8.6 Attaching Contracts to Values

The contract-out form treats modules as units of blame. The module that defines the provided variable is expected to meet the positive (co-variant) positions of the contract. Each module that imports the provided variable must obey the negative (contra-variant) positions of the contract. Each contract-expr in a contract-out form is effectively moved to the end of the enclosing module, so a contract-expr can refer to variables that are defined later in the same module.

Only uses of the contracted variable outside the module are checked. Inside the module, no contract checking occurs.

The rename form of contract-out exports the first variable (the internal name) with the name specified by the second variable (the external name).

The struct form of contract-out provides a structure-type definition, and each field has a contract that dictates the contents of the fields. The structure-type definition must appear before the provide clause within the enclosing module. If the structure type has a parent, the second struct form (above) must be used, with the first name referring to the structure type to export and the second name referring to the parent structure type. Unlike a struct definition, however, all of the fields (and their contracts) must be listed. The contract on the fields that the sub-struct shares with its parent are only used in the contract for the sub-struct’s constructor, and the selector or mutators for the super-struct are not provided. The exported structure-type name always doubles as a constructor, even if the original structure-type name does not act as a constructor. If the #:omit-constructor option is present, the constructor is not provided.

The #:∃, #:exists, #:∀, and #:forall clauses define new abstract contracts. The variables are bound in the remainder of the contract-out form to new contracts that hide the values they accept and ensure that the exported functions are treated parametrically. See new-∃/c and new-∀/c for details on how the clauses hide the values.

The implementation of contract-out uses syntax-property to attach properties to the code it generates that records the syntax of the contracts in the fully expanded program. Specifically, the symbol 'provide/contract-original-contract is bound to vectors of two elements, the exported identifier and a syntax object for the expression that produces the contract controlling the export.

It re-exports id, but with positive blame associated to the module containing recontract-out instead of the location of the original site of id.

This can be useful when a public module wants to export an identifier from a private module but where any contract violations should be reported in terms of the public module instead of the private one.

Replacing the use of recontract-out with just recip would result in a contract violation blaming the private module.

8.6.1 Nested Contract Boundaries

The first with-contract form cannot appear in expression position. All names defined within the first with-contract form are visible externally, but those names listed in the wc-export list are protected with the corresponding contract. The body of the form allows definition/expression interleaving if its context does.

The second with-contract form must appear in expression position. The final body expression should return the same number of values as the number of contracts listed in the result-spec, and each returned value is contracted with its respective contract. The sequence of body forms is treated as for let.

The blame-id is used for the positive positions of contracts paired with exported ids. Contracts broken within the with-contract body will use the blame-id for their negative position.

If a free-var-list is given, then any uses of the free variables inside the body will be protected with contracts that blame the context of the with-contract form for the positive positions and the with-contract form for the negative ones.

The define/contract form treats the individual definition as a contract region. The definition itself is responsible for positive (co-variant) positions of the contract, and references to id outside of the definition must meet the negative positions of the contract. Since the contract boundary is between the definition and the surrounding context, references to id inside the define/contract form are not checked.

If a free-var-list is given, then any uses of the free variables inside the body will be protected with contracts that blame the context of the define/contract form for the positive positions and the define/contract form for the negative ones.

The define-struct/contract form only allows a subset of the struct-option keywords: #:mutable, #:transparent, #:auto-value, #:omit-define-syntaxes, #:property and #:omit-define-values.

Unlike the specification of a contract, an invariant-assertion does not establish a boundary between two parties. Instead, it simply attaches a logical assertion to the value. Because the form uses contract machinery to check the assertion, the surrounding module is treated as the party to be blamed for any violations of the assertion.

This means, for example, that the assertion is checked on recursive calls, when an invariant is used on the right-hand side of a definition:

Added in version 6.0.1.11 of package base.

8.6.2 Low-level Contract Boundaries

The identifier id is defined as a macro transformer that consults the context of its use to determine the name for negative blame assignment (using the entire module where a reference appears as the negative party).

The positive party defaults to the module containing the use of define-module-boundary-contract, but can be specified explicitly via the #:pos-source keyword.

The source location used in the blame error messages for the location of the place where the contract was put on the value defaults to the source location of the use of define-module-boundary-contract, but can be specified via the #:srcloc argument, in which case it can be any of the things that the third argument to datum->syntax can be.

The contract expression adds the contract specified by contract-expr to the value produced by to-protect-expr. The result of a contract expression is the result of the to-protect-expr expression, but with the contract specified by contract-expr enforced on to-protect-expr.

The values of positive-blame-expr and negative-blame-expr indicate how to assign blame for positive and negative positions of the contract specified by contract-expr. They may be any value, and are formatted as by display for purposes of contract violation error messages.

If specified, source-location-expr indicates the source location reported by contract violations. The expression must produce a srcloc structure, syntax object, #f, or a list or vector in the format accepted by the third argument to datum->syntax.