On this page:
enum
define-enum
enum->string
enum->pretty-string
enum-value?
enum-value/ c
in-enum
in-enum/ pretty
enum-prettify
enum-list
enum-complement
enum-case

8 Enumerations (revised)

 (require (planet untyped/unlib/enumeration))
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.

(define-enum enum-id (value-clause ...) keyword-arg ...)
 
value-clause = value-id
  | [value-id pretty-expr]
  | [value-id value-expr pretty-expr]
     
value-expr = any
  | _
     
pretty-expr = string?
     
keyword-arg = #:plural id
Binds enum-id to an enumeration macro that can be used:

  • in argument position to refer to an enum struct;

  • in procedure call position to retrieve a value from the enumeration.

Examples:

  > (define-enum options
      ([a 'option1 "first option"]
       [b "second option"]
       c))
  > (options a)

  option1

  > (options b)

  "second option"

  > (options c)

  c

  > (map (lambda (val)
           (enum-prettify options val))
         (list (options a) (options b) (options c)))

  ("first option" "b" "c")

The identifier _ can be used as a value-expr as a shorthand for 'value-id. This is useful when writing value clauses where the value-id and the pretty-expr are the most important parts.

Examples:

  > (define-enum options
      ([a _ "first option"]
       [b _ "second option"]
       [c _ "third option"]))
  > (options a)

  a

  > (options b)

  b

  > (options c)

  c

  > (map (lambda (val)
           (enum-prettify options val))
         (list (options a) (options b) (options c)))

  ("first option" "second option" "third option")

(enum->string enum [separator])  string?
  enum : enum?
  separator : string? = ", "
Returns a string representation of (enum-values enum), useful for including in debugging output. separator is used to separate the enum values in the return value.

Examples:

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

  "car, boat, plane"

(enum->pretty-string enum [separator])  string?
  enum : enum?
  separator : string? = ", "
Returns a string representation of (enum-pretty-values enum), useful for describing the possible values to a user. separator is used to separate the enum values in the return value.

Examples:

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

  "car, boat, plane"

(enum-value? enum value)  boolean?
  enum : enum?
  value : any
Returns #t if value is a member of (enum-values enum).

(enum-value/c enum)  flat-contract?
  enum : enum?
Returns a contract that accepts values from (enum-values enum).

(in-enum enum val ...)
Like in-list but iterates through (enum-list enum val ...).

Examples:

  > (define-enum vehicle (car boat plane))
  > (for/list ([val (in-enum vehicle)])
      val)

  (car boat plane)

(for/list ([val (in-enum vehicle car plane)]) val)]

(in-enum/pretty enum val ...)
Like in-list but iterates through the pretty versions of (enum-list enum val ...).

Examples:

  > (define-enum vehicle (car boat plane))
  > (for/list ([val (in-enum/pretty vehicle)])
      val)

  ("car" "boat" "plane")

(for/list ([val (in-enum vehicle car plane)]) val)]

Examples:

  > (define-enum vehicle (car boat plane))
  > (enum-value? vehicle 'car)

  #t

  > (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.

(enum-list enum value ...)
Expands to a list of values from enum.

Examples:

  > (define-enum vehicles (car boat plane))
  > (enum-list vehicles car boat)

  (car boat)

(enum-complement enum value ...)
Expands to a list of all values in enum apart from the specified values.

Examples:

  > (define-enum vehicles (car boat plane))
  > (enum-complement vehicles car boat)

  (plane)

(enum-case enum value clause ...)
 
clause = [(value ...) expr ...]
  | [else expr ...]
Like case but each value must be a value from enum. If an else expression is not provided, the values must cover the complete enumeration.

Examples:

  > (define-enum vehicles (car boat plane))
  > (define (flies? vehicle)
      (enum-case vehicles vehicle
        [(plane) #t]
        [(car boat) #f]))
  > (flies? 'car)

  #f

  > (flies? 'plane)

  #t