Jeremy Fairbank
April 26, 2019
87

# Lambda Squared 2019: Solving the Boolean Identity Crisis

Powerful in its simplicity, the boolean can be limiting. In this talk, we will look at examples where booleans obscure the meaning of code and make it harder to maintain. You will learn patterns such as ADTs to write clear code without booleans and gain greater empathy for your teammates and users.

April 26, 2019

## Transcript

}

12. ### { fetching: false, success: false, dog: null, error: true, errorMessage:

'Ruh roh!', }
13. ### { fetching: true, success: true, dog: { name: 'Tucker' },

error: true, errorMessage: 'Uh oh!', } ¯\_(ツ)_/¯ Invalid State
14. ### { fetching: true, success: true, dog: { name: 'Tucker' },

error: true, errorMessage: 'Uh oh!', } ¯\_(ツ)_/¯ Invalid State
15. ### if (props.error) { ... } else if (props.fetching) { ...

} else if (props.success) { ... } else { ... }

19. ### – Robert Harper “There is no information carried by a

Boolean beyond its value, and that’s the rub.”

of the code.

25. ### Boolean Algebra George Boole x ∧ y x ∨ y

x ∨ (y ∧ z) ¬x
26. ### Boolean Values True && True True || False True ||

(True && False) not True

28. ### Premise 1: If it’s raining then it’s cloudy. Premise 2:

It’s raining. Conclusion: It’s cloudy. Propositional Logic
29. ### Premise 1: If it’s raining then it’s cloudy. Premise 2:

It’s raining. Conclusion: It’s cloudy. PROPOSITIONS
30. ### prop·o·si·tion a statement that expresses a concept that can be

true or false
31. ### Boolean ≠ Proposition A proposition, p, that is true is

not the same as saying p is equal to true.

38. ### bookFlight city isPremiumCustomer = if isPremiumCustomer then ... else ...

Regular customer?
39. ### viewOpacitySlider = viewSlider True viewVolumeSlider = viewSlider False view model

= div [] [ viewOpacitySlider , viewVolumeSlider ] ?

41. ### viewSlider msg isHorizontal = if isHorizontal then ... else ...

Implicit vertical slider
42. ### – Robert Martin “Boolean arguments loudly declare that the function

does more than one thing. They are confusing and should be eliminated.”

= ...

= ...

= ...

49. ### bookFlight city isPremiumCustomer hasCheckedLuggage preferWindow = let luggageCost = if

hasCheckedLuggage then ... else ... in if isPremiumCustomer then if hasCheckedLuggage then ... else ... else if hasCheckedLuggage then ... else ...
50. ### – Martin Fowler “…an API should be written to make

it easier for the caller, so if we know where the caller is coming from we should design the API with that information in mind.”

53. ### bookFlight city customerType = case customerType of "Premium" -> ...

"Regular" -> ... _ -> ...
54. ### bookFlight city customerType = case customerType of "Premium" -> ...

"Regular" -> ... _ -> ...
55. ### bookFlight "TYS" "premium" bookFlight "TYS" "economical" bookFlight "TYS" "" Primitive

Obsession Revisited

57. ### type CustomerType = Premium | Regular bookFlight "TYS" Premium bookFlight

"TYS" Regular
58. ### bookFlight city customerType = case customerType of Premium -> ...

Regular -> ...
59. ### type Angle = Degrees Float | Radians Float rotateFuelRod fuelRod

(Degrees 30) rotateControlRod controlRod (Radians pi)
60. ### rotateFuelRod fuelRod angle = case angle of Degrees amount ->

... Radians amount -> ... rotateControlRod controlRod angle = case angle of Degrees amount -> ... Radians amount -> ...

68. ### time = if doYouKnowTheTime person then tellMeTheTime person else """

Does anybody really know what time it is? """
69. ### time = if doYouKnowTheTime person then tellMeTheTime person else """

Does anybody really know what time it is? """
70. ### time = if doYouKnowTheTime person then tellMeTheTime person else """

Does anybody really know what time it is? """
71. ### time = if doYouKnowTheTime person then tellMeTheTime person else """

Does anybody really know what time it is? """

person

74. ### – Conor McBride “To make use of a Boolean you

have to know its provenance so that you can know what it means.”

