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