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Diffstat (limited to 'txr.1')
| -rw-r--r-- | txr.1 | 63 |
1 files changed, 63 insertions, 0 deletions
@@ -22983,6 +22983,69 @@ Note: the dirty flag can be used to support support the caching of values derived from an object's slots. The derived values don't have to be re-computed while an object remains clean. +.NP* Equality Substitution + +In object-based or object-oriented programming, sometimes it is necessary for a +new data type to provide its own notion of equality: its own requirements for +when two distinct instances of the type are considered equal. Furthermore, +types sometimes have to implement their own notion, also, of inequality: the +requirements for the manner in which one instance is considered lesser or +greater than another. + +\*(TL structures implement a concept called +.IR "equality substitution" +which provides a simple, unified way for the implementor of an object to +encode the requirements for both equality and inequality. +Equality substitution allows for objects to be used as keys in a hash +table according to the custom equality, without the programmer being +burdened with the responsibility of developing a custom hashing function. + +An object participates in equality substitution by implementing the +.code equal +method. The +.code equal +method takes no arguments other than the object itself. It returns +a representative value which is used in place of that object +for the purposes of +.code equal +comparison. + +Whenever an object which supports equality substitution is used as an argument +of any of the functions +.codn equal , +.codn nequal , +.codn greater , +.codn less , +.codn gequal , +.code lequal +or +.codn hash-equal , +the +.code equal +method of that object is invoked, and the return value of that method +is taken in place of that object. + +The same is true if an object which supports equality substitution is used as a +key in an +.code :equal-based +hash table. + +The substitution is applied repeatedly: if the return value of the object's +.code equal +method is an object which itself supports equality substitution, +than that returned object's method is invoked on that object +to fetch its equality substitute. This repeats as many times as necessary +until an object is determined which isn't a structure that supports +equality substitution. + +Once the equality substitute is determined, then the given function proceeds +with the replacement object. Thus for example +.code equal +compares the replacement object in place of the original, and an +.code :equal-based +hash table uses the replacement object as the key for the purposes of +hashing and comparison. + .coNP Macro @ defstruct .synb .mets (defstruct >> { name | >> ( name << arg *)} < super |