In Midje, I have reason to create a type Metaconstant whose instances are
"equal" to a symbol with the same name. Here's the relevant bits of the
definition:
(deftype Metaconstant [name storage]
Object
(equals [this that]
(if (instance? (class this) that)
(= (.name this) (.name that))
(= (.name this) that)))
clojure.lang.IPersistentCollection
(equiv [this that]
(println "equiv is called with" (type this) (type that))
(.equals this that)))
That's fine when a Metaconstant is compared to a symbol, but I thought I'd have
to engage in hackery when a symbol is compared to a Metaconstant. That is:
(println "= symbol metaconstant?")
(println (= '...name... (Metaconstant. '...name... {}))) ; expect false
(println "= metaconstant symbol?")
(println (= (Metaconstant. '...name... {}) '...name... )) ; expect true
Surprisingly, both directions produce true. That's because the order in which
`.equiv` is called doesn't seem to depend on the order of arguments to `=`:
= symbol metaconstant?
equiv is called with midje.ideas.t-m.Metaconstant clojure.lang.Symbol
true
= metaconstant symbol?
equiv is called with midje.ideas.t-m.Metaconstant clojure.lang.Symbol
true
Why is .equiv called with reordered arguments? And can I depend on that
behavior going forward?
I see this behavior with 1.2.0, 1.2.1, and 1.3.0-beta1.
-----
Brian Marick, Artisanal Labrador
Contract programming in Ruby and Clojure
Occasional consulting on Agile
www.exampler.com, www.twitter.com/marick
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