On Aug 14, 12:34 am, Stuart Halloway <[email protected]>
wrote:
> > Following Stuart's suggestion, I *could* just add a protocol
> > called "PrettyPrintable" with one method and implement it on some
> > of the new node types, but now I can't just call "pretty-print" on
> > any node: I need to write another function that checks if it's a
> > PrettyPrintable first and calls something default if not.
>
> You can just extend PrettyPrintable to Object.
Just to thrash this out in my own head I've developed the scenario
below as a hypothetical (and obviously trivial) graph node library
evolves through several versions. I've used extend-type on Object to
retro-actively provide default implementations for older clients.
Adding to Object works, but doesn't feel right: as libraries grow,
they'll start bloating out the method sets on the global Object type.
One idea that I tried was to use extend-type on a protocol, say to
extend any Node to be a PrettyPrintableNode. Obviously this didn't
work, and I'm not sure it actually makes semantic sense, but it's
interesting that was my intuitive action.
Example below, interested in any thoughts.
Matthew.
---
;; A node has a data attachment and (possibly) children
(defprotocol Node
(data [n])
(children [n]))
(deftype SimpleNode [d]
Node
(data [n] d)
(children [n] []))
;; In version 2, I add want to add pretty-printing
(defprotocol PrettyPrintableNode
(pretty-print [n]))
;; Make anything potentially pretty-print'able
(extend-type Object
PrettyPrintableNode
(pretty-print [n] (str "A node: " (data n))))
;; Later, in version 3, I decide nodes also need a tag
(defprotocol TaggedNode
(tag [n]))
;; Default to node's data if no tag
(extend-type Object
TaggedNode
(tag [n] (data n)))
;; A new type of node that has a settable tag
(deftype SuperNode [d t]
Node
(data [n] d)
(children [n] [])
PrettyPrintableNode
(pretty-print [n] (str "Node: tag " (tag n)
", data: " (data n)))
TaggedNode
(tag [n] t))
---
Examples of use
> (def n (SimpleNode. "Matt"))
> (def n2 (SuperNode. "Matt" "a tag"))
> (tag n)
"Matt"
> (tag n2)
"a tag"
> (pretty-print n2)
"Node: tag a tag, data: Matt"
> (pretty-print n)
"A node: Matt"
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