This is very helpful. I think my false assumption was that structural
contracts weren't enforced, even with protocols. Your example is very
helpful.
On 30 January 2012 11:43, Timo Mihaljov <t...@mihaljov.info> wrote:
> As I understand the problem in your example, you do have a definition
> for what an Insurable is, but it's implicit: there's no way to check
> whether an object is insurable and thus no way to check which parts of
> the code break when the notion of being Insurable changes. Luckily
> Clojure provides us with the tools needed to make the definition
> explicit.
>
> In your Java example you used an interface to define what an Insurable
> is. In Clojure you can do the same with a protocol:
>
> (defprotocol Insurable
> (cost-to-replace-with-new [insurable]))
>
> (def insurable? (partial satisfies? Insurable))
>
> Whenever you expect to receive an insurable, you can use a
> precondition to make this assumption explicit, like BG and Meikel
> suggested:
>
> (defn calculate-insurance [& insurables]
> {:pre [(every? insurable? insurables)]}
> ...)
>
> Note that the Insurable objects are completely abstract; we don't know
> or care what kind of data structure are used to implement them. This
> means that we can't build valid Insurables by accident, like you did
> in your example by calling FuncA to FuncE, but that's a *good* thing,
> because data structures built like that are very brittle, as you
> noted.
>
> To wrap up the example, here are a two very different implementations
> of Insurable. Note how everything is modeled in terms of domain
> concepts instead of "maps with keys :x, :y and :z". We manipulate the
> data structures with completely normal Clojure code, but keep the
> knowledge of those data structures in one place only instead of
> spreading it all over the codebase. This makes the code much more
> robust and amenable to change.
>
> (defn make-simple-insurable [cost]
> {:post [(insurable? %)]}
> (reify Insurable
> (cost-to-replace-with-new [_]
> cost)))
>
> (defrecord CompositeInsurable [parts]
> Insurable
> (cost-to-replace-with-new [composite-insurable]
> (->> (:parts composite-insurable)
> (map cost-to-replace-with-new)
> (reduce +))))
>
> (defn make-composite-insurable [& parts]
> {:pre [(every? insurable? parts)]
> :post [(insurable? %)]}
> (CompositeInsurable. parts))
>
> --
> Timo
>
> --
> You received this message because you are subscribed to the Google
> Groups "Clojure" group.
> To post to this group, send email to clojure@googlegroups.com
> Note that posts from new members are moderated - please be patient with
> your first post.
> To unsubscribe from this group, send email to
> clojure+unsubscr...@googlegroups.com
> For more options, visit this group at
> http://groups.google.com/group/clojure?hl=en
>
--
You received this message because you are subscribed to the Google
Groups "Clojure" group.
To post to this group, send email to clojure@googlegroups.com
Note that posts from new members are moderated - please be patient with your
first post.
To unsubscribe from this group, send email to
clojure+unsubscr...@googlegroups.com
For more options, visit this group at
http://groups.google.com/group/clojure?hl=en
