"       NAME            infinity

	AUTHOR          manchester

	FUNCTION        Provides a class of infinities

	ST-VERSION      2.2

	PREREQUISITES   

	CONFLICTS

	DISTRIBUTION    world

	VERSION         1

	DATE    22 Jan 1989

SUMMARY

This is a set of changes that implements infinity in the Number hierarchy.  

I obtained the original changes from the author of an article in comp.lang.smalltalk.

I have just installed it in my image and I have found two small omissions

which are corrected in what is below; there might be others.  Arithmetic

between infinities is not defined but magnitude comparisons are implemented.

Claus: fixed some minor bugs (args to errorUndefinedResult:) and some

       wrong comments.

       Changed retry:coercing: to match ST/X

"!

!Point methodsFor: 'testing'!

isFinite

	^x isFinite and: [y isFinite]!

isInfinite

	^x isInfinite or: [y isInfinite]! !

!Number methodsFor: 'testing'!

isFinite

	^true!

isInfinite

	^false! !

!Number methodsFor: 'coercing'!

retry: aSymbol coercing: aNumber

    "Arithmetic represented by the symbol, aSymbol,

    could not be performed with the receiver and the argument,

    aNumber, because of the differences in representation.  Coerce either

    the receiver or the argument, depending on which has higher generality, and

    try again.  If the symbol is the equals sign, answer false if the argument

    is not a Number.  If the generalities are the same, create an error message."

    |myGenerality otherGenerality|

    (aSymbol == #=) ifTrue:[

	(aNumber respondsTo:#generality) ifFalse:[^ false]

    ] ifFalse:[

	(aNumber respondsTo:#generality) ifFalse:[

	    self error:'retry:coercing: argument is not a number'.

	    ^ self

	]

    ].

    myGenerality := self generality.

    otherGenerality := aNumber generality.

    (myGenerality > otherGenerality) ifTrue:[

	^ self perform:aSymbol with:(self coerce:aNumber)

    ].

    (myGenerality < otherGenerality) ifTrue:[

	aNumber isInfinite ifTrue: [

	    ^ aNumber retryReverseOf:aSymbol with:self

	].

	^ (aNumber coerce:self) perform:aSymbol with:aNumber

    ].

    self error:'retry:coercing: oops - same generality'

! !

Number subclass: #Infinity

	instanceVariableNames: 'positive '

	classVariableNames: 'NegativeInfinity PositiveInfinity '

	poolDictionaries: ''

	category: 'Magnitude-Numbers'!

Infinity comment:

'I have two instances representing positive and negative infinity.

Instance Variables :-

	positive <Boolean>      :       if true the instance represents positive

					infinity. if false, negative infinity'

!

!Infinity methodsFor: 'arithmetic'!

* aNumber

	"Multiply the receiver by the argument and answer with the result."

	aNumber isInfinite ifTrue: [

	    self errorUndefinedResult: #*

	].

	^self

!

+ aNumber

	"Add the receiver by the argument and answer with the result."

	(aNumber isInfinite and: [aNumber ~~ self]) ifTrue: [

	    self errorUndefinedResult: #+

	].

	^self

!

- aNumber

	"subtracet aNumber from the receiver answer with the result."

	(aNumber isInfinite) ifTrue: [

	    self errorUndefinedResult: #-

	].

	^self

!

/ aNumber

	"Divide the receiver by the argument and answer with the result."

	(aNumber isInfinite or: [aNumber = 0]) ifTrue: [

	    self errorUndefinedResult: #/

	].

	^self

! !

!Infinity methodsFor: 'comparing'!

< aNumber

	"Positive infinity is greater than any number than positive infinity.

	 Analogously, negative infinity is less than any other number other

	 than negative infinity"

	aNumber == self ifTrue: [^false].

	^ positive not!

= aNumber

	^aNumber == self

! !

!Infinity methodsFor: 'testing'!

isFinite

	^false

!

isInfinite

	^true

! !

!Infinity methodsFor: 'coercing'!

generality

	"Infinities are more general than scalars, but not more general than

	 vectors (e.g. Points)"

	^85

!

retryReverseOf: aSymbol with: aNumber

	(aSymbol == #* or: [aSymbol == #+]) ifTrue: [

	    ^self perform: aSymbol with: aNumber

	].

	(aSymbol == #/ and: [aNumber isFinite]) ifTrue: [^0].

	(aSymbol == #< and: [aNumber isFinite]) ifTrue: [^positive].

	(aSymbol == #> and: [ aNumber isFinite ]) ifTrue: [^positive not].

	(aSymbol == #= and: [ aNumber isFinite ])ifTrue: [ ^false ]. 

	self errorUndefinedResult: aSymbol

! !

!Infinity methodsFor: 'printing'!

printOn: aStream

	aStream nextPutAll: self class name.

	aStream nextPutAll:(positive

				ifTrue: [' positive']

				ifFalse: [' negative'])

! !

!Infinity methodsFor: 'errors'!

errorUndefinedResult: messageName

	self error: 'Undefined result in an Infinity ', messageName

! !

!Infinity methodsFor: 'private'!

setPositive: aBoolean

	positive := aBoolean

! !

!Infinity class methodsFor: 'class initialization'!

initialize

	"Infinity initialize"

	PositiveInfinity := self basicNew setPositive: true.

	NegativeInfinity := self basicNew setPositive: false

! !

!Infinity class methodsFor: 'instance creation'!

negative

	"Return the unique instance of negative infinity"

	^NegativeInfinity

!

new

	self shouldNotImplement

!

positive

	"Return the unique instance of positive infinity"

	^PositiveInfinity

! !

Infinity initialize!

"COPYRIGHT.

 The above file is a Manchester Goodie protected by copyright.

 These conditions are imposed on the whole Goodie, and on any significant

 part of it which is separately transmitted or stored:

	* You must ensure that every copy includes this notice, and that

	  source and author(s) of the material are acknowledged.

	* These conditions must be imposed on anyone who receives a copy.

	* The material shall not be used for commercial gain without the prior

	  written consent of the author(s).

 Further information on the copyright conditions may be obtained by

 sending electronic mail:

	To: goodies-lib@cs.man.ac.uk

	Subject: copyright

 or by writing to The Smalltalk Goodies Library Manager, Dept of

 Computer Science, The University, Manchester M13 9PL, UK

 (C) Copyright 1992 University of Manchester

 For more information about the Manchester Goodies Library (from which 

 this file was distributed) send e-mail:

	To: goodies-lib@cs.man.ac.uk

	Subject: help 

"!