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"
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COPYRIGHT (c) 2002 by eXept Software AG
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All Rights Reserved
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This software is furnished under a license and may be used
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only in accordance with the terms of that license and with the
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inclusion of the above copyright notice. This software may not
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be provided or otherwise made available to, or used by, any
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other person. No title to or ownership of the software is
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hereby transferred.
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"
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"{ Package: 'stx:libbasic2' }"
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1515
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ProtoObject subclass:#LazyValue
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instanceVariableNames:'result block'
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classVariableNames:'AccessLock'
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poolDictionaries:''
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category:'Kernel-Processes'
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!
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!LazyValue class methodsFor:'documentation'!
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copyright
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"
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COPYRIGHT (c) 2002 by eXept Software AG
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All Rights Reserved
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This software is furnished under a license and may be used
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only in accordance with the terms of that license and with the
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inclusion of the above copyright notice. This software may not
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be provided or otherwise made available to, or used by, any
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other person. No title to or ownership of the software is
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hereby transferred.
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"
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!
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documentation
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"
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I represent an expression which might not be needed.
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Any messages sent to me will force the evaluation (once).
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The value is remembered.
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Can be used to simulate non-strict programming languages.
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[see also:]
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Block Future LazyValue
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[author:]
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Claus Gittinger
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"
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!
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examples
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"
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[exBegin]
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|x|
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x := LazyValue block:[ Transcript flash. 1234 ].
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Delay waitForSeconds:4.
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Transcript showCR:x printString
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[exEnd]
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listFrom:n - an infinite list of integers starting with n
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[exBegin]
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|gen infiniteList|
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gen := [:n |
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Cons
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car:n
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cdr:( LazyValue block:[gen value:n+1] )
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].
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infiniteList := gen value:1.
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1 to:10 do:[:i |
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Transcript showCR:infiniteList car.
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infiniteList := infiniteList cdr.
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].
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[exEnd]
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filterOdd:l - an infinite list filtering odd numbers from another list
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[exBegin]
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|gen infiniteList genOdd infiniteOdd|
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gen := [:n |
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Cons
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car:n
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cdr:( LazyValue block:[gen value:n+1] )
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].
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infiniteList := gen value:1.
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genOdd := [:l |
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|lR el|
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lR := l.
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[ el := lR car. lR := lR cdr. el odd ] whileFalse:[].
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Cons
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car:el
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cdr:( LazyValue block:[genOdd value:lR] )
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].
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infiniteOdd := genOdd value:infiniteList.
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1 to:10 do:[:i |
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Transcript showCR:infiniteOdd car.
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infiniteOdd := infiniteOdd cdr.
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].
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[exEnd]
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powersOf:n - an infinite list of powers of n
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[exBegin]
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|genPowersOf infiniteListOfPowers|
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genPowersOf := [:base |
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|powersOfBase|
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powersOfBase :=
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[:n |
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Cons
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car:n
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cdr:( LazyValue block:[powersOfBase value:n*base] )
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].
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powersOfBase value:1.
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].
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infiniteListOfPowers := genPowersOf value:2.
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1 to:10 do:[:i |
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Transcript showCR:infiniteListOfPowers car.
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infiniteListOfPowers := infiniteListOfPowers cdr.
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].
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[exEnd]
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merge2:a _:b - merge 2 lists.
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[exBegin]
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|genMerge2 l gen infiniteList genOdd infiniteOdd genEven infiniteEven genMerge|
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gen := [:n |
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Cons
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car:n
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cdr:( LazyValue block:[gen value:n+1] )
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].
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infiniteList := gen value:1.
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genOdd := [:l |
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|lR el|
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lR := l.
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[ el := lR car. lR := lR cdr. el odd ] whileFalse:[].
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Cons
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car:el
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cdr:( LazyValue block:[genOdd value:lR] )
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].
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infiniteOdd := genOdd value:infiniteList.
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genEven := [:l |
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|lR el|
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lR := l.
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[ el := lR car. lR := lR cdr. el even ] whileFalse:[].
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Cons
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car:el
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cdr:( LazyValue block:[genEven value:lR] )
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].
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infiniteEven := genEven value:infiniteList.
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genMerge2 := [:a :b |
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|nextA nextB|
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nextA := a car.
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nextB := b car.
