Elm package source code - elm

Question

I'd like to see the source code of elm-lang/core that is in use in my project.
My project has:
import Json.Decode exposing (..)
Right now elm compiler says
Cannot find variable `Json.Decode.Decoder`.
`Json.Decode` does not expose `Decoder`.
From github source I can see that it's exposing Decoder. Would like to see if I have the wrong version of Elm or something.
Just in case - my elm-package.json has
"dependencies": {...
"elm-lang/core": "5.1.1 <= v < 6.0.0",
...
},
"elm-version": "0.18.0 <= v < 0.19.0"

Answer 1

Your example use in the comments shows that you are using Decoder like this:
on "load" (Decode.Decoder (toString model.gifUrl)
That is indeed a compiler error. While the Json.Decode package exposes a Decoder type, it does not expose a Decoder constructor. This is known as an opaque type, meaning you can't construct a Decoder value yourself, but only by using functions from the Json.Decode package. Opaque types can be exposed by having a module defined like this:
module Foo exposing (MyOpaqueType)
You can specify which constructors are exposed in one of the following ways:
-- only exposes Constructor1 and Constructor2
module Foo exposing (MyType(Constructor1, Constructor2))
-- exposes all constructors of MyType
module Foo exposing (MyType(..))
By your example code, I am inferring that you want some Msg to occur when an image is fully loaded. If that were the case, then you could use something like this:
type Msg
= ...
| ImageLoaded String
viewImage model =
img [ src model.gifUrl, on "load" (Json.Decode.succeed (ImageLoaded model.gifUrl)) ] []
Here is an example of how to handle both the image load and error events.