Signatures

A signature is either a signature description or a bound signature identifier:

  (sig-element   ) 
  signature-identifier

  sig-element is one of:
     variable 
     (struct base-identifier (field-identifier   ) omission   ) 
     (open signature) 
     (unit identifier : signature)

  omission is one of:
     -selectors 
     -setters 
     (- variable) 

Together, the element descriptions determine the set of elements that compose the signature:

• The simple variable form adds a variable name to the new signature.
• The struct form expands into the list of variable names generated by a define-struct expression with the given base-identifier and field-identifiers.

  The actual structure type can contain additional fields; if a field identifier is omitted, the corresponding selector and setter names are not added to the signature. Optional omission specifications can omit other kinds of names: -selectors omits all field selector variables. -setters omits all field setter variables, and (- variable) omits a specific generated variable.

• The open form copies all of the elements of another signature into the new signature description.
• The unit form creates a sub-signature within the new signature. A signature that includes a unit clause corresponds to a signed compound unit that exports an embedded unit. (Embedded units are described in section 7.2.4 and section 7.3.3.)

The names of all elements in a signature must be distinct. Two signatures match when they contain the same element names, and when a name in both signatures is either a variable name in both signatures or a sub-signature name in both signatures such that the sub-signatures match. The order of elements within a signature is not important. A source signature satisfies a destination signature when the source signature has all of the elements of the destination signature, but the source signature may have additional elements.

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

  (define-signature signature-identifier signature) 

The let-signature form binds a signature to an identifier within a body of expressions:

  (let-signature identifier signature body-expr   ) 

Embedded define-signature expressions are transformed into let-signature expressions.

• Flattening Signatures