5.13.1 Constructors

Procedure: make-eq-hashtable

Procedure: make-eq-hashtable k

Return a newly allocated mutable hashtable that accepts arbitrary objects as keys, and compares those keys with eq?. If an argument is given, the initial capacity of the hashtable is set to approximately k elements.

Procedure: make-eqv-hashtable

Procedure: make-eqv-hashtable k

Return a newly allocated mutable hashtable that accepts arbitrary objects as keys, and compares those keys with eqv?. If an argument is given, the initial capacity of the hashtable is set to approximately k elements.

Procedure: make-hashtable hash-function equiv

Procedure: make-hashtable hash-function equiv k

hash-function and equiv must be procedures. hash-function should accept a key as an argument and should return a non–negative exact integer object. equiv should accept two keys as arguments and return a single value. Neither procedure should mutate the hashtable returned by make-hashtable.

The make-hashtable procedure returns a newly allocated mutable hashtable using hash-function as the hash function and equiv as the equivalence function used to compare keys. If a third argument is given, the initial capacity of the hashtable is set to approximately k elements.

Both hash-function and equiv should behave like pure functions on the domain of keys. For example, the string-hash and string=? procedures are permissible only if all keys are strings and the contents of those strings are never changed so long as any of them continues to serve as a key in the hashtable. Furthermore, any pair of keys for which equiv returns true should be hashed to the same exact integer objects by hash-function.

Implementation responsibilities: The implementation must check the restrictions on hash-function and equiv to the extent performed by applying them as described.

NOTE Hashtables are allowed to cache the results of calling the hash function and equivalence function, so programs cannot rely on the hash function being called for every lookup or update. Furthermore any hashtable operation may call the hash function more than once.