Seeing a positive correspondence between people who disdain generic
functions and people who do not know what exactly generic functions are,
I decided to write a little primer in hopes FUD propagation will make
way for an augmented discussion.
---
Somewhat simplified, generic function is a set of functions bound to a
single symbol. Where things get interesting is how a particular
function* (let's call it specialisation) gets selected (dispatched). The
process is usually deterministic, following a set of rules. Three
predominant dispatch strategies are:
1) type dispatch (also called multiple dispatch and overloading)
A function is selected depending on types of arguments passed.
Examples: C, Py3k?, PyProtocols.dispatch.on
2) identity dispatch
Usually used in conjunction with type dispatch
A function is selected depending on concrete instances of
arguments passed.
Examples: CLOS
3) predicate dispatch
A function is selected depending on arbitrary (logical)
expressions.
Examples: Cecil, Mathematica, PyProtocols.distpatch.generic
The aforementioned rules usually also establishing a selection hierarchy
("more specific" is often the governing relation) to reduce ambiguity.
While having more than one applicable specialisation may seem like a bad
thing, it can actually be very useful. Enter function combination.
Strictly speaking function combination is not a part of generic
functions, therefore let's limit the scope to what PyProtocols (and
CLOS) offer us.
The simplest combination strategy is allowing the selected
specialisation to call the next most specific specialisation. What makes
function combination transcend generic functions are secondary
functions. These allow us to define (generic) functions that get called
automatically in relation to another function (called primary). Hence we
can have secondary functions be called before, after or around (same as
composition) a primary function.
Some common "misconceptions":
1) Generic functions are an alternative to "normal" (Java) OOP
False. Message passing is nothing more than type dispatch on first
(implicit) argument.
2) There is nothing I can't do without generic functions
True, however considering lambda calculus (or Turing completeness) this
can be said for most language constructs.
References:
http://www.gigamonkeys.com/book/object-reorientation-generic-functions.html
http://peak.telecommunity.com/DevCenter/CombiningResults
http://en.wikipedia.org/wiki/Multiple_dispatch
http://en.wikipedia.org/wiki/Generic_functions
* term method is often used
Cheers,
Simon
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