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lazyset(3tcl) Lazy evaluation for variables and arrays lazyset(3tcl)


lazyset - Lazy evaluation


package require Tcl 8.5

package require lazyset ?1?

::lazyset::variable ?-array boolean? ?-appendArgs boolean? variableName commandPrefix


The lazyset package provides a mechanism for deferring execution of code until a specific variable or any index of an array is referenced.


::lazyset::variable ?-array boolean? ?-appendArgs boolean? variableName commandPrefix
Arrange for the code specified as commandPrefix to be executed when the variable whose name is specified by variableName is read for the first time. If the optional argument -array boolean is specified as true, then the variable specified as variableName is treated as an array and attempting to read any index of the array causes that index to be set by the commandPrefix as they are read. If the optional argument -appendArgs boolean is specified as false, then the variable name and subnames are not appended to the commandPrefix before it is evaluated. If the argument -appendArgs boolean is not specified or is specified as true then 1 or 2 additional arguments are appended to the commandPrefix. If -array boolean is specified as true, then 2 arguments are appended corresponding to the name of the variable and the index, otherwise 1 argument is appended containing the name of variable. The commandPrefix code is run in the same scope as the variable is read.


	::lazyset::variable page {apply {{name} {
		package require http
		set token [http::geturl]
		set data [http::data $token]
		return $data
	puts $page
	::lazyset::variable -array true page {apply {{name index} {
		package require http
		set token [http::geturl $index]
		set data [http::data $token]
		return $data
	puts $page(
	::lazyset::variable -appendArgs false simple {
		return -level 0 42
	puts $simple


Roy Keene


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Copyright (c) 2018 Roy Keene
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