case dispatch in Lisp

case picks a branch by comparing one value against literal keys with eql.

;; case tests keys with eql, so the clauses stay bare symbols
(defun unit-label (measure)
  (case measure
    (celsius "degrees C")
    (millibar "mb")
    ((meter meters) "m above sea level")
    (otherwise "unrecognized unit")))

;; one clause can list several keys, and otherwise catches the rest
(defun log-reading (measure value)
  (format t "~a ~a~%" value (unit-label measure)))

(log-reading 'celsius 21)
(log-reading 'millibar 1013)
(log-reading 'meters 348)
(log-reading 'furlong 2)

How it works

1. unit-label matches the symbols celsius and millibar to their printed units.
2. The ((meter meters) "m above sea level") clause accepts either spelling of one key.
3. otherwise catches anything unlisted, which is what (log-reading 'furlong 2) hits.

Keywords and builtins used here

• case
• defun
• format

The run, in numbers

Lines

16

Characters to type

472

Tokens

70

Three-star pace

80 tpm

At the three-star pace of 80 tokens a minute, this run takes about 52 seconds.

Type this snippet

Step 2 of 3 in Control flow, step 8 of 27 in Language basics.

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