16 Enumerations

(require (planet untyped/unlib/enum))

Utilities for defining simple enumerations of booleans, symbols and integers. These are useful wherever you would normally use a small collection of literals to represent possible values of a variable, and test for value equality with eq?. The define-enum form binds the literals to Scheme identifiers so the compiler catches typos that might otherwise take a long time to debug.

(struct   enum   (name values pretty-values))

name : symbol?

values : (listof (U boolean? symbol? integer?))

pretty-values : (listof string?)

An enumeration. For each symbol in values there is a human-readable string equivalent in pretty-values.

(enum->string enum) → string?

enum : enum?

Returns a string representation of (enum-values enum), useful for including in debugging output.

Examples:

> (define-enum vehicle (car boat plane))

define-enum vehicle: binding discrepancy: car is predefined

and not eq? to car

> (enum->string vehicle)

"boat, plane"

(enum->pretty-string enum) → string?

enum : enum?

Returns a string representation of (enum-pretty-values enum), useful for describing the possible values to a user.

Examples:

> (define-enum vehicle (car boat plane))

define-enum vehicle: binding discrepancy: car is predefined

and not eq? to car

> (enum->pretty-string vehicle)

"boat, plane"

(enum-value? enum value) → boolean?

enum : enum?

value : any

Returns #t if value is a member of (enum-values enum).

Examples:

> (define-enum vehicle (car boat plane))

define-enum vehicle: binding discrepancy: car is predefined

and not eq? to car

> (enum-value? vehicle 'car)

#f

> (enum-value? vehicle 'apple)

#f

(enum-prettify enum value [default]) → string?

enum : enum?

value : symbol?

default   :   (U string? (-> string?))       =   (cut raise-exn exn:fail:contract ...)

Returns the pretty equivalent of value. If value is not found in enum, default is used instead:

• if default is a procedure, it is called to determine the return value;

• if default is not a procedure, it is returned.

(define-enum enum-id (value-clause ...) keyword-arg ...)

value-clause   =   value-id     |   [value-id pretty-expr]     |   [value-id value-expr pretty-expr]           value-expr   =   (U boolean? symbol? integer?)           pretty-expr   =   string?           keyword-arg   =   #:plural id

Binds the following identifiers:

• enum-id: an enumeration struct;

• value-id (one binding per value): the values of the enumeration, each a symbol;

• enum-id?: a predicate that recognises the values;

• enum-id-out: a provide form that provides all of the above.

If value-expr and pretty-expr are unspecified for a value, they are created from value-id.

Examples:

> (define-enum vehicle (car boat plane))

define-enum vehicle: binding discrepancy: car is predefined

and not eq? to car

> car

#<procedure:car>

> boat

boat

> plane

plane

> (vehicle? car)

reference to undefined identifier: vehicle?

> (vehicle? 'apple)

reference to undefined identifier: vehicle?

The optional #:prefix argument affects the names of the value identifiers (car, boat and so on) bound by the macro:

Examples:

> (define-enum vehicle (car boat plane) #:prefix vehicle-)

> vehicle-car

car

> vehicle-boat

boat

> vehicle-plane

plane