Preventing incorrect use with modules

In this section we will move the HTML generation library to its own module.

Modules

Each Haskell source file is a module. The module name should have the same name as the source file and should start with a capital letter. Sub-directories should also be part of the name and we use . do denote a sub-directory. We'll see that in the next section.

The only exception to the rule are entry points to the program - modules with the name 'Main' that define main in them. Their source file names could have any name they want.

A module declaration looks like this:

module <module-name>
  ( <export-list>
  )
  where

The export list can be omitted if you want to export everything defined in the module, but we don't. We will list exactly the functions and type we want to export. This will give us control on how people can use our tiny library.

We will create a new source file named Html.hs and add the following module declaration code at the top of the file:

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

Note that we do not export:

  1. The constructors for our new types, only the types themselves. If we wanted to export the constructors as well we would've written Html(Html) or Html(..). This way the user cannot create their own Structure simply by writing Structure "Hello".

  2. Internal functions used by the library, such as el and getStructureString.

And we will also move the HTML related functions from our hello.hs file to this new Html.hs file:

newtype Html
  = Html String

newtype Structure
  = Structure String

type Title
  = String

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

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

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

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

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

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

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

Now, anyone importing our module (using the import statement which can be used below module declarations but above any other declaration), will only be able to import what we export.

Add the following code at the top of the hello.hs file:

import Html

The hello.hs file should now look like this:

-- hello.hs

import Html

main :: IO ()
main = putStrLn (render myhtml)

myhtml :: Html
myhtml =
  html_
    "My title"
    ( append_
      (h1_ "Header")
      ( append_
        (p_ "Paragraph #1")
        (p_ "Paragraph #2")
      )
    )

And the Html.hs file should look like this:

-- Html.hs

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

newtype Html
  = Html String

newtype Structure
  = Structure String

type Title
  = String

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

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

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

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

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

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

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