SRFI-128: Comparators

This SRFI provides comparators, which bundle a type test predicate, an equality predicate, an ordering predicate, and a hash function (the last two are optional) into a single Scheme object. By packaging these procedures together, they can be treated as a single item for use in the implementation of data structures.

  1. SRFI-128: Comparators
  2. Installation
  3. SRFI Description
  4. Comparators
    1. Predicates
    2. Constructors
    3. Standard hash functions
    4. Bounds and salt
    5. Default comparators
    6. Accessors and invokers
    7. Comparison predicates
    8. Syntax
  5. Repository
  6. Version History
  7. License

Installation

$ chicken-install srfi-128

or

$ chicken-install srfi-128 -test

if you want to run the tests for the egg in addition.

SRFI Description

For a full description of this SRFI, see the full SRFI document. This documentation covers the API only.

Comparators

Predicates

[procedure] (comparator? obj)

Returns #t if obj is a comparator, and #f otherwise.

[procedure] (comparator-ordered? comparator)

Returns #t if comparator has a supplied ordering predicate, and #f otherwise.

[procedure] (comparator-hashable? comparator)

Returns #t if comparator has a supplied hash function, and #f otherwise.

Constructors

The following comparator constructors all supply appropriate type test predicates, equality predicates, ordering predicates, and hash functions based on the supplied arguments. They are allowed to cache their results: they need not return a newly allocated object, since comparators are pure and functional. In addition, the procedures in a comparator are likewise pure and functional.

[procedure] (make-comparator type-test equality ordering hash)

Returns a comparator which bundles the type-test, equality, ordering, and hash procedures provided. However, if ordering or hash is #f, a procedure is provided that signals an error on application. The predicates comparator-ordered? and/or comparator-hashable?, respectively, will return #f in these cases.

Here are calls on make-comparator that will return useful comparators for standard Scheme types:

(make-comparator boolean? boolean=? (lambda (x y) (and (not x) y)) boolean-hash)

will return a comparator for booleans, expressing the ordering #f < #t and the standard hash function for booleans.

(make-comparator real? = < (lambda (x) (exact (abs x))))

will return a comparator expressing the natural ordering of real numbers and a plausible (but not optimal) hash function.

(make-comparator string? string=? string<? string-hash)

will return a comparator expressing the implementation's ordering of strings and the standard hash function.

(make-comparator string? string-ci=? string-ci<? string-ci-hash)

will return a comparator expressing the implementation's case-insensitive ordering of strings and the standard case-insensitive hash function.

[procedure] (make-pair-comparator car-comparator cdr-comparator)

This procedure returns comparators whose functions behave as follows:

[procedure] (make-list-comparator element-comparator type-test empty? head tail)

This procedure returns comparators whose functions behave as follows:

[procedure] (make-vector-comparator element-comparator type-test length ref)

This procedure returns comparators whose functions behave as follows:

Here is an example, which returns a comparator for byte vectors:

(make-vector-comparator
  (make-comparator exact-integer? = < number-hash)
  bytevector?
  bytevector-length
  bytevector-u8-ref)
[procedure] (make-eq-comparator)
[procedure] (make-eqv-comparator)
[procedure] (make-equal-comparator)

These procedures return comparators whose functions behave as follows:

These comparators accept circular structure (in the case of equal-comparator, provided the implementation's equal? predicate does so) and NaNs.

Standard hash functions

These are hash functions for some standard Scheme types, suitable for passing to make-comparator. Users may write their own hash functions with the same signature. However, if programmers wish their hash functions to be backward compatible with the reference implementation of SRFI 69, they are advised to write their hash functions to accept a second argument and ignore it.

[procedure] (boolean-hash obj)
[procedure] (char-hash obj)
[procedure] (char-ci-hash obj)
[procedure] (string-hash obj)
[procedure] (string-ci-hash obj)
[procedure] (symbol-hash obj)
[procedure] (number-hash obj)

These are suitable hash functions for the specified types. The hash functions char-ci-hash and string-ci-hash treat their argument case-insensitively. Note that while symbol-hash may return the hashed value of applying symbol->string and then string-hash to the symbol, this is not a requirement.

Bounds and salt

The following macros allow the callers of hash functions to affect their behavior without interfering with the calling signature of a hash function, which accepts a single argument (the object to be hashed) and returns its hash value. They are provided as macros so that they may be implemented in different ways: as a global variable, a SRFI 39 or R7RS parameter, or an ordinary procedure, whatever is most efficient in a particular implementation.

[syntax] (hash-bound)

Hash functions should be written so as to return a number between 0 and the largest reasonable number of elements (such as hash buckets) a data structure in the implementation might have. What that value is depends on the implementation. This value provides the current bound as a positive exact integer, typically for use by user-written hash functions. However, they are not required to bound their results in this way.

