bookFlight city isPremiumCustomer hasCheckedLuggage preferWindow =
let
luggageCost =
if hasCheckedLuggage then
...
else
...
in
if isPremiumCustomer then
if hasCheckedLuggage then
...
else
...
else if hasCheckedLuggage then
...
else
...
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No content
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– 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.”
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Write code for humans
Not computers
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No content
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bookFlight "TYS" "Premium"
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bookFlight city customerType =
case customerType of
"Premium" -> ...
"Regular" -> ...
_ -> ...
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bookFlight city customerType =
case customerType of
"Premium" -> ...
"Regular" -> ...
_ -> ...
rotateFuelRod fuelRod angle =
case angle of
Degrees amount -> ...
Radians amount -> ...
rotateControlRod controlRod angle =
case angle of
Degrees amount -> ...
Radians amount -> ...
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rotate rod angle =
case angle of
Degrees amount -> ...
Radians amount -> ...
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rotate rod angle =
case angle of
Degrees amount -> ...
Radians amount -> ...
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rotateInDegrees fuelRod 30
rotateInRadians controlRod pi
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rotateInDegrees fuelRod 30
rotateInRadians controlRod pi
Still encourages primitive obsession
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rotate rod angle =
case angle of
Degrees amount ->
rotateInDegrees rod amount
Radians amount ->
rotateInRadians rod amount
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Make APIs
UNDERSTANDABLE and
CONVENIENT
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Function
True
False
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No content
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time =
if doYouKnowTheTime person then
tellMeTheTime person
else
"""
Does anybody really know
what time it is?
"""
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time =
if doYouKnowTheTime person then
tellMeTheTime person
else
"""
Does anybody really know
what time it is?
"""
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time =
if doYouKnowTheTime person then
tellMeTheTime person
else
"""
Does anybody really know
what time it is?
"""
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time =
if doYouKnowTheTime person then
tellMeTheTime person
else
"""
Does anybody really know
what time it is?
"""
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time =
if doYouKnowTheTime person then
tellMeTheTime person
else
tellMeTheTime person
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Boolean Blindness
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– Conor McBride
“To make use of a Boolean you
have to know its provenance so
that you can know what it
means.”
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Get your organic,
homegrown
booleans from
Rhode Island!
Providence, RI
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PROVENANCE
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function findDogByName(name, dogs) {
if (name in dogs) {
return `${name} is a ${dogs[name].breed}`;
} else {
return 'Heck! No pupper found.';
}
}
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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."
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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."
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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."
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canDivide numerator denominator =
denominator /= 0
divisionResult numerator denominator =
if canDivide numerator denominator then
"The result is "
++ toString (numerator / denominator)
else
"Could not divide"
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canDivide numerator denominator =
denominator /= 0
divisionResult numerator denominator =
if canDivide numerator denominator then
"The result is "
++ toString (numerator / denominator)
else
"Could not divide"
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divisionResult numerator denominator =
if canDivide numerator denominator then
"The result is "
++ toString (numerator / denominator)
else
"The result is "
++ toString (numerator / denominator)
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– Dan Licata
“Boolean tests let you look,
options let you see.”
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Alternative
Return Values
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type Maybe a
= Just a
| Nothing
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findDogByName name dogs =
case Dict.get name dogs of
Just dog ->
name ++ " is a " ++ dog.breed
Nothing ->
"Heck! No pupper found."
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case whatTimeIsIt person of
Just time -> ...
Nothing -> ...
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case whatTimeIsIt person of
Just time -> ...
Nothing -> ...
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type TellTime
= Time String
| NoWatch
| NoPhone
| NoSundial
case whatTimeIsIt person of
Time time -> ...
NoWatch -> ...
NoPhone -> ...
NoSundial -> ...
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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
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See with Custom Types
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{ fetching = False
, success = True
, dog = Just { name = "Tucker" }
, error = False
, errorMessage = ""
}
Back to State
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if model.error then
...
else if model.fetching then
...
else if model.success then
...
else
...
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if model.fetching then
...
else if model.success then
...
else
...
type RemoteDoggo
= Ready
| Fetching
| Success Dog
| Error String
type alias Model =
{ dog : RemoteDoggo
, ...
}
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type RemoteDoggo
= Ready
| Fetching
| Success Dog
| Error String
type alias Model =
{ dog : RemoteDoggo
, ...
}
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type RemoteDoggo
= Ready
| Fetching
| Success Dog
| Error String
type alias Model =
{ dog : RemoteDoggo
, ...
}
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type RemoteDoggo
= Ready
| Fetching
| Success Dog
| Error String
type alias Model =
{ dog : RemoteDoggo
, ...
}
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{ dog = Ready, ... }
{ dog = Fetching, ... }
{ dog = Success { name = "Tucker" }, ... }
{ dog = Error "Uh oh!", ... }
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view : Model -> Html Msg
view model =
case model.dog of
Ready -> viewFindDog model
Fetching -> viewSpinner
Success dog -> viewDog dog
Error error -> viewError error
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view : Model -> Html Msg
view model =
case model.dog of
Ready -> viewFindDog model
Fetching -> viewSpinner
Success dog -> viewDog dog
-- Error error -> viewError error
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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!
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type alias Todo =
{ isComplete : Bool
, isEditing : Bool
}
type alias Customer =
{ isPlatinum : Bool
, isGold : Bool
, isSilver : Bool
}
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type TodoStatus
= Complete
| Editing
type alias Todo =
{ status : TodoStatus }
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type AccountType
= Platinum
| Gold
| Silver
type alias Customer =
{ accountType : AccountType }
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UT
Impossible state becomes impossible*
Code becomes understandable
*trademark Richard Feldman
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But wait,
There’s MORE!
AS SEEN ON
U I
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Save to archive
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Save to archive
A wild checkbox appears!
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Save to archive
✓
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Save to archive
?
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We design APIs for callers
So we should design UIs for users
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hu·mane
having or showing compassion or
benevolence
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Save to archive
?
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Save to archive
Save to inbox
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Save to archive
Save to inbox
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Save to archive
Save to inbox
Verb labels = when will it happen?
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Saved to archive
Saved to inbox
Adjective labels = describe the final result.
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Goodbye, boolean?
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B A L A N C E
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EMPATHY
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Jeremy Fairbank
@elpapapollo / jfairbank
Slides: bit.ly/ls-elm-bool