"

 COPYRIGHT (c) 2002 by eXept Software AG

              All Rights Reserved

 This software is furnished under a license and may be used

 only in accordance with the terms of that license and with the

 inclusion of the above copyright notice.   This software may not

 be provided or otherwise made available to, or used by, any

 other person.  No title to or ownership of the software is

 hereby transferred.

"

"{ Package: 'stx:libbasic2' }"

nil subclass:#LazyValue

	instanceVariableNames:'result block'

	classVariableNames:'AccessLock'

	poolDictionaries:''

	category:'Kernel-Processes'

!

!LazyValue class methodsFor:'documentation'!

copyright

"

 COPYRIGHT (c) 2002 by eXept Software AG

              All Rights Reserved

 This software is furnished under a license and may be used

 only in accordance with the terms of that license and with the

 inclusion of the above copyright notice.   This software may not

 be provided or otherwise made available to, or used by, any

 other person.  No title to or ownership of the software is

 hereby transferred.

"

!

documentation

"

    I represent an expression which might not be needed.

    Any messages sent to me will force the evaluation (once).

    The value is remembered.

    Can be used to simulate non-strict programming languages.

    [see also:]

        Block Future

    [author:]

        Claus Gittinger

"

!

examples

"

  listFrom:n - an infinite list of integers starting with n

                                                        [exBegin]

    |gen infiniteList|

    gen := [:n |

                        Cons 

                            car:n 

                            cdr:( LazyValue block:[gen value:n+1] )

                   ].

    infiniteList := gen value:1.

    1 to:10 do:[:i |

        Transcript showCR:infiniteList car.

        infiniteList := infiniteList cdr.

    ].

                                                        [exEnd]

  filterOdd:l - an infinite list filtering odd numbers from another list

                                                        [exBegin]

    |gen infiniteList genOdd infiniteOdd|

    gen := [:n |

                        Cons 

                            car:n 

                            cdr:( LazyValue block:[gen value:n+1] )

                   ].

    infiniteList := gen value:1.

    genOdd := [:l |

                        |lR el|

                        lR := l.

                        [ el := lR car. lR := lR cdr. el odd ] whileFalse:[].

                        Cons 

                            car:el 

                            cdr:( LazyValue block:[genOdd value:lR] )

                   ].

    infiniteOdd := genOdd value:infiniteList.

    1 to:10 do:[:i |

        Transcript showCR:infiniteOdd car.

        infiniteOdd := infiniteOdd cdr.

    ].

                                                        [exEnd]

  powersOf:n - an infinite list of powers of n

                                                        [exBegin]

    |genPowersOf infiniteListOfPowers|

    genPowersOf := [:base |

                   |powersOfBase|

                   powersOfBase :=

                       [:n |

                            Cons 

                                car:n 

                                cdr:( LazyValue block:[powersOfBase value:n*base] )

                       ].

                   powersOfBase value:1.

                ].

    infiniteListOfPowers := genPowersOf value:2.

    1 to:10 do:[:i |

        Transcript showCR:infiniteListOfPowers car.

        infiniteListOfPowers := infiniteListOfPowers cdr.

    ].

                                                        [exEnd]

  merge2:a _:b - merge 2 lists.

                                                        [exBegin]

    |genMerge2 l gen infiniteList genOdd infiniteOdd genEven infiniteEven genMerge|

    gen := [:n |

                        Cons 

                            car:n 

                            cdr:( LazyValue block:[gen value:n+1] )

                   ].

    infiniteList := gen value:1.

    genOdd := [:l |

                        |lR el|

                        lR := l.

                        [ el := lR car. lR := lR cdr. el odd ] whileFalse:[].

                        Cons 

                            car:el 

                            cdr:( LazyValue block:[genOdd value:lR] )

                   ].

    infiniteOdd := genOdd value:infiniteList.

    genEven := [:l |

                        |lR el|

                        lR := l.

                        [ el := lR car. lR := lR cdr. el even ] whileFalse:[].

