@elpapapollo / jfairbank
Building
Web Apps with Elm
Jeremy Fairbank
Follow instructions here to get setup!
github.com/jfairbank/elm-workshop
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Goals
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Goals
• Benefits of Elm
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Goals
• Benefits of Elm
• Elm Syntax
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Goals
• Benefits of Elm
• Elm Syntax
• Functional Programming
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Goals
• Benefits of Elm
• Elm Syntax
• Functional Programming
• Build web apps with Elm
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Format
• Some slides.
• Coding.
• Ask questions!
• Code along if you want.
- Exercises
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Follow Along
github.com/jfairbank/elm-workshop
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elm
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elm
• Created in 2012 Evan Czaplicki
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elm
• Created in 2012 Evan Czaplicki
• Web and UI focused
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elm
• Created in 2012 Evan Czaplicki
• Web and UI focused
• Compiles to JavaScript
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elm
• Created in 2012 Evan Czaplicki
• Web and UI focused
• Compiles to JavaScript
• Influences: Haskell, SML, OCaml, F#
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elm
• Created in 2012 Evan Czaplicki
• Web and UI focused
• Compiles to JavaScript
• Influences: Haskell, SML, OCaml, F#
• Functional
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elm
• Created in 2012 Evan Czaplicki
• Web and UI focused
• Compiles to JavaScript
• Influences: Haskell, SML, OCaml, F#
• Functional
• Statically Typed
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What’s all the fuss about?
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undefined is not a function
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No runtime exceptions in
practice.
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The argument to function `greet` is causing a mismatch.
11| greet 42
^^
Function `greet` is expecting the argument to be:
String
But it is:
number
Compile time static type checks
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Fast
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Fast
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Fast
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Functional
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Expressive and Declarative
Concise and intent-revealing
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Expressive and Declarative
Fewer lines of code!
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function doubleNumbers(numbers) {
const doubled = [];
const l = numbers.length;
for (let i = 0; i < l; i++) {
doubled.push(numbers[i] * 2);
}
return doubled;
}
doubleNumbers([1, 2, 3]);
// [2, 4, 6]
×
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doubleNumbers list =
List.map ((*) 2) list
doubleNumbers myList -- [2, 4, 6]
✓
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Pure Functions
Predictable and easier to test
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add x y = x + y
add 2 2 == 4
add 2 2 == 4
add 2 2 == 4
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Immutable Data
No mutation bugs or inconsistent state
conference =
{ name = "JazzCon" }
nextConference =
{ conference | name = "ConnectJS" }
✓
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Built-in Framework
The Elm Architecture
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Built-in Framework
Unidirectional with organized state
changes
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Update
View
Model
Messages
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Sold?
Let’s learn Elm!
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Plan
9:00 - 9:15 Intro
9:15 - 9:45 Code: Syntax and Functional Programming
9:45 - 9:55 The Elm Architecture
9:55 - 10:30 Code: Building a Todo List
10:30 - 11:00 Code: Exercise and Break
11:00 - 11:10 HTTP: Commands and Decoders
11:10 - 11:45 Code: Fetching a GitHub User
11:45 - 12:00 Code: Exercise
12:00 - 12:15 Code: Components
12:15 - 12:30 Wrap Up
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Coding Time!
Elm Syntax
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The Elm Architecture
Model-View-Update
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Controller
View
Model
Model
Model
Model
Model
View
View
View
View
MVC
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View
Model
Model
Model
Model
Model
View
Model
View
Model
View
Model
View
Model
Model
Two-way Data Binding
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Update
View
Model
Messages
Unidirectional
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Update
View
Model
Messages
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Model
Single source of truth
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Model
• String
• Int
• Record
• List
• Etc.
