For the time being "part 2" remains a work in progress.

Page 113(117):
4.3.3 Decoding JSON HTTP Responses >> Http.get

A Warning to readers:

When looking at the official documentation make sure you are looking at
http://package.elm-lang.org/packages/elm-lang/http/1.0.0/Http

It is easy to get sidetracked to
http://package.elm-lang.org/packages/evancz/elm-http/3.0.1/Http
by
http://package.elm-lang.org/packages/evancz/elm-http/latest/Http


Yes, the the pipeline style of initialCmd is concise but is also wonderfully opaque from a learning perspective.

{-
initialCmd : Cmd Msg
initialCmd =
    let
        decoder = list photoDecoder
        request = Http.get "http://elm-in-action.com/photos/list.json" decoder
        -- Http.get : String -> Decoder a -> Request a
    in
        Http.send LoadPhotos request

        -- Http.send : (Result Error a -> msg) -> Request a -> Cmd msg

        -- i.e. LoadPhotos is a data constructor which creates a Msg type instance
        -- from a (Result Http.Error (List Photo)) type instance.
        -- So LoadPhotos satisfies (Result Error a -> msg) 
        -- when a = (List Photo), msg = Msg

or more concisely:
-}

initialCmd : Cmd Msg
initialCmd =
    list photoDecoder
        |> Http.get "http://elm-in-action.com/photos/list.json"
        |> Http.send LoadPhotos

At this point it occurs to me that given its relationship to the Photo type the function name viewThumbnail is more than a little misleading - it should be renamed to something like viewPhoto - or alternately rename the Photo type to Thumbnail if the Thumbnail URI template is http://elm-in-action.com/{filename}.jpeg and the Photo URI template is http://elm-in-action.com/large/{filename}.jpeg.