2015-09-13 05:28:39 +00:00
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(ns koans.23-meta
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2015-07-12 20:03:34 +00:00
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(:require [koan-engine.core :refer :all]))
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(def giants
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(with-meta 'Giants
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{:league "National League"}))
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(meditations
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"Some objects can be tagged using the with-meta function"
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(= __ (meta giants))
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"Or more succintly with a reader macro"
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(= __ (meta '^{:division "West"} Giants))
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"While others can't"
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(= __ (try
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(with-meta
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2
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{:prime true})
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(catch ClassCastException e
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"This doesn't implement the IObj interface")))
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"Notice when metadata carries over"
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(= __ (meta (merge '^{:foo :bar} {:a 1 :b 2}
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{:b 3 :c 4})))
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"And when it doesn't"
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(= __ (meta (merge {:a 1 :b 2}
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'^{:foo :bar} {:b 3 :c 4})))
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"Metadata can be used as a type hint to avoid reflection during runtime"
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(= __ (#(.charAt ^String % 0) "Cast me"))
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"You can directly update an object's metadata"
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(= 8 (let [giants
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(with-meta
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'Giants
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{:world-series-titles (atom 7)})]
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(swap! (:world-series-titles (meta giants)) __)
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@(:world-series-titles (meta giants))))
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"You can also create a new object from another object with metadata"
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(= {:league "National League" :park "AT&T Park"}
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(meta (vary-meta giants
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assoc __ __)))
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"But it won't affect behavior like equality"
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(= __ (vary-meta giants dissoc :league))
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"Or the object's printed representation"
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(= __ (pr-str (vary-meta giants dissoc :league))))
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