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nextA < nextB ifTrue:[
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Cons
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car:nextA
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cdr:( LazyValue block:[genMerge2 value:(a cdr) value:b] )
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] ifFalse:[
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Cons
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car:nextB
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cdr:( LazyValue block:[genMerge2 value:a value:(b cdr)] )
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].
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].
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l := genMerge2 value:infiniteOdd value:infiniteEven.
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1 to:10 do:[:i |
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Transcript showCR:l car.
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l := l cdr.
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].
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[exEnd]
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"
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!
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examples2
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"
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Attention: for the examples below to work,
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you have to enable the Parsers AllowLazyValueExtension and
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the parsers AllowFunctionCallSyntaxForBlockEvaluation.
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(set the corresponding classVariables in Parser to true).
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Parser classVarAt:#AllowSTXSyntaxExtensions put:true.
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Parser classVarAt:#AllowLazyValueExtension put:true.
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Parser classVarAt:#AllowFunctionCallSyntaxForBlockEvaluation put:true.
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notice that this is just syntactic sugar: the 'FOO(arg)' notation is
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the same as 'FOO value:arg', and !!block is the same as 'LazyValue block:block'.
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BTW: the # is the binary Cons-message.
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[exBegin]
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|x|
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x := !![ Transcript flash. 1234 ].
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Delay waitForSeconds:4.
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Transcript showCR:x printString
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[exEnd]
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listFrom:n - an infinite list of integers starting with n
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[exBegin]
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|GEN infiniteList|
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GEN := [:n | n # !![ GEN( n+1) ] ].
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infiniteList := GEN(1).
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1 to:10 do:[:i |
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Transcript showCR:infiniteList car.
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infiniteList := infiniteList cdr.
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].
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[exEnd]
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filterOdd:l - an infinite list filtering odd numbers from another list
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[exBegin]
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|GEN infiniteList GENOdd infiniteOdd|
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GEN := [:n | n # !![ GEN(n+1) ] ].
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infiniteList := GEN(1).
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GENOdd := [:l |
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|lR el|
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lR := l.
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[
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el := lR car.
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lR := lR cdr.
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el odd
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] whileFalse:[].
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el # !![ GENOdd(lR) ]
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].
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infiniteOdd := GENOdd(infiniteList).
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1 to:10 do:[:i |
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Transcript showCR:infiniteOdd car.
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infiniteOdd := infiniteOdd cdr.
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].
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[exEnd]
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powersOf:n - an infinite list of powers of n
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[exBegin]
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|GENPowersOf infiniteListOfPowers|
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GENPowersOf := [:base |
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|powersOfBase|
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powersOfBase := [:n | n # !![powersOfBase(n*base)]].
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powersOfBase(1)
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].
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infiniteListOfPowers := GENPowersOf(2).
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1 to:10 do:[:i |
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Transcript showCR:infiniteListOfPowers car.
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infiniteListOfPowers := infiniteListOfPowers cdr.
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].
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[exEnd]
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merge2:a _:b - merge 2 lists.
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the genODD and genEVEN setup is just for testing.
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[exBegin]
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|GENMerge2 l GEN infiniteList GENFILTER GENOdd infiniteOdd GENEven infiniteEven GENMerge|
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GEN := [:n | n # !![GEN(n+1)] ].
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infiniteList := GEN(1).
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GENFILTER := [:l :filter|
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|lR el|
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lR := l.
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[ el := lR car. lR := lR cdr. filter(el) ] whileFalse:[].
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el # !![ GENFILTER(lR. filter) ]
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].
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GENOdd := [:l | GENFILTER(l. [:el | el odd])].
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infiniteOdd := GENOdd(infiniteList).
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GENEven := [:l | GENFILTER(l. [:el | el even])].
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infiniteEven := GENEven value:infiniteList.
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GENMerge2 := [:a :b |
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|nextA nextB|
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nextA := a car.
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nextB := b car.
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nextA < nextB ifTrue:[
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nextA # !![ GENMerge2(a cdr . b)]
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] ifFalse:[
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nextB # !![ GENMerge2(a . b cdr)]
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].
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].
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l := GENMerge2(infiniteOdd .infiniteEven).
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1 to:10 do:[:i |
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Transcript showCR:l car.
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l := l cdr.
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].
