#{title} ^{headTags} <body>^{bodyTags} |] -- Tell yesod to use the SqlPersist backend by default instance YesodPersist GuestBook where type YesodPersistBackend GuestBook = SqlPersist -- We use runDB to more easily thread our database -- connection into functions in the Handler monad. runDB action = getYesod >>= runSqlConn action . _db -- Provide a default pattern for the internationalisation machinery. instance RenderMessage GuestBook FormMessage where renderMessage _ _ = defaultFormMessage -- show Models.hs -- Define our data models & create a function to setup our tables. -- (Uses a generous helping of Template Haskell) share [mkPersist sqlSettings, mkMigrate "migrateAll"] [persistLowerCase| Signature name Text message Text date UTCTime deriving Show |] -- show Routes.hs -- More TH, this time generating Type-Safe Routes -- This will generate a sum datatype that describes -- your app's routes and datatypes. mkYesod "GuestBook" [parseRoutes| / HomeR GET /sign SubmitR GET POST |] -- show Forms.hs -- Simple form for creating Signatures entryForm = renderBootstrap $ Signature <$> areq textField "Name" Nothing <*> areq textField "Message" Nothing <*> lift (liftIO getCurrentTime) -- show Views.hs -- Our homepage view, querying and displaying signatures -- from our all-important guestbook getHomeR = do -- Query from the db, annotating type so the compiler knows what we're querying signatures :: [Entity Signature] <- runDB $ selectList [] [Asc SignatureDate] -- Render out the template defaultLayout [whamlet| <h1>Welcome to the Guestbook! <table .table.table-bordered> <thead> <tr> <th> Name <th> Message <th> When <tbody>  $forall Entity _ signature <- signatures <tr> <td> #{signatureName signature} <td> #{signatureMessage signature} <td> #{fTime $ signatureDate signature}  <p><a href=@{SubmitR}>Sign Guestbook. |] where -- A quick and dirty time formatter fTime :: UTCTime -> String fTime = formatTime defaultTimeLocale "%F %X %Z" renderEntryForm widget enctype = [whamlet| <form method=POST action=@{SubmitR} enctype=#{enctype}> <table>^{widget} <div .form-actions> <button .btn.btn-primary type=submit> Sign Guestbook |] -- The GET handler for our signing page getSubmitR = do (formWidget, enctype) <- generateFormPost entryForm defaultLayout [whamlet| <h1> Sign The Guestbook! ^{renderEntryForm formWidget enctype} <p><a href=@{HomeR}>Back Home. |] -- The POST handler for our signing page postSubmitR = do -- run the Posted form, and retrieve the values ((result, formWidget), enctype) <- runFormPost entryForm case result of FormSuccess signature@(Signature name message _) -> do -- Insert our Signature object into the Database runDB $ insert signature -- Render out the success template defaultLayout [whamlet| <h2> Successfully Submitted! <blockquote> #{message} <br>-- #{name} <p><a href=@{HomeR}>Back to Guestbook. |] FormFailure _ -> defaultLayout [whamlet| <h2> OOPS! Please, Try again. ^{renderEntryForm formWidget enctype} |] FormMissing -> redirect SubmitR -- show Main.hs -- We'll use a temporary in-memory database for this example main = withSqliteConn ":memory:" $ \conn -> do time <- liftIO getCurrentTime runNoLoggingT $ runResourceT $ flip runSqlConn conn $ do -- Create the db tables (running our auto-generated migration) runMigration migrateAll -- Create some example data insert $ Signature "Johnny" "Hello, Haskeller" time insert $ Signature "James" "Guestbook? What is this, Geocities?" time insert $ Signature "Judy" "Wow, totally Retro" time -- Initialise our webserver and begin serving content -- Here we bind our database connection into our application state. warpEnv $ GuestBook { _db = conn } ``` And there you go, A simple application to curate your very own piece of internet nostalgia. You can find out more about yesod by reading the docs at [YesodWeb](http://www.yesodweb.com/book)

Here i'm going to show you how to build a super simple web app using haskell and yesod. For the purposes of the tutorial, this is a single piece of code, however you would split the code among multiple filesif you were to build on this project. Everyone knows that the internet peaked in the 90s and so today we're going to build somthing that no 90s website can be without, the Guest Book. ``` active haskell web -- show Imports and Pragmas {-# LANGUAGE OverloadedStrings, FlexibleContexts, ScopedTypeVariables, GeneralizedNewtypeDeriving, TemplateHaskell, GADTs, TypeFamilies, QuasiQuotes, MultiParamTypeClasses #-} -- As usual, a generous helping of pragmas and imports. -- These will need to be placed at the top of your source files. -- Everything you need to work with Yesod, exported from one package import Yesod -- Standard platform datatypes & functions import Data.Text (Text) import Data.Time (UTCTime, getCurrentTime) import Data.Time.Format (formatTime) import