What is the Reader Monad Good For ?

Readme.md

What is the Reader Monad Good For?

haskell-simple-webpage-hierarchy.md

• view
  • page
    • topnav
    • content (needs email)
      • left
      • right
        • article
          • widget (needs email)

haskell-simple-webpage-reader-monad-do-notation.hs

main :: IO ()
main = do
  putStrLn "what is your email address?"
  email <- getLine
  putStrLn . show $ runReader view email

view :: Reader Email Html
view = do
  page' <- page
  return $ div
    [ page'
    ]

page :: Reader Email Html
page = do
  content' <- content
  return $ div
    [ topNav
    , content'
    ]

topNav :: Html
topNav =
  div
    [ h1 [ "OurSite.com" ]
    ]

content :: Reader Email Html
content = do
  email <- ask
  right' <- right
  return $ div
    [ h1 [ "Custom Content for " ++ email ]
    , left
    , right'
    ]

left :: Html
left =
  div
    [ p [ "this is the left side" ]
    ]

right :: Reader Email Html
right = do
  article' <- article
  return $ div
    [ article'
    ]

article :: Reader Email Html
article = do
  widget' <- widget
  return $ div
    [ p [ "this is an article" ]
    , widget'
    ]

widget :: Reader Email Html
widget = do
  email <- ask
  return $ div
    [ p [ "Hey " ++ email ++ ", we've got a great offer for you!" ]
    ]

haskell-simple-webpage.hs

main :: IO ()
main =
  putStrLn "what is your email address?" >>
    getLine >>= \email ->
      putStrLn . show $ view email

view :: Email -> Html
view email =
  div
    [ page email
    ]

page :: Email -> Html
page email =
  div
    [ topNav
    , content email
    ]

topNav :: Html
topNav =
  div
    [ h1 [ "OurSite.com" ]
    ]

content :: Email -> Html
content email =
  div
    [ h1 [ "Custom Content for " ++ email ]
    , left
    , right email
    ]

left :: Html
left =
  div
    [ p [ "this is the left side" ]
    ]

right :: Email -> Html
right email =
  div
    [ article email
    ]

article :: Email -> Html
article email =
  div
    [ p [ "this is an article" ]
    , widget email
    ]

widget :: Email -> Html
widget email =
  div
    [ p [ "Hey " ++ email ++ ", we've got a great offer for you!" ]
    ]