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[exEnd]
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powersOf2 - generate 2^i for all i >= 0
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[exBegin]
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|l GENPowersOf powersOf2|
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GENPowersOf := [:base |
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|powersOfBase|
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powersOfBase := [:n | n # !![powersOfBase value:n*base] ].
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powersOfBase value:1.
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].
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powersOf2 := GENPowersOf value:2.
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1 to:10 do:[:i |
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Transcript showCR:powersOf2 car.
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powersOf2 := powersOf2 cdr.
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].
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[exEnd]
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genPoly - generate 2^i + 3^j + 5^k for all i,j,k
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unfinished.
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[exBegin]
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|GENPowersOf genMerge2 l gen infiniteList powersOf2 powersOf3 powersOf5
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genAdd genMerge3|
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gen := [:n | Cons car:n cdr:( !![gen value:n+1] ) ].
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infiniteList := gen value:1.
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GENPowersOf := [:base |
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|powersOfBase|
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powersOfBase := [:n | n # !![powersOfBase value:n*base] ].
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powersOfBase(1).
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].
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powersOf2 := GENPowersOf(2). '2^i' for all i
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powersOf3 := GENPowersOf(3). '3^j' for all j
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powersOf5 := GENPowersOf(5). '5^k' for all k
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genMerge3 := [:a :b :c|
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|min nextA nextB nextC rA rB rC|
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nextA := a car.
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nextB := b car.
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nextC := c car.
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[ (nextB := rB car) = nextA ] whileTrue:[ rB := rB cdr ].
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nextA < nextB ifTrue:[
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'a is smallest'.
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min := nextA.
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rA := rA cdr.
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] ifFalse:[
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'b is smallest'.
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min := nextB.
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rB := rB cdr.
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].
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Cons
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car:min
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cdr:!![genMerge2 value:rA value:rB]
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].
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genMerge3 := [:a :b :c|
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genMerge2(genMerge2(a . b) . c).
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].
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l := genMerge3 value:powersOf2 value:powersOf3 value:powersOf5.
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1 to:10 do:[:i |
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Transcript showCR:l car.
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l := l cdr.
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].
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[exEnd]
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"
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! !
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!LazyValue class methodsFor:'initialization'!
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initialize
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AccessLock := RecursionLock forMutualExclusion
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! !
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!LazyValue class methodsFor:'instance creation'!
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block:aBlock
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^ self new block:aBlock
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! !
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409 |
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!LazyValue methodsFor:'error handling'!
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411 |
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doesNotUnderstand:aMessage
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block notNil ifTrue:[
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self _evaluate_
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].
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^ aMessage sendTo:result
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! !
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!LazyValue methodsFor:'evaluation'!
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_evaluate_
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block notNil ifTrue:[
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AccessLock critical:[
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|b|
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(b := block) notNil ifTrue:[
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block := nil.
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result := b value.
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].
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].
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].
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^ result
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433 |
! !
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434 |
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435 |
!LazyValue methodsFor:'printing'!
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436 |
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437 |
displayString
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438 |
block isNil ifTrue:[
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439 |
^ 'LazyValue (unevaluated)'
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440 |
].
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441 |
^ 'LazyValue (evaluated)'
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442 |
!
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443 |
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1042
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printOn:aStream
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445 |
aStream nextPutAll:'(lazy)'
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|
446 |
! !
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|
447 |
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448 |
!LazyValue methodsFor:'private access'!
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449 |
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450 |
block:aBlock
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451 |
block := aBlock.
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|
452 |
! !
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453 |
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454 |
!LazyValue methodsFor:'queries'!
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455 |
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class
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457 |
^ self _evaluate_ class
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|
458 |
!
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459 |
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460 |
isLazyValue
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|
461 |
^ block notNil
|
|
462 |
! !
|
|
463 |
|
|
464 |
!LazyValue methodsFor:'testing'!
|
|
465 |
|
|
466 |
isBehavior
|
|
467 |
^ self _evaluate_ isBehavior
|
|
468 |
! !
|
|
469 |
|
|
470 |
!LazyValue class methodsFor:'documentation'!
|
|
471 |
|
|
472 |
version
|
1515
|
473 |
^ '$Header: /cvs/stx/stx/libbasic2/LazyValue.st,v 1.5 2005-01-26 13:54:57 stefan Exp $'
|
1042
|
474 |
! !
|
1307
|
475 |
|
1042
|
476 |
LazyValue initialize!
|