Exposing internal functionality (Internal modules)

We have now built a very small but convenient and safe way to write HTML code in Haskell. This is something that we could (potentially) publish as a library and share with the world by uploading it to a package repository such as Hackage. Users interested in our library could use a package manager to include it in their project and build their own HTML pages.

It is important to note that users are building their projects against the API that we expose to them, and the package manager doesn't generally provide access to the source code, so they can't, for example, modify the Html module (that we expose) in their project directly without jumping through some hoops.

Because we wanted our Html EDSL to be safe, we hid the internal implementation from the user, and the only way to interact with the library is via the API we provide.

This provides the safety we wanted to provide, but in this case, it also blocks the user from extending our library in their own project with things we haven't implemented yet, such as lists or code blocks.

When a user runs into trouble with a library (such as missing features) the best course of action usually is to open an issue in the repository or submit a pull request, but sometimes the user needs things to work now.

We admit that we are not perfect and can't think of all use cases for our library. Sometimes the restrictions we add are too great and may limit the usage of advanced users who know how things work under the hood and need certain functionality to use our library.

Internal modules

For that, we can expose internal modules to provide some flexibility for advanced users. Internal modules are not a language concept but rather a (fairly common) design pattern (or idiom) in Haskell.

Internal modules are simply modules named <something>.Internal, which export all of the functionality and implementation details in that module.

Instead of writing the implementation in (for example) the Html module, we write it in the Html.Internal module, which will export everything. Then we will import that module in the Html module and write an explicit export list to only export the API we'd like to export (as before).

Internal modules are considered unstable and risky to use by convention. If you end up using one yourself when using an external Haskell library, make sure to open a ticket in the library's repository after the storm has passed!

Let's make the changes

We will create a new directory named Html and inside it a new file named Internal.hs. The name of this module should be Html.Internal.

This module will contain all of the code we previously had in the Html module, but we will change the module declaration in Html.Internal and omit the export list:

-- Html/Internal.hs

module Html.Internal where

...

And now in Html.hs, we will remove the code that we moved to Html/Internal.hs and in its stead we'll import the internal module:

-- Html.hs

module Html
  ( Html
  , Title
  , Structure
  , html_
  , p_
  , h1_
  , append_
  , render
  )
  where

import Html.Internal

Now, users of our library can still import Html and safely use our library, but if they run into trouble and have a dire need to implement unordered lists to work with our library, they could always work with Html.Internal instead.

-- Html.hs

module Html
  ( Html
  , Title
  , Structure
  , html_
  , p_
  , h1_
  , append_
  , render
  )
  where

import Html.Internal

-- Html/Internal.hs

module Html.Internal where

-- * Types

newtype Html
  = Html String

newtype Structure
  = Structure String

type Title
  = String

-- * EDSL

html_ :: Title -> Structure -> Html
html_ title content =
  Html
    ( el "html"
      ( el "head" (el "title" (escape title))
        <> el "body" (getStructureString content)
      )
    )

p_ :: String -> Structure
p_ = Structure . el "p" . escape

h1_ :: String -> Structure
h1_ = Structure . el "h1" . escape

append_ :: Structure -> Structure -> Structure
append_ c1 c2 =
  Structure (getStructureString c1 <> getStructureString c2)

-- * Render

render :: Html -> String
render html =
  case html of
    Html str -> str

-- * Utilities

el :: String -> String -> String
el tag content =
  "<" <> tag <> ">" <> content <> "</" <> tag <> ">"

getStructureString :: Structure -> String
getStructureString content =
  case content of
    Structure str -> str

escape :: String -> String
escape =
  let
    escapeChar c =
      case c of
        '<' -> "&lt;"
        '>' -> "&gt;"
        '&' -> "&amp;"
        '"' -> "&quot;"
        '\'' -> "&#39;"
        _ -> [c]
  in
    concat . map escapeChar

Summary

For our particular project, Internal modules aren't necessary. Because our project and the source code for the HTML EDSL are part of the same project, and we have access to the Html module directly, we can always go and edit it if we want (and we are going to do that throughout the book).

However, if we were planning to release our HTML EDSL as a library for other developers to use, it would be nice to also expose the internal implementation as an Internal module. Just so we can save some trouble for potential users!

In a later chapter, we will see how to create a package from our source code.