Thanks to all of you who responded.
So, I think my original thesis was correct: I'm clearly misconstruing
something quite fundamental here ;)
And I can see now my original example was clumsy: for example
something like PrettyPrintable *should* be an orthogonal protocol to
Node. (Not to mention the example I had in mind was actually a tree
not a graph :/)
Anyway, to either clarify, or further muddy, the waters here's another
scenario, for an actual graph library this time, assuming you'll
indulge me further.
---
;; A node has a data attachment and (possibly) out-edges
(defprotocol Node
(data [n])
(out-edges [n]))
;; A client's implementation of Node using library 1.0's protocol
(deftype SimpleNode [d]
Node
(data [n] d)
(out-edges [n] []))
;; In version 2, I decide nodes may also report their in-edges. I'd
;; actually like to add "in-edges" to Node, but can't because that
;; would break existing clients.
(defprotocol Node2
(in-edges [n]))
;; A client's new type of node using library v 2.0's protocols
(deftype SimpleNode2 [d in out]
Node
(data [n] d)
(out-edges [n] out)
Node2
(in-edges [n] in))
;; Now, if I want any node's in-edges, I can't just call "in-edges"
;; because Node implementations won't have it, so I compute them in
;; that case.
(defn node-in-edges [n]
(if (isa? Node2)
(in-edges n)
(compute-node-in-edges n)))
;; OR, I could use extend-type to allow in-edges to be called on
;; anything, and use a default implementation for Node's.
(extend-type Object
Node2
(in-edges [n]
(if (isa? Node)
(compute-node-in-edges n)
(<throw an exception?>))))
---
In contrast, if I could have added in-edges to Node in v 2.0, with an
accompanying default implementation for existing clients, then I could
have saved quite a lot of messing about.
Am I still on the wrong track here?
Matthew.
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