On 13.11.2009, at 17:07, Rich Hickey wrote:
> Admittedly, it is a difference from multimethods. With protocols, both
> protocols and their functions/methods are immutable. Redefining or
> extending a protocol modifies only the protocol and fn vars. I prefer
> that, and don't consider the above behavior a problem. What do others
> think?
For most applications the difference doesn't matter. Having protocols
as immutable values bound to vars that change with every extend could
lead to both interesting use cases and undesirable surprises when used
with threads. Threads can have thread-local implementations of
protocols, intentionally or by mistake.
What makes this behaviour a bit disturbing is the fact that a var in a
namespace that refers to a protocol or a method is changed from code
in another namespace. This is of course possible otherwise as well,
but highly unusual. Consider this example:
(defprotocol Foo
(bar [x]))
(def bar nil)
Now in some other namespace, at some later time:
(extend Object
Foo
{:bar (fn [x] x)})
This will redefine bar in the first namespace to a method of Foo.
While I don't expect such a situation to be frequent, it is certainly
highly unexpected. All the more so since the code in the second
namespace doesn't even mention the symbol bar that it changes.
Konrad.
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