Initializing an empty file value in Elm

问题

I am using Elm (0.18) and imported simonh1000's FileReader library. To store a file value, we use the following type:

import Json.Decode as Json exposing (Decoder, Value)
...
{-| An ArrayBuffer is a Elm Json Value.
-}
type alias FileContentArrayBuffer =
    Value

I want to initialize my model with an empty placeholder. I do this as follows:

type alias Model = 
  {
     username : String
   , filecontent: FileContentArrayBuffer
  }

initialModel : Model
initialModel = 
  {
     username = "mark"
   , filecontent = Nothing
  }

But the compiler gives me this error:

The type annotation for `initialModel` says it is a:

    Model

But the definition (shown above) is a:

    { username : String
    , filecontent : Maybe a
    }

回答1:

Since Json.Decode.Value is an alias for Json.Encode.Value, if you really want to initialize a Value type as a JSON {}, you can do the following:

filecontent = Json.Encode.object []

However, I think in your case, it makes more sense to refactor to a Maybe FileContentArrayBuffer field type, since, what would you do with an Value type that decodes to {} anyway? A Nothing value definitely seems more fitting and idiomatic.

来源：https://stackoverflow.com/questions/47541291/initializing-an-empty-file-value-in-elm