Language Differences

This document describes the ways in which Open Dylan differs from the specification in the Dylan Reference Manual.

Note that in addition to the intentional differences documented below, there are several known bugs related to compliance with the language standard as documented in the DRM.

Table Protocol

For efficiency, Open Dylan adopts a slightly different table protocol to that described by the DRM. Hashing functions take an additional hash-state argument and merge it into the hash-state result. The function merge-hash-codes is replaced by merge-hash-ids because hash-states are merged as part of the hashing process. The constant $permanent-hash-state is no longer required; the same effect can be achieved by returning the argument hash-state unchanged as the result hash-state. Finally, object-hash has been altered to use the new protocol.

This section describes the items that have been changed. We also provide a table-extensions module.

table-protocol Open Generic function

Returns functions used to implement the iteration protocol for tables.

Signature:

table-protocol table => test-function hash-function

Parameters:

  • table – An instance of <table>.

Values:
Discussion:

Returns the functions used to iterate over tables. These functions are in turn used to implement the other collection operations on <table>.

The test-function is used to compare table keys. It returns true if, according to the table’s equivalence predicate, the keys are members of the same equivalence class. Its signature must be:

test-function *key1* *key2* => *boolean*

The hash-function must compute the hash code of a key. Its signature must be:

hash-function *key* *initial-state* => *id* *result-state*

In this signature, initial-state is an instance of <hash-state>. The hash function computes the hash code of key, using the hash function that is associated with the table’s equivalence predicate. The hash code is returned as two values: an integer id and a hash-state result-state. This result-state is obtained by merging the initial-state with the hash-state that results from hashing key. The result-state may or may not be == to initial-state. The initial-state may be modified by this operation.

merge-hash-ids Function

Returns a hash ID created by merging two hash IDs.

Signature:

merge-hash-ids id1 id2 #key ordered => merged-id

Parameters:
Values:
Discussion:

Computes a new hash ID by merging the argument hash IDs in some implementation-dependent way. This can be used, for example, to generate a hash ID for an object by combining hash IDs of some of its parts.

The ordered argument determines whether the algorithm used to merge the IDs is permitted to be order-dependent. If false (the default), the merged result must be independent of the order in which the arguments are provided. If true, the order of the arguments matters because the algorithm used need not be either commutative or associative. It is best to provide a true value for ordered when possible, as this may result in a better distribution of hash IDs. However, ordered must only be true if that will not cause the hash function to violate the second constraint on hash functions, described in the DRM.

object-hash Function

The hash function for the equivalence predicate ==.

Signature:

object-hash object initial-state => hash-id result-state

Parameters:
Values:
Discussion:

Returns a hash code for object that corresponds to the equivalence predicate ==.

This function is a useful tool for writing hash functions in which the object identity of some component of a key is to be used in computing the hash code.

It returns a hash ID (an integer) and the result of merging the initial state with the associated hash state for the object, computed in some implementation-dependent manner.