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hashtuplescommon-lisphashtable
ealfonso
ealfonso

Reputation: 7302

multiple-value hash common lisp

How can I hash pairs or triples of 'eq-able objects like symbols or ints?

In python I can use tuples as dictionary keys, is there a way to do this in lisp without resorting to an 'equal test?

Upvotes: 2

Views: 305

Answers (1)

Joshua Taylor
Joshua Taylor

Reputation: 85843

While some implementations might provide provisions for custom hash table functions, the standard only defines four:

18.1.1 Hash-Table Operations

There are four kinds of hash tables: those whose keys are compared with eq, those whose keys are compared with eql, those whose keys are compared with equal, and those whose keys are compared with equalp.

That means that if you want to use the standard hash tables, then you'll probably need to use an equal or equalp hash table. I do notice that you wrote:

How can I hash pairs or triples of 'eq-able objects like symbols or ints?

While symbols can be compared reliably with eq, you shouldn't compare numbers with eq. The documentation of eq says:

numbers with the same value need not be eq, … An implementation is permitted to make "copies" of characters and numbers at any time. The effect is that Common Lisp makes no guarantee that eq is true even when both its arguments are "the same thing" if that thing is a character or number.

and gives this example:

(eq 3 3)
;   =>  true
; OR=>  false

However, if you are working with (small) tuples of integers, you could easily hash on a function of them. E.g., the tuple (a,b,c) could be mapped to 2a×3b×5c. Since a function like that would generate unique numbers which are comparable with eql, you could use an eql hash table.

Another option for such a mapping function (that would work with symbols, too) would be to use sxhash. It's a standard hashing function that should produce identical values for equal values. How it works, and what exactly it does is not really specified at all, but it has the advantage that it's stable across Lisp images of the same implementation (e.g., run one version of SBCL today and tomorrow, and sxhash will return the same result for an equal object). Of course, it's possible that an equal-hash-table is just doing this for you already, so your mileage might vary.

Upvotes: 2

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