Hello
In the laser game tutorial by Stephan B Wessels I was intrerested by this code:
directionFor: aSymbol ^ self subclasses detect: [:cls | cls directionSymbol = aSymbol]
The code appears here at the bottom of the page: http://squeak.preeminent.org/tut2007/html/038.html
What I read this code as doing, in this example is: return an symbol object which is a sublass instance varable, if it is the same as aSymbol.
What would happen if more than one subclass object had a match?
Also how else can subclasses detect: be used? It looks very interesting.
Grunt
The message #subclasses returns a collection of Class objects The message #detect: returns the first object in that collection that matches the criteria in the block
Since detect is the last message sent before the return, one class object will be returned.
If more than one matches, the code will never know, since detect: returns on the first match. It won't evaluate to find the other match. That is, it short circuits the iteration.
If you want to return a collection of matches, send the #select: message instead of #detect:.
Nothing to do with class instance variables (those are rare birds -- misused as often as they are needed).
Also realize that the message isn't "subclasses detect". They are two separate messages. #detect: works with any collection. #subclasses returns a collection.
On 7/31/07, gruntfuttuck gruntfuttuck@gmail.com wrote:
Hello
In the laser game tutorial by Stephan B Wessels I was intrerested by this code:
directionFor: aSymbol ^ self subclasses detect: [:cls | cls directionSymbol = aSymbol]
The code appears here at the bottom of the page: http://squeak.preeminent.org/tut2007/html/038.html
What I read this code as doing, in this example is: return an symbol object which is a sublass instance varable, if it is the same as aSymbol.
What would happen if more than one subclass object had a match?
Also how else can subclasses detect: be used? It looks very interesting.
Grunt
-- View this message in context: http://www.nabble.com/subclasses-detect-tf4196202.html#a11934239 Sent from the Squeak - Beginners mailing list archive at Nabble.com.
Beginners mailing list Beginners@lists.squeakfoundation.org http://lists.squeakfoundation.org/mailman/listinfo/beginners
Thank you, that was very clear and helpful :-)
David Mitchell-10 wrote:
The message #subclasses returns a collection of Class objects The message #detect: returns the first object in that collection that matches the criteria in the block
Since detect is the last message sent before the return, one class object will be returned.
If more than one matches, the code will never know, since detect: returns on the first match. It won't evaluate to find the other match. That is, it short circuits the iteration.
If you want to return a collection of matches, send the #select: message instead of #detect:.
Nothing to do with class instance variables (those are rare birds -- misused as often as they are needed).
Also realize that the message isn't "subclasses detect". They are two separate messages. #detect: works with any collection. #subclasses returns a collection.
On 7/31/07, gruntfuttuck gruntfuttuck@gmail.com wrote:
Hello
In the laser game tutorial by Stephan B Wessels I was intrerested by this code:
directionFor: aSymbol ^ self subclasses detect: [:cls | cls directionSymbol = aSymbol]
The code appears here at the bottom of the page: http://squeak.preeminent.org/tut2007/html/038.html
What I read this code as doing, in this example is: return an symbol object which is a sublass instance varable, if it is the same as aSymbol.
What would happen if more than one subclass object had a match?
Also how else can subclasses detect: be used? It looks very interesting.
Grunt
-- View this message in context: http://www.nabble.com/subclasses-detect-tf4196202.html#a11934239 Sent from the Squeak - Beginners mailing list archive at Nabble.com.
Beginners mailing list Beginners@lists.squeakfoundation.org http://lists.squeakfoundation.org/mailman/listinfo/beginners
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David Mitchell -
gruntfuttuck