On Jan 4, 9:40 pm, Raoul Duke rao...@gmail.com wrote:
It is interfaces that are more restrictive than protocols, and extend
can't fix that. This ability to extend an existing type to a protocol
is a main reason protocols exist.
i guess i'm horribly confused about the right mental model for all of
this, apologies.
e.g. it sounds like the only way i can get a type to participate in an
interface is if i control the source file that has the deftype. i
thought in previous versions of the new branch i could use extend to
get a type i don't own to participate in an interface, which seems
desirable. maybe i am horribly confabulating. perhaps the idea would
be to make a new protocol that 'mirrors' the already existing
interface somehow?
Broadly speaking yes; you can only have a type participate in an
interface if you control the type; because interfaces work via Java
methods; i.e. you invoke them with . syntax, like: (.compareTo c1
c2). Protocols, on the other hand, register their methods as
clojure functions, which you call as a normal function, e.g. (compare-
to c1 c2). In order to make a type support an interface, you'd
literally have to change the class to add definitions for the
interface methods, which you can't (for the purposes of this
dicussion) do once the class has been loaded. Since protocol
methods are just Clojure functions, there is under-the-hood
machinery that allows them to support implementation of a protocol on
a type via a number of different mechanisms; protocol methods
implemented inside a deftype construct are translated into traditional
java-style methods, while protocol methods implemented with extend
generate normal clojure functions which are then hooked into the
protocol system (this is the reason that protocols implemented inside
deftype are faster than those implemented with extend).
This is what Rich means when he says that It is interfaces that are
more restrictive than protocols. The problem of adding an
interface to a type is often referred to as the expression problem,
particularly in the Clojure documentation around types / protocols.
Hope this helps (apologies if anything's glaringly wrong...),
-DTH
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