.13 of package base: Added the #:list-contract? argument.
Changed in version 6.1.1.4: Allow generate to return contract-random-generate-fail.
Changed in version 6.90.0.30: Added the #:equivalent argument.

8.7.3 Obligation Information in Check Syntax

Check Syntax in DrRacket shows obligation information for contracts according to syntax-propertys that the contract combinators leave in the expanded form of the program. These properties indicate where contracts appear in the source and where the positive and negative positions of the contracts appear.

To make Check Syntax show obligation information for your new contract combinators, use the following properties (some helper macros and functions are below):

• 'racket/contract:contract : (vector/c symbol? (listof syntax?) (listof syntax?))
  This property should be attached to the result of a transformer that implements a contract combinator. It signals to Check Syntax that this is where a contract begins.

  The first element in the vector should be a unique (in the sense of eq?) value that Check Syntax can use a tag to match up this contract with its subpieces (specified by the two following syntax properties).

  The second and third elements of the vector are syntax objects from pieces of the contract, and Check Syntax will color them. The first list should contain subparts that are the responsibility of parties (typically modules) that provide implementations of the contract. The second list should contain subparts that are the responsibility of clients.

  For example, in (->* () #:pre #t any/c #:post #t), the ->* and the #:post should be in the first list and #:pre in the second list.

• 'racket/contract:negative-position : symbol?
  This property should be attached to sub-expressions of a contract combinator that are expected to be other contracts. The value of the property should be the key (the first element from the vector for the 'racket/contract:contract property) indicating which contract this is.

  This property should be used when the expression’s value is a contract that clients are responsible for.

• 'racket/contract:positive-position : symbol?
  This form is just like 'racket/contract:negative-position, except that it should be used when the expression’s value is a contract that the original party should be responsible for.

• 'racket/contract:contract-on-boundary : symbol?
  The presence of this property tells Check Syntax that it should start coloring from this point. It expects the expression to be a contract (and, thus, to have the 'racket/contract:contract property); this property indicates that this contract is on a (module) boundary.

  (The value of the property is not used.)

• 'racket/contract:internal-contract : symbol?
  Like 'racket/contract:contract-on-boundary, the presence of this property triggers coloring, but this is meant for use when the party (module) containing the contract (regardless of whether or not this module exports anything matching the contract) can be blamed for violating the contract. This comes into play for ->i contracts, since the contract itself has access to values under contract via the dependency.