Smalltalk Best Practice Patterns Volume 1: Coding - Free

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Conditional ExpressionHow do you format conditional expressions where both branches assign or return a value?Most programming languages make a distinction between statements that work solely by side effectand expressions that return values. For example, control structures in C and Pascal work only bycontrolling how other statements execute.In Smalltalk there are no pure statements. All control structures are implemented in terms ofmessages, and all messages return values. This leads to the possibility of using the value of controlstructures.Programmers new to Smalltalk are likely to be surprised the first time they encounter loops orconditionals used as an expression. New Smalltalkers are likely to write:self isInitializedifTrue: [cost := self calculateCost]ifFalse: [cost := 0]These expressions can be translated into the following without changing the meaning:cost := self isInitializedifTrue: [self calculateCost]ifFalse: [0]Is the simpler form worth the possibility of confusion for beginners? It more directly communicatesthe intent of the expression. You don’t mean “there are two paths of expression, one of which setsthe value of cost to the result of sending myself calculateCost and the other of which sets the valueof cost to 0,” you mean “set cost to one of two values, either the result of sending myselfcalculateCost or 0.”Format conditionals so their value is used where it clearly expresses the intent of the method.Assignment and return are often found in both branches of a conditional. Look for opportunities tofactor both to the outside of the conditional.Here is an example of a return on both branches of a conditional:costself isInitializedifTrue: [^self calculateCost]ifFalse: [^0]If I write code like this, I don’t mean “here are two alternative paths of execution,”, I mean, “hereare two alternative values to be returned.” Thus, a Conditional Expression expresses my intentmore clearly:cost^self isInitializedifTrue: [self calculateCost]ifFalse: [0]I commonly see code in which both sides of a conditional expression evaluate to a Boolean. Startwith this:Coding Patterns page 132 of 147 9/30/2006
aCollection isEmptyifTrue: [empty := true]ifFalse: [empty := false]Using Conditional Expression we first factor out the assignment:empty := aCollection isEmptyifTrue: [true]ifFalse: [false]We can go a step further and eliminate the conditional entirely. The following code is equivalent tothe preceding:empty := aCollection isEmptyYou may be able to express one or both branches of the conditional more explicitly by using aComposed Method.Coding Patterns page 133 of 147 9/30/2006
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Smalltalk Best Practice PatternsVol
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STATE..............................
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PrefaceThis preface will explain wh
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AcknowledgmentsI would like to than
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Many of the patterns tell you what
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day to day in development and docum
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What’s Missing?There are a few co
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AdoptionHow can you learn and use t
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Why Patterns WorkHere is the Big As
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Role of PatternsReadingThese patter
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BehaviorObjects model the world thr
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Composed MethodYou are implementing
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Complete Creation MethodA Composed
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ConversionHow do you convert inform
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Converter Creation MethodYou need t
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Query MethodA Composed Method has h
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Execute Around MethodHow do you rep
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MessagesMessages are the heartbeat
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Choosing MessageYou are using a Mes
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Decomposing MessageYou are using a
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Intention Revealing SelectorYou may
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Integer>>addFloat: aFloat^self asFl
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Class: SuperFindingClientsuperclass
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Extending SuperYou are using Super.
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DelegationA Composed Method needs w
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Self DelegationYou are using Delega
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Pluggable BehaviorHow can you param
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Pluggable BlockYou need complex Plu
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marriedMenAndUnmarriedWomen^self ma
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Instance VariablesI wrote the secti
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Variable StateHow do you represent
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Timer>>defaultMillisecondPeriod^100
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Default Value MethodYou have a comp
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Direct Variable AccessYou need to a
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Getting MethodYou are using Lazy In
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Collection Accessor MethodYou are u
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Enumeration MethodYou are using Ind
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Role Suggesting Instance Variable N
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Temporary VariableA Composed Method
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Caching Temporary VariableA Perform
Page 82 and 83: Reusing Temporary VariableTemporary
Page 84 and 85: CollectionsThe collection hierarchy
Page 86 and 87: CollectionHow do you represent a on
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Page 90 and 91: | s |s := Set new.Like an OrderedCo
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Page 102 and 103: IsEmptyHow do you test if a collect
Page 104 and 105: ConcatentationHow do you put two co
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Page 108 and 109: Select/RejectHow do you execute cod
Page 110 and 111: Inject:into:You need an Enumeration
Page 112 and 113: Duplicate Removing SetYou have a Co
Page 114 and 115: StackHow do you implement a stack?O
Page 116 and 117: implementation of queues in Pascal
Page 118 and 119: Lookup CacheHow do you optimize com
Page 120 and 121: Parsing StreamHow do you write a si
Page 122 and 123: ClassesThere is probably no coding
Page 124 and 125: Qualified Subclass NameYou have cre
Page 126 and 127: FormattingNo other topic generates
Page 128 and 129: Type Suggesting Parameter NameYou a
Page 130 and 131: Rectangular BlockYou are writing an
Page 134 and 135: Simple Enumeration ParameterWhat sh
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Page 138 and 139: Point>>printOn: aStreamaStreamprint
Page 140 and 141: Development ExampleIn this chapter,
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Page 144 and 145: MoneySum>>printOn: aStreammonies do
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Smalltalk Best Practice Patterns Volume 1: Coding - Free

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