https://github.com/bigbinary/elm-reader

Reader type in Elm
https://github.com/bigbinary/elm-reader

elm

Last synced: 2 months ago
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Reader type in Elm

• Host: GitHub
• URL: https://github.com/bigbinary/elm-reader
• Owner: bigbinary
• License: mit
• Created: 2018-06-06T13:34:12.000Z (over 6 years ago)
• Default Branch: master
• Last Pushed: 2021-04-20T20:43:52.000Z (over 3 years ago)
• Last Synced: 2024-09-30T05:22:33.800Z (3 months ago)
• Topics: elm
• Language: Elm
• Homepage: http://package.elm-lang.org/packages/bigbinary/elm-reader/latest
• Size: 9.77 KB
• Stars: 7
• Watchers: 39
• Forks: 2
• Open Issues: 2
• Metadata Files:
  • Readme: README.md
  • License: LICENSE

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README

        
# Reader

A ```Reader``` is a handy type that is used for providing a shared "environment" to computations.

It helps to solve the problem of passing around the same values or configurations to many functions.

It is also often used as a way of doing dependency injections. In such cases by using a Reader the concern of passing the relevant context is mostly handled at the very peripheri of the application

Consider the following

```elm

type alias DogImages =

    { status : String

    , message : List String

    }

    

requestDogBreed : String -> String -> Json.Decode.Decoder a -> Http.Request a

requestDogBreed apiKey apiPath decoder =

    Http.get ("https://dog.ceo/api/breed/" ++ apiPath ++ "&apiKey=" ++ apiKey) decoder

requestShibaImages :  String -> String -> Http.Request DogImages

requestShibaImages apiKey queryParameters =

    requestDogBreed apiKey ("shiba/images" ++ queryParameters) decodeDogImages

requestOneShibaImage : String -> Http.Request DogImages

requestOneShibaImage apiKey =

    requestShibaImages apiKey "?limit=10"

```

If you look at ```requestOneShibaImage``` you’ll see ```apiKey``` is not used, but is passed through to the other functions until it reaches ```requestDogBreed``` where it is actually used. Such kind of boilerplate can be avoided with the usage of a ```Reader``` type

Instead of ```apiKey``` being passed down to each function, we can use a ```Reader``` and rewrite this in such a way that the context will get passed implicitly.

```elm

requestDogBreed : String -> Json.Decode.Decoder a -> Reader String (Http.Request a)

requestDogBreed apiPath decoder =

    Reader (\apiKey -> Http.get ("https://dog.ceo/api/breed/" ++ apiPath ++ "&apiKey=" ++ apiKey) decoder )

requestShibaImages : String -> Reader String (Http.Request DogImages)

requestShibaImages queryParameters =

    requestDogBreed ("shiba/images" ++ queryParameters) decodeDogImages

requestOneShibaImage : Reader String (Http.Request DogImages)

requestOneShibaImage =

    requestShibaImages "?limit=1"

```

Now the intermediate functions ```requestShibaImages``` and ```requestOneShibaImage``` no longer have to take in and pass ```apiKey``` around.

When we finally want to send the Http Request we can use the ```run``` function to pass in the context to the Reader and retrive the value i.e the final Request

```elm

type Msg

    = LoadShibaImages (Result Http.Error DogImages)

send : Cmd Msg

send =

    Http.send LoadShibaImages <|

        run requestOneShibaImage "123456"

```

## About BigBinary

![BigBinary](https://raw.githubusercontent.com/bigbinary/bigbinary-assets/press-assets/PNG/logo-light-solid-small.png?raw=true)

elm-reader is maintained by [BigBinary](https://www.BigBinary.com). BigBinary is a software consultancy company. We build web and mobile applications using Ruby on Rails, React.js, React Native and Elm.