7.9 Unit Contracts
sig-block = (tagged-sig-id [id contract] ...) | tagged-sig-id
A unit contract wraps a unit and checks both its imported and exported identifiers to ensure that they match the appropriate contracts. This allows the programmer to add contract checks to a single unit value without adding contracts to the imported and exported signatures.
The unit value must import a subset of the import signatures and export a superset of the export signatures listed in the unit contract. Any identifier which is not listed for a given signature is left alone. Variables used in a given contract expression first refer to other variables in the same signature, and then to the context of the unit/c expression.
(define-unit/contract unit-id (import sig-spec-block ...) (export sig-spec-block ...) init-depends-decl unit-body-expr-or-defn ...)
sig-spec-block = (tagged-sig-spec [id contract] ...) | tagged-sig-spec
The define-unit/contract form defines a unit compatible with link inference whose imports and exports are contracted with a unit contract. The unit name is used for the positive blame of the contract.