diff options
Diffstat (limited to 'txr.1')
| -rw-r--r-- | txr.1 | 61 |
1 files changed, 59 insertions, 2 deletions
@@ -13607,8 +13607,10 @@ The .code equal function is less strict still than .codn eql . -In general, it recurses into some -kinds of aggregate objects to perform a structural equivalence check. +In general, it recurses into some kinds of aggregate objects to perform a +structural equivalence check. For struct types, it also supports customization +via equality substitution. See the Equality Substitution section under +Structures. Firstly, if .meta left-obj @@ -13830,6 +13832,15 @@ correct behavior when is regarded as an empty list, since the empty list is lexicographically prior to a nonempty list. +If either argument is a structure for which the +.code equal +method is defined, the method is invoked on that argument, and the +value returned is used in place of that argument for performing +the comparison. Structures with no +.code equal +method cannot participate in a comparison, resulting in an error. +See the Equality Substitution section under Structures. + Finally, if either of the arguments has a type other than the above discussed types, the situation is an error. @@ -18201,6 +18212,47 @@ is evaluated only once: (mapcar s list) <--> (mapcar (meth s lambda) list) .cble +.NP* Equality Substitution + +Normally, two struct values are not considered the same under the +.code equal +function unless they are the same objects. + +However, if a method named +.code equal +is defined for a structure type, via a static slot, then instances of +that structure type support +.IR "equality substitution" . + +The +.code equal +method must take exactly one argument: the structure object. +Moreover, the method must never return +.codn nil . + +When a struct which supports equality substitution is compared using +.codn equal , +.codn less +or +.codn greater , +its +.code equal +method is invoked, and the return value is used in place of that +structure for the purposes of the comparison. + +The same applies when an struct is hashed using the +.code hash-equal +function, or implicitly by an +.code :equal-hash +hash tables. + +Note: if an +.code equal +method is defined or redefined with different semantics for a struct +type whose instances have already been inserted as keys in an +.code :equal-based +hash table, searches for those keys will not work reliably. + .coNP Macro @ defstruct .synb .mets (defstruct >> { name | >> ( name << arg *)} < super @@ -29473,6 +29525,11 @@ each produce the same integer hash value. In all other circumstances, the hash values of two distinct objects are unrelated, and may or may not be the same. +Object of struct type may support custom hashing by way of defining +an equality substitution via an +.code equal +method. See the Equality Substitution section under Structures. + .coNP Functions @, hash_keys @, hash_values @ hash_pairs and @ hash_alist .synb .mets (hash-keys << hash ) |