Any data type
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Update
View
Model
Messages
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View
Projection of model
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UI as a Function
Model
Virtual
DOM
View
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elm
Virtual
DOM
Todo List
Learn Elm
Build awesome Elm apps
Learn functional programming
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Todo List
Learn Elm
Build awesome Elm apps
Learn functional programming
Model
Delete first todo item
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View
Model
Notice missing item in virtual DOM list,
so just remove corresponding
.
elm
Virtual
DOM
Todo List
Build awesome Elm apps
Learn functional programming
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Update
View
Model
Messages
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Messages
Standardized application events
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elm
app
model
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elm
app
model
Commands
HTTP
Dates
Random #’s
Subscriptions
WebSockets
Browser Window
Mouse Position
Events
Text Input
Mouse Click
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elm
app
model
Commands
HTTP
Dates
Random #’s
Subscriptions
WebSockets
Browser Window
Mouse Position
Events
Text Input
Mouse Click
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Update
View
Model
Messages
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Update
Responds to messages
and creates new state
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Update
Model
New
Model Command
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model
Update View
elm
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model
Update View
elm
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model
Update View
elm
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model
Update View
elm
!
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model
Update View
elm
!
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model
Update View
elm
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model
Update View
elm
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model
Update View
elm
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model
Update View
elm
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Coding Time!
Building a Todo List
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Exercise(s)
• Allow adding new todo items.
• Underneath todo list add a button to add a new todo item to the todo
list.
• The new todo item should have a blank description and should start off
in an Editing state.
• Hint: you’ll probably want the :: or ++ operators.
• Allow deleting todo items.
• Add delete button next to edit button.
• Hint: editing/saving/completing required List.map. Deletion will
require List.filter.
• Allow marking a complete todo item as incomplete.
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Working with HTTP
Commands and Decoders
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Update
Model
New
Model Command
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Update
Model
New
Model Command
?
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Side
Effects
I/O
Mutable
State
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Commands
Describe side effects
instead of doing them
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HTTP Commands
Update
Command elm
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What about the response?
Update
elm
JSON
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What about the response?
Update
elm
JSON
Dynamic Payloads
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What about the response?
Update
elm
JSON
Dynamic Payloads
Static Types
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Decoders
Transform arbitrary JSON to
static types
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user : User
user =
{ id = 1
, name = "Jeremy"
}
{
"id": 1,
"name": "Jeremy"
}
Decoder
type alias User =
{ id : Int, name : String }
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{
"id": 1
}
type alias User =
{ id : Int, name : String }
Decoder
?
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{
"id": 1
}
type alias User =
{ id : Int, name : String }
Decoder
?
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{
"id": 1
}
Decoder BadPayload
Err
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{
"id": 1
}
Decoder BadPayload
Err
Result type
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Result
type Result error value
= Ok value
| Err error
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Result
type Result error value
= Ok value
| Err error
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Result
type Result error value
= Ok value
| Err error
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user =
{ id = 1
, name = "Jeremy"
}
{
"id": 1,
"name": "Jeremy"
}
Decoder
Ok
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{
"id": 1,
"name": "Jeremy"
}
“id”
int
“name”
string
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{
"id": 1,
"name": "Jeremy"
}
“id”
int
“name”
string
Field decoders
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{
"id": 1,
"name": "Jeremy"
}
“id”
int
“name”
string
1
Ok
"Jeremy"
Ok
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1
Ok
"Jeremy"
Ok
User
Decoder
user =
{ id = 1
, name = "Jeremy"
}
Ok
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1
Ok
User
Decoder
BadPayload
Err
BadPayload
Err
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Coding Time!
Fetching a GitHub User
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Exercise(s)
• Add a loading screen.
• Display something like “Fetching user …”
• Hint: what state should you update and when?
• Display another field or two for the user.
• Hint: Remember to update the User type and the userDecoder.
• Decode GitHub error messages.
• Use login nouser for testing.
• Hint: you need to decode response.body in the Http.BadStatus
branch.
• Hint: you’ll probably want the decodeString and field functions from:
package.elm-lang.org/packages/elm-lang/core/5.1.1/Json-Decode
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Coding Time!
Components
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Components vs. Flat Structure
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Components vs. Flat Structure
• Components
• Pros
• Perfect for independent,
reusable applications
• Single-page applications
• Cons
• Nested complexity
• More boilerplate
• Cumbersome parent-child
communication
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Components vs. Flat Structure
• Components
• Pros
• Perfect for independent,
reusable applications
• Single-page applications
• Cons
• Nested complexity
• More boilerplate
• Cumbersome parent-child
communication
• Flat Structure
• Flatter state
• Use function and module helpers
• Pros
• No parent-child
communication ceremony
• Easier to follow flow of
application
• Cons
• Larger models
• More Msg values