                        Cons 

                            car:el 

                            cdr:( LazyValue block:[genEven value:lR] )

                   ].

    infiniteEven := genEven value:infiniteList.

    genMerge2 := [:a :b |

                   |nextA nextB|

                   nextA := a car.

                   nextB := b car.

                   nextA < nextB ifTrue:[

                       Cons 

                           car:nextA 

                           cdr:( LazyValue block:[genMerge2 value:(a cdr) value:b] )

                   ] ifFalse:[

                       Cons 

                           car:nextB 

                           cdr:( LazyValue block:[genMerge2 value:a value:(b cdr)] )

                   ].

                ].

    l := genMerge2 value:infiniteOdd value:infiniteEven.

    1 to:10 do:[:i |

        Transcript showCR:l car.

        l := l cdr.

    ].

                                                        [exEnd]

  genPoly - generate 2^i + 3^j + 5^k for all i,j,k

  not finished.

                                                        [exBegin]

    |genMerge2 l gen genPowersOf infiniteList powersOf2 powersOf3 powersOf5|

    gen := [:n |

                        Cons 

                            car:n 

                            cdr:( LazyValue block:[gen value:n+1] )

                   ].

    infiniteList := gen value:1.

    genPowersOf := [:base |

                   |powersOfBase|

                   powersOfBase :=

                       [:n |

                            Cons 

                                car:n 

                                cdr:( LazyValue block:[powersOfBase value:n*base] )

                       ].

                   powersOfBase value:1.

                ].

    powersOf2 := genPowersOf value:2.

    powersOf3 := genPowersOf value:3.

    powersOf5 := genPowersOf value:5.

    genAdd := [:n :l |

                   |gen|

                   gen := [:l |

                               Cons 

                                   car:(car l) * n 

                                   cdr:( LazyValue block:[gen value:(l cdr)] )

                          ].

                   gen value:l

              ].

    genMerge2 := [:a :b |

                   |min nextA nextB rA rB|

                   rA := a.

                   rB := b.

                   nextA := rA car.

                   [ (nextB := rB car) = nextA ] whileTrue:[ rB := rB cdr ].

                   nextA < nextB ifTrue:[

                       'a is smallest'.

                       min := nextA.

                       rA := rA cdr.

                   ] ifFalse:[

                       'b is smallest'.

                       min := nextB.

                       rB := rB cdr.

                   ].

                   Cons 

                       car:min 

                       cdr:( LazyValue block:[genMerge2 value:rA value:rB] )

                ].

    l := genMerge2 value:powersOf2 value:powersOf3 value:powersOf5.

    1 to:10 do:[:i |

        Transcript showCR:l car.

        l := l cdr.

    ].

                                                        [exEnd]

"

! !

!LazyValue class methodsFor:'initialization'!

initialize

    AccessLock := RecursionLock forMutualExclusion

! !

!LazyValue class methodsFor:'instance creation'!

block:aBlock

    ^ self new block:aBlock

! !

!LazyValue methodsFor:'error handling'!

doesNotUnderstand:aMessage

    block notNil ifTrue:[

        self _evaluate_

    ].

    ^ aMessage sendTo:result

! !

!LazyValue methodsFor:'evaluation'!

_evaluate_

    block notNil ifTrue:[

        AccessLock critical:[

            |b|

            (b := block notNil) ifTrue:[

                block := nil.

                result := b value.

            ].

        ].

    ].

    ^ result

! !

!LazyValue methodsFor:'printing'!

printOn:aStream

    aStream nextPutAll:'(lazy)'

! !

!LazyValue methodsFor:'private access'!

block:aBlock

    block := aBlock.

! !

!LazyValue methodsFor:'queries'!

class

    ^ self _evaluate_ class

!

isLazyValue

    ^ block notNil

! !

!LazyValue methodsFor:'testing'!

isBehavior

    ^ self _evaluate_ isBehavior

! !

!LazyValue class methodsFor:'documentation'!

version

    ^ '$Header: /cvs/stx/stx/libbasic2/LazyValue.st,v 1.1 2002-06-07 15:23:46 cg Exp $'

! !

LazyValue initialize!