System.Locale (defaultTimeLocale) -- Using the `persistent` database layer import Database.Persist import Database.Persist.TH import Database.Persist.Sqlite -- Monad helpers import Control.Applicative ((<$>),(<*>)) import Control.Monad.IO.Class (liftIO) import Control.Monad.Logger (runNoLoggingT) import Control.Monad.Trans.Resource (runResourceT) -- show Types.hs -- Our datatype that represents our App -- the fields of this type represent pieces of the -- global environment in which it runs. data GuestBook = GuestBook { _db :: Connection } -- Declare that our App is a Yesod site -- This will provide you with sensible defaults -- and your website's defaultLayout instance Yesod GuestBook where defaultLayout contents = do PageContent title headTags bodyTags <- widgetToPageContent $ do addStylesheetRemote "//netdna.bootstrapcdn.com/twitter-bootstrap/2.3.2/css/bootstrap-combined.min.css" toWidget [cassius| body padding: 0px 10px |] contents hamletToRepHtml [hamlet| $doctype 5 #{title} ^{headTags} <body>^{bodyTags} |] -- Tell yesod to use the SqlPersist backend by default instance YesodPersist GuestBook where type YesodPersistBackend GuestBook = SqlPersist -- We use runDB to more easily thread our database -- connection into functions in the Handler monad. runDB action = getYesod >>= runSqlConn action . _db -- Provide a default pattern for the internationalisation machinery. instance RenderMessage GuestBook FormMessage where renderMessage _ _ = defaultFormMessage -- show Models.hs -- Define our data models & create a function to setup our tables. -- (Uses a generous helping of Template Haskell) share [mkPersist sqlSettings, mkMigrate "migrateAll"] [persistLowerCase| Signature name Text message Text date UTCTime deriving Show |] -- show Routes.hs -- More TH, this time generating Type-Safe Routes -- This will generate a sum datatype that describes -- your app's routes and datatypes. mkYesod "GuestBook" [parseRoutes| / HomeR GET /sign SubmitR GET POST |] -- show Forms.hs -- Simple form for creating Signatures entryForm = renderBootstrap $ Signature <$> areq textField "Name" Nothing <*> areq textField "Message" Nothing <*> lift (liftIO getCurrentTime) -- show Views.hs -- Our homepage view, querying and displaying signatures -- from our all-important guestbook getHomeR = do -- Query from the db, annotating type so the compiler knows what we're querying signatures :: [Entity Signature] <- runDB $ selectList [] [Asc SignatureDate] -- Render out the template defaultLayout [whamlet| <h1>Welcome to the Guestbook! <table .table.table-bordered> <thead> <tr> <th> Name <th> Message <th> When <tbody>  $forall Entity _ signature <- signatures <tr> <td> #{signatureName signature} <td> #{signatureMessage signature} <td> #{fTime $ signatureDate signature}  <p><a href=@{SubmitR}>Sign Guestbook. |] where -- A quick and dirty time formatter fTime :: UTCTime -> String fTime = formatTime defaultTimeLocale "%F %X %Z" renderEntryForm widget enctype = [whamlet| <form method=POST action=@{SubmitR} enctype=#{enctype}> <table>^{widget} <div .form-actions> <button .btn.btn-primary type=submit> Sign Guestbook |] -- The GET handler for our signing page getSubmitR = do (formWidget, enctype) <- generateFormPost entryForm defaultLayout [whamlet| <h1> Sign The Guestbook! ^{renderEntryForm formWidget enctype} <p><a href=@{HomeR}>Back Home. |] -- The POST handler for our signing page postSubmitR = do -- run the Posted form, and retrieve the values ((result, formWidget), enctype) <- runFormPost entryForm case result of FormSuccess signature@(Signature name message _) -> do -- Insert our Signature object into the Database runDB $ insert signature -- Render out the success template defaultLayout [whamlet| <h2> Successfully Submitted! <blockquote> #{message} <br>-- #{name} <p><a href=@{HomeR}>Back to Guestbook. |] FormFailure _ -> defaultLayout [whamlet| <h2> OOPS! Please, Try again. ^{renderEntryForm formWidget enctype} |] FormMissing -> redirect SubmitR -- show Main.hs -- We'll use a temporary in-memory database for this example main = withSqliteConn ":memory:" $ \conn -> do time <- liftIO getCurrentTime runNoLoggingT $ runResourceT $ flip runSqlConn conn $ do -- Create the db tables (running our auto-generated migration) runMigration migrateAll -- Create some example data insert $ Signature "Johnny" "Hello, Haskeller" time insert $ Signature "James" "Guestbook? What is this, Geocities?" time insert $ Signature "Judy" "Wow, totally Retro" time -- Initialise our webserver and begin serving content -- Here we bind our database connection into our application state. warpEnv $ GuestBook { _db = conn } ``` And there you go, A simple application to curate your very own piece of internet nostalgia. You can find out more about yesod by reading the docs at [YesodWeb](http://www.yesodweb.com/book)