On 04 Oct 2011, at 12:52, Mariano Martinez Peck wrote:
Hi guys. If I tell you the selector is Dictionary >> #at:ifAbsentPut: what would you expect the second parameter to be? the value. So, one would do: Dictionary new at: #foo ifAbsentPut: 4
But if you see Dictionary >>
at: key ifAbsentPut: aBlock "Return the value at the given key. If key is not included in the receiver store the result of evaluating aBlock as new value."
^ self at: key ifAbsent: [self at: key put: aBlock value]
so it expects a Block. Ok, we are in Smalltalk, so implementing #value is enough.
Well..the previous example works, but only because we have an ugly Object >> value that returns self. If I put instances of subclasses from ProtoObjects (proxies), that do not work anymore.
So...my question is we do Dictionary at: #foo put: 4, why #at:ifAbsentPut: expects a block and not directly the value? in which case I need a block instead of the value object directly ?
You are right, it feels a bit silly. However, have a look at the senders (there are many). Often the block holds an expensive operation when using the dictionary as a cache: cache at: key ifAbsent: [ backEnd get: key ] Then it feels quite elegant. Sven