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Creating a Hash with Immutable Keys or Values

Problem

You would like to have a hash table whose values or keys couldn't be modified once set.

Solution

The procedure make-immutable-hash-table creates an immutable hash table. It receives two parameters, the first one must be a list of pairs, where the car of each pair is a key and the cdr the value. The second parameter is optional, if it is the symbol 'equal, the created hash table compares keys using equal?; in other case it compares keys using eq?.

(let ((t (make-immutable-hash-table '((1 . "one") (2 . "two")))))
  (display (hash-table-get t 1))
  ;; The following raises an exception:
  (hash-table-put! t 3 "three"))

Discussion

You can check whether a hash table is mutable or not using the predicate immutable?.

Comments about this recipe

Contributors

-- KarlaRamirez - 19 May 2004