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Tag 2.0.0
Committed At 2018-08-30 02:21:03 UTC




    An Elm union type and library to encapsulate any primitive in a single type


    Oftentimes, you find yourself tailoring decoders for all the different schemas or records that you are expecting from the back end. But what if you simply come up with a decoder that stores those records, say, as a Dict? And forget for the moment about the type of each member of the record. You can deal with that later. You won't worry about the names of the fields in your record, either.

    All of that postponed work can be encoded in your updaters or views. You look for a field with a certain name, of a certain type, in your dictionary. If not found then no update happens. If it should have been found, then you can start debugging your code (or your back end code).


    The sample data in the following example has been taken from the RAML 200 Tutorial.

    Save this code in a file called Demo.elm,

    module Demo exposing (schema, albumId)
    import Json.Encode as Encode
    import Json.Decode as Decode exposing (decodeValue, string, field, bool, dict)
    import Boxed exposing (Boxed(..))
    import Boxed.Json
    import Dict exposing (Dict)
    o = Encode.object
    s = Encode.string
    b = Encode.bool
    i =
    albumIdSample = o
      [ ("type" , s "string")
      , ("required" , b True)
      , ("minLength" , i 36)
      , ("maxLength" , i 36)
    sample = o
      [ ("type" , s "object")
      , ("$schema" , s "")
      , ("id" , s "")
      , ("required" , b True)
      , ("properties" , o
          [ ("songTitle" , o
              [ ("type" , s "string")
              , ("required" , b True)
          , ("albumId" , albumIdSample)
    type alias Schema = 
      { type_: String 
      , schema_: String
      , id : String 
      , required : Bool
      , properties : Dict String (Boxed ())
    invalidSchema : Schema 
    invalidSchema = 
      { type_ = "" 
      , schema_ = ""
      , id = "" 
      , required = False
      , properties = Dict.empty
    schemaDecoder =
      Decode.map5 Schema 
        (field "type" string)
        (field "$schema" string)
        (field "id" string)
        (field "required" bool)
        (field "properties" (dict Boxed.Json.decoder))
    schema : Schema    
    schema = 
      decodeValue schemaDecoder sample
      |> Result.withDefault invalidSchema
    albumId : Boxed ()    
    albumId = 
      decodeValue Boxed.Json.decoder albumIdSample
      |> Result.withDefault Null

    Run elm-repl and type as follows

    > import Demo exposing (..)
    > import Boxed exposing (..)
    > import Boxed.Dictionary exposing (apply, mapGet)
    > import Maybe exposing (withDefault)
    > import Dict exposing(get)
    > get "songTitle" |> withDefault Null |> apply (get "type")
    Just (Str "string") : Maybe.Maybe (Boxed.Boxed ())
    > mapGet asInt "maxLength" albumId
    Just 36 : Maybe.Maybe Int
    > mapGet asBool "required" albumId
    Just True : Maybe.Maybe Bool

    A field like properties, seen above as member of Schema, is found many times in API specifications. And, although the top level structure of Schema may remain pretty much the same, the contents of the properties member can be expected to be more dynamic. And that is where we can use the Boxed type as a catch-all, maybe even for rapid prototyping.