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Category:Smalltalk: Difference between revisions
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[some code] fork "starts a new thread"</lang>
=== Return from a Block ===
it should be noted that a "ˆ" (return) inside a block will return from the enclosing method, NOT only from the block and that this is an essential semantic property of the return (technically, it may be a long return from a deeply nested call hierarchy, involving unwind actions).
This makes it possible to pass a block to eg. collections to enumerate elements up-to and until some condition is met. For example, if we need a helper method, which returns the first element from a dataset which meets some condition, we can write:
<lang smalltalk>
findSomeElementWhichMeetsCondition:conditionBlock thenDo:actionBlock ifNone:failBlock
dataSet do:[:eachElement |
(conditionBlock value:eachElement) ifTrue:[
^ actionBlock value:eachElement
]
].
^ failBlock value</lang>
Here, a block is passed to the dataSet's "do:" method, which will return (if invoked inside the "do:") from the containing findSomeElement method.
If a block-return (as eg. in JavaScript) would one return from itself, this wasn't possible, and ugly workarounds (like exceptions or long-jumps) were needed.
There are rare situations, where an explicit block return is needed (for example, to break out of a loop in the middle, without returning from the method). For this, block provides a special "valueWithExit" method, so you can write:
<lang smalltalk>1 to:10 do:[:outerLoopsI |
[:exit |
1 to:10 do:[:innerLoopsI |
...
someCondition ifTrue:exit
]
] valueWithExit
]</lang>
=== Exceptions and Handlers ===
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