How can I refactor two functions into one function that takes a generic argument? - elm

Question

How can I refactor two functions into one function that has a generic parameter?
Example:
getVideo : Video -> Post
getVideo video =
let
(Video post) =
video
in
post
getPodcast : Podcast -> Post
getPodcast podcast =
let
(Podcast post) =
podcast
in
post
I would like to do something like this:
getPodcast : 'a -> Post
getPodcast 'a =
let
('a post) =
'a
in
post
Appendix:
type Video
= Video Post
type Podcast
= Podcast Post

Answer 1

You cannot have such an open-ended generic function in Elm. Here are two options:
Use a container type
You can create a container type that has a constructor for each of its valid types:
type PostContainer
= VideoContainer Video
| PodcastContainer Podcast
Now your getPost function consists of a case statement which returns the appropriate post.
getPost : PostContainer -> Post
getPost container =
case container of
VideoContainer (Video post) ->
post
PodcastContainer (Podcast post) ->
post
Include the post type in the Post value
Let's say your Post object looks like this:
type alias Post =
{ name : String
, body : String
}
You could create an enumeration of post types like this:
type PostType = Video | Podcast
You could redefine Post to include the type:
type alias Post =
{ name : String
, body : String
, postType : PostType
}
Or, if you choose to keep the post body separate from the type, you could do something like this:
type alias PostContents =
{ name : String
, body : String
}
type Post = Post PostType PostContents
and your getPostContents function would simply be
getPostContents : Post -> PostContents
getPostContents _ contents =
contents