Purely Functional Web Apps (Extended Version, 16.02.2016)
Transcript of Purely Functional Web Apps (Extended Version, 16.02.2016)
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FUNCTIONALWEB APPS
Michał Płachta@miciek
PURELY
SUCHPURE
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Let’s create a full-blown web application
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MAIN DEV GOALS■ explicit external dependencies■ explicit side effects■ explicit configuration■ explicit error handling
pure functions & strong typesSolution:
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Gitlab
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Merge Request StatsGitlab Our app
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Application Architecture
Gitlab Server
Merge RequestFetcher
MR StatsWeb Service
In MemoryStorage
MR StatsWeb Application
data flow
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Haskell & Elm
Gitlab Server
Merge RequestFetcher
MR StatsWeb Service
In MemoryStorage
MR StatsWeb Application
data flow
Elm
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One slide Haskell tutorial
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Using Servant asHTTP Client
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Haskell Servant■ set of libraries■ for HTTP related stuff■ DRY■ type safe■ type level DSL
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Let’s start with the Fetcher
Gitlab Server
Merge RequestFetcher
MR StatsWeb Service
In MemoryStorage
MR StatsWeb Application
data flow
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Gitlab API
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Merge Request JSON
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Merge Request Data Type
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API Type
/api/v3/projects/{projectId}/merge_requests/?page={page}
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apiQuery Function Type
URL to query
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queryApi Return Type
EitherT
ServantError
IO
Paged
[MergeRequest]
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IO Type
IO
Paged
[MergeRequest]
■ side effecting■ an action■ e.g. get something
from the server■ can’t get rid of it
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Quick IO Side NoteWhat’s the difference?
Examples from Haskell
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EitherT
EitherT
ServantError IO
■ can be either Left or Right■ Left contains ServantError■ Right contains IO a
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ServantError
■ FailureResponse
■ DecodeFailure
■ UnsupportedContentType
■ ConnectionError
■ InvalidContentTypeHeader
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apiQuery implementation
client is a Servant function that looks at our type and provides the implementation
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Nicer API with AppConfig
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pagedMergeRequests function
■ Maybe a type can be○ Just a
○ Nothing
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allMergeRequests function
uses
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allMergeRequests implementation
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Side Note: Side Effects without IO?
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Merge Request Fetcher module
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Same story: Comments
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Business Logic: Stats Calculations
■ title■ create date■ last update date■ upvotes■ downvotes■ list of comments
■ Merge Request object■ time to merge (calculated)■ comments quantity (calculated)■ url to the MR
Merge Request (from Gitlab) Merge Request Stats (ours)
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calculateStats function
no IO!
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Now onto: In Memory Storage
Gitlab Server
Merge RequestFetcher
MR StatsWeb Service
In MemoryStorage
MR StatsWeb Application
data flow
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StatsStorage
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main function
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fetchMRsAndSaveStats function
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Serving the stats
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We did it!Merge Request
FetcherMR Stats
Web ServiceIn Memory
Storage
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Automatic JS client and Markdown!
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Let’s do the frontenddata flow
Gitlab Server
Merge RequestFetcher
MR StatsWeb Service
In MemoryStorage
MR StatsWeb Application
■ using Elm■ merge request stats in a table■ dynamically updated each 5 seconds
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Elm architecture
Elm Runtime
Model
update
Action
NewModel
view
Effect
HtmlObject
Elm Rendering
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Elm architecture in types
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Our Model
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Our Actions
■ UpdateList is generated inside the app■ Tick is generated outside of the app
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update Function
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mergeRequestsFetchEffect
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view = virtual DOM objects
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Effects are Data
Elm Runtime
MRs &refresh
time
update
Tick orUpdate
ListAction
MRs &refresh
time
view
Tick or FetchEffect
MRs TableObject
Elm Rendering
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DEMO TIME!
Fame & Glory
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Time Travel Debugger
See for yourself: http://debug.elm-lang.org/edit/Mario.elm
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Links■ Backend project: https://github.com/miciek/mr-stats-haskell-
servant■ Frontend project: https://github.com/miciek/mr-stats-frontend-
elm■ Elm architecture tutorial: https://github.com/evancz/elm-
architecture-tutorial■ Haskell Servant: https://github.com/haskell-servant/servant■ Let’s be mainstream - user-focused design in Elm: https://www.
youtube.com/watch?v=oYk8CKH7OhE■ Monad Transformers: https://www.youtube.com/watch?
v=pzouxmWiemg■ Make the backend team jealous - Elm in Production: https://www.
youtube.com/watch?v=FV0DXNB94NE■ Children do projects using Elm: http://outreach.mcmaster.
ca/tutorials/shapes/shapes.html
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PURELY FUNCTIONAL WEB APPS
Michał Płachtawww.michalplachta.com
@miciek
THANK YOU!