A type system for a stream processing framework

types.elm

-- A user searched or changed to a new page of search results.
type Search = { query : SearchQuery, searchId : SearchId }

-- A user clicked a search result.
type Click = { searchId : SearchId, pageId : PageId }

-- A user viewed an article.
type PageView = { pageId : PageId }

type SearchQuery = String
type PageId = String
type SearchId = String

searches : Stream SessionId Search
clicks : Stream SessionId Click
pageviews : Stream SessionId PageView

searchClickThroughs : Stream SearchQuery PageId
searchClickThroughs =
  let
    searchClicks : Stream SearchId (Search, Click)
    searchClicks = joinBy .searchId searches clicks
  in
    searchClicks
      |> map (\(search, click) -> { query = search.query, pageId = click.pageId })
      |> keyBy .query
      |> map .pageId