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1.5.3 Concatenation Using `:' in Argument List

What has been said so far about arguments is not quite true. A special string concatenation construction is allowed in argument lists for widgets, widget constructors and control functions.

First we introduce the function tk-conc which takes an arbitrary number of arguments, which may be symbols, strings or numbers, and concatenates these into a string. The print names of symbols are converted to lower case, and package names are ignored.

 
(tk-conc "a" 1 :b 'cd "e") ==> "a1bcde"

One could use tk-conc to construct arguments for widget functions. But even though tk-conc has been made quite efficient, it still would involve the creation of a string. The : construct avoids this. In a call to a widget function, a widget constructor, or a control function you may remove the call to tk-conc and place : in between each of its arguments. Those functions are able to understand this and treat the extra arguments as if they were glued together in one string, but without the extra cost of actually forming that string.

 
(tk-conc a b c .. w) <==> a : b : c : ... w
(setq i 10)
(.hello :configure :text i : " pies")
(.hello :configure :text (tk-conc i  " pies"))
(.hello :configure :text (format nil "~a pies" i))

The last three examples would all result in the text string being "10 pies", but the first method is the most efficient. That call will be made with no string or cons creation. The GC Monitor example, is written in such a way that there is no creation of cons or string types during normal operation. This is particularly useful in that case, since one is trying to monitor usage of conses by other programs, not its own usage.


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