[syntax] (hash-salt)

A salt is random data in the form of a non-negative exact integer used as an additional input to a hash function in order to defend against dictionary attacks, or (when used in hash tables) against denial-of-service attacks that overcrowd certain hash buckets, increasing the amortized O(1) lookup time to O(n). Salt can also be used to specify which of a family of hash functions should be used for purposes such as cuckoo hashing. This macro provides the current value of the salt, typically for use by user-written hash functions. However, they are not required to make use of the current salt.

The initial value is implementation-dependent, but must be less than the value of (hash-bound), and should be distinct for distinct runs of a program unless otherwise specified by the implementation. Implementations may provide a means to specify the salt value to be used by a particular invocation of a hash function.

Default comparators

[procedure] (make-default-comparator)

Returns a comparator known as a default comparator that accepts Scheme values and orders them in some implementation-defined way, subject to the following conditions:

Default comparators use default-hash as their hash function.

[procedure] (default-hash obj)

This is the hash function used by default comparators, which accepts a Scheme value and hashes it in some implementation-defined way, subject to the following conditions:

[procedure] (comparator-register-default! comparator)

Registers comparator for use by default comparators, such that if the objects being compared both satisfy the type test predicate of comparator, it will be employed by default comparators to compare them. Returns an unspecified value. It is an error if any value satisfies both the type test predicate of comparator and any of the following type test predicates: boolean?, char?, null?, pair?, symbol?, bytevector?, number?, string?, vector?, or the type test predicate of a comparator that has already been registered.

This procedure is intended only to extend default comparators into territory that would otherwise be undefined, not to override their existing behavior. In general, the ordering of calls to comparator-register-default! should be irrelevant. However, implementations that support inheritance of record types may wish to ensure that default comparators always check subtypes before supertypes.

Accessors and invokers

[procedure] (comparator-type-test-predicate comparator)
[procedure] (comparator-equality-predicate comparator)
[procedure] (comparator-ordering-predicate comparator)
[procedure] (comparator-hash-function comparator)

Return the four procedures of comparator.

[procedure] (comparator-test-type comparator obj)

Invokes the type test predicate of comparator on obj and returns what it returns. More convenient than comparator-type-test-predicate, but less efficient when the predicate is called repeatedly.

[procedure] (comparator-check-type comparator obj)

Invokes the type test predicate of comparator on obj and returns true if it returns true, but signals an error otherwise. More convenient than comparator-type-test-predicate, but less efficient when the predicate is called repeatedly.

[procedure] (comparator-hash comparator obj)

Invokes the hash function of comparator on obj and returns what it returns. More convenient than comparator-hash-function, but less efficient when the function is called repeatedly.

Note: No invokers are required for the equality and ordering predicates, because =? and <? serve this function.

Comparison predicates

[procedure] (=? comparator object1 object2 object3 ...)
[procedure] (<? comparator object1 object2 object3 ...)
[procedure] (>? comparator object1 object2 object3 ...)
[procedure] (<=? comparator object1 object2 object3 ...)
[procedure] (>=? comparator object1 object2 object3 ...)

These procedures are analogous to the number, character, and string comparison predicates of Scheme. They allow the convenient use of comparators to handle variable data types.

These procedures apply the equality and ordering predicates of comparator to the objects as follows. If the specified relation returns #t for all objecti and objectj where n is the number of objects and 1 <= i < j <= n, then the procedures return #t, but otherwise #f. Because the relations are transitive, it suffices to compare each object with its successor. The order in which the values are compared is unspecified.

Syntax

[syntax] (comparator-if<=> [ <comparator> ] <object1> <object2> <less-than> <equal-to> <greater-than>)

It is an error unless <comparator> evaluates to a comparator and <object1> and <object2> evaluate to objects that the comparator can handle. If the ordering predicate returns true when applied to the values of <object1> and <object2> in that order, then <less-than> is evaluated and its value returned. If the equality predicate returns true when applied in the same way, then <equal-to> is evaluated and its value returned. If neither returns true, <greater-than> is evaluated and its value returned.

If <comparator> is omitted, a default comparator is used.

Repository

Github

Version History

0.7
Includes bug fix for make-list-comparator from Marc Nieper-Wisskirchen
0.6
Removed hardcoded .so in setup file
0.5
Standard README.org added to all SRFIs
0.4
Update to fix SRFI-128 mustard
0.3
Packages egg without extraneous files
0.2
Fixes bugs in setup
0.1
Initial release

License

Copyright (C) John Cowan (2016). All Rights Reserved.

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.