Signatures

A signature expression is a sequence of signature elements:

 
  (sig-element   ) 

Each signature element has one of the following forms:

 
   id 
   (struct id (id   ) omission   ) 
   (unit id : signature) 
   (open signature) 

• The simple id form adds a run-time variable to the signature.
• The struct form expands into the list of run-time variables generated by a define-struct expression with the given base identifier and field identifiers. A field identifier can be omitted to avoid exporting/importing the corresponding selector and setter identifiers. The omission sub-clauses are optional; each omission can either be -selectors (all field selector identifiers are omitted), -setters (all field setter identifiers are omitted) or (- id) (a specific generated id is ommitted).
• The unit form creates a sub-signature within the signature. When a signature includes a unit clause, it can only apply to a compound signed unit that exports an entire sub-unit.
• The open form textually includes another signature (creating sub-signatures only when the included signature has sub-signatures).

Two signatures exactly match when they have the same variable identifiers and matching sub-signatures with the same names all in the same order. A signature conservatively matches another signature when it has the same names in the same order, but also contains additional names.

The define-signature form binds a signature to an identifier:

 
  (define-signature id signature) 

In unit and open clauses of a signature, signature can either be an inline signature specification or a bound signature identifier. The same is true for define-signature and all occurrences of a signature in unit/sig, compound-unit/sig,  invoke-unit/sig,  invoke-open-unit/sig, and  unit->unit/sig expressions.

The let-signature form binds a signature to an identifier within a body of expressions. Embedded define-signature expressions are transformed into let-signature expressions.