77. ### function findDogByName(name, dogs) { if (name in dogs) { return

`\${name} is a \${dogs[name].breed}`; } else { return 'Heck! No pupper found.'; } }
78. ### findDogByName name dogs = if Dict.member name dogs then case

Dict.get name dogs of Just dog -> name ++ " is a " ++ dog.breed Nothing -> "Heck! No pupper found." else "Heck! No pupper found."
79. ### findDogByName name dogs = if Dict.member name dogs then case

Dict.get name dogs of Just dog -> name ++ " is a " ++ dog.breed Nothing -> "Heck! No pupper found." else "Heck! No pupper found."
80. ### findDogByName name dogs = if Dict.member name dogs then case

Dict.get name dogs of Just dog -> name ++ " is a " ++ dog.breed Nothing -> "Heck! No pupper found." else "Heck! No pupper found."
81. ### canDivide numerator denominator = denominator /= 0 divisionResult numerator denominator

= if canDivide numerator denominator then "The result is " ++ toString (numerator / denominator) else "Could not divide"
82. ### canDivide numerator denominator = denominator /= 0 divisionResult numerator denominator

= if canDivide numerator denominator then "The result is " ++ toString (numerator / denominator) else "Could not divide"
83. ### divisionResult numerator denominator = if canDivide numerator denominator then "The

result is " ++ toString (numerator / denominator) else "The result is " ++ toString (numerator / denominator)

you see.”

87. ### findDogByName name dogs = case Dict.get name dogs of Just

dog -> name ++ " is a " ++ dog.breed Nothing -> "Heck! No pupper found."

...

...
90. ### type TellTime = Time String | NoWatch | NoPhone |

NoSundial case whatTimeIsIt person of Time time -> ... NoWatch -> ... NoPhone -> ... NoSundial -> ...
91. ### divide numerator denominator = case denominator of 0 -> Err

"Divide by zero" _ -> Ok (numerator / denominator) divisionResult numerator denominator = case divide 4 2 of Ok result -> "The result is " ++ toString result Err error -> "Could not divide: " ++ error

93. ### { fetching = False , success = True , dog

= Just { name = "Tucker" } , error = False , errorMessage = "" } Back to State
94. ### if model.error then ... else if model.fetching then ... else

if model.success then ... else ...

...
96. ### { fetching = True , success = True , dog

= { name = "Tucker" } , error = True , errorMessage = "Uh oh!" }

99. ### type RemoteDoggo = Ready | Fetching | Success Dog |

Error String type alias Model = { dog : RemoteDoggo , ... }
100. ### type RemoteDoggo = Ready | Fetching | Success Dog |

Error String type alias Model = { dog : RemoteDoggo , ... }
101. ### type RemoteDoggo = Ready | Fetching | Success Dog |

Error String type alias Model = { dog : RemoteDoggo , ... }
102. ### type RemoteDoggo = Ready | Fetching | Success Dog |

Error String type alias Model = { dog : RemoteDoggo , ... }
103. ### { dog = Ready, ... } { dog = Fetching,

... } { dog = Success { name = "Tucker" }, ... } { dog = Error "Uh oh!", ... }
104. ### view : Model -> Html Msg view model = case

model.dog of Ready -> viewFindDog model Fetching -> viewSpinner Success dog -> viewDog dog Error error -> viewError error
105. ### view : Model -> Html Msg view model = case

model.dog of Ready -> viewFindDog model Fetching -> viewSpinner Success dog -> viewDog dog -- Error error -> viewError error
106. ### This `case` does not have branches for all possibilities. 47|>

case model.dog of 48|> Ready -> viewFindDog model 49|> 50|> Fetching -> viewSpinner 51|> 52|> Success dog -> viewDog dog You need to account for the following values: Main.Error _ Add a branch to cover this pattern!
107. ### type alias Todo = { isComplete : Bool , isEditing

: Bool } type alias Customer = { isPlatinum : Bool , isGold : Bool , isSilver : Bool }
108. ### type TodoStatus = Complete | Editing type alias Todo =

{ status : TodoStatus }
109. ### type AccountType = Platinum | Gold | Silver type alias

Customer = { accountType : AccountType }

Feldman

for users

121. ### Save to archive Save to inbox Verb labels = when

will it happen?
122. ### Saved to archive Saved to inbox Adjective labels = describe

the final result.