Comparators (reduced)


John Cowan


This SRFI is currently in final status. Here is an explanation of each status that a SRFI can hold. To provide input on this SRFI, please send email to srfi-128@nospamsrfi.schemers.org. To subscribe to the list, follow these instructions. You can access previous messages via the mailing list archive.

Post-finalization note: Because of the extremely high cost of conforming to the first and third conditions of default-hash, implementers may disregard those conditions and examine only a bounded portion of the argument.


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.


None at present.


The four procedures above have complex dependencies on one another, and it is inconvenient to have to pass them individually to other procedures that might or might not make use of all of them. For example, a set implementation by its nature requires only an equality predicate, but if it is implemented using a hash table, an appropriate hash function is also required if the implementation does not provide one; alternatively, if it is implemented using a tree, procedures specifying a total order are required. By passing a comparator rather than a bare equality predicate, the set implementation can make use of whatever procedures are available and useful to it.

This SRFI is a simplified and enhanced rewrite of SRFI 114, and shares some of its design rationale and all of its acknowledgements. The largest change is the replacement of the comparison procedure with the ordering procedure. This allowed most of the special-purpose comparators to be removed. In addition, many of the more specialized procedures, as well as all but one of the syntax forms, have been removed as unnecessary.

Special thanks to Taylan Ulrich Bayırlı/Kammer, whose insistence that SRFI 114 was unacceptable inspired this redesign. Jörg Wittenberger added Chicken-specific type declarations, which I have moved to comparators.scm, as it is a Chicken-specific library. He also provided Chicken-specific metadata and setup commands. Comments from Shiro Kawai, Alex Shinn, and Kevin Wortman guided me to the current design for bounds and salt.


The procedures in this SRFI are in the (srfi 128) library (or (srfi :128) on R6RS), but the sample implementation currently places them in the (comparators) library. This means it can't be used alongside SRFI 114, but there's no reason for anyone to do that.


A comparator is an object of a disjoint type. It is a bundle of procedures that are useful for comparing two objects in a total order. It is an error if any of the procedures have side effects. There are four procedures in the bundle:

It is also the programmer's responsibility to ensure that all four procedures provide the same result whenever they are applied to the same object(s) (in the sense of eqv?), unless the object(s) have been mutated since the last invocation. In particular, they must not depend in any way on memory addresses in implementations where the garbage collector can move objects in memory.


The comparator objects defined in this SRFI are not applicable to circular structure or to NaNs, or to objects containing any of these. Attempts to pass any such objects to any procedure defined here, or to any procedure that is part of a comparator defined here, is an error except as otherwise noted.



(comparator? obj)

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

(comparator-ordered? comparator)

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

(comparator-hashable? comparator)

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


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.

(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-pair-comparator car-comparator cdr-comparator)

This procedure returns comparators whose functions behave as follows.

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

This procedure returns comparators whose functions behave as follows:

(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-comparator exact-integer? = < number-hash)




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.

(boolean-hash obj)

(char-hash obj)

(char-ci-hash obj)

(string-hash obj)

(string-ci-hash obj)

(symbol-hash obj)

(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.

(hash-bound) [syntax]

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.

(hash-salt) [syntax]

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


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.

(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:

(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

(comparator-type-test-predicate comparator)

(comparator-equality-predicate comparator)

(comparator-ordering-predicate comparator)

(comparator-hash-function comparator)

Return the four procedures of comparator.

(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.

(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.

(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

(=? comparator object1 object2 object3 ...)

(<? comparator object1 object2 object3 ...)

(>? comparator object1 object2 object3 ...)

(<=? comparator object1 object2 object3 ...)

(>=? 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.


(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.


The sample implementation is found in the repository of this SRFI. It contains the following files.


Copyright (C) John Cowan (2015). All Rights Reserved.

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Editor: Arthur A. Gleckler