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Chris Biscardi

Getting Started With Snap (and User Authentication): Part 2


Git Repo part 1

The file structure now looks like this:

- abc.cabal
- log/
- access.log
- error.log
- snaplets/
- heist/
- templates/
- _login.tpl
- _new_user.tpl
- base.tpl
- index.tpl
- login.tpl
- new_user.tpl
- userform.tpl
- src/
- Application.hs
- Main.hs
- Site.hs
- static/
- screen.css

abc.cabal includes our dependencies and build information. This file is read when we run cabal install

log includes two files that log out what browsers access the site and what errors occur.

snaplets is where our snaplets store their files. In this case we only have heist there, which contains a templates folder that includes the templates we use to render the site.

static includes a simple stylesheet that is served when we visit the site.

src is the folder we’re currently concerned with. It includes three files:


This file contains some boilerplate for dynamic recompilation of a snap site. We’ll be leaving this file alone.


Here we define our App datatype with the snaplets we will be using.

data App = App
{ _heist :: Snaplet (Heist App)
, _sess :: Snaplet SessionManager
, _auth :: Snaplet (AuthManager App)

In this case we are using heist, sessions and authentication.

Then we make a call to makeLenses

makeLenses ''App

makeLenses does some things under the hood like creating getters/setters and changing the names for the snaplets in our app to remove the underscore. This means that _auth will be referred to as auth in the rest of our app, including in src/Site.hs.

We then define an instance for the Heist Snaplet so we don’t have to use with heist every time we want to render a template (More about this later).

instance HasHeist App where
heistLens = subSnaplet heist

and finally we declare a type alias so that we can use AppHandler instead of the longer Handler App App in the type signatures for our routes.

type AppHandler = Handler App App

This is where the meat of our site lives. The routing code, handlers and initialization for the entire app.

The first route is "/login" which uses handleLoginSubmit.

("/login", with auth handleLoginSubmit)

with auth lets us work in the auth Snaplet for handleLoginSubmit.

handleLoginSubmit :: Handler App (AuthManager App) ()
handleLoginSubmit =
loginUser "login" "password" Nothing
(_ -> handleLogin err) (redirect "/")
err = Just "Unknown user or password"

Since we’re working in the auth Snaplet our type signature for the handler has the type Handler App (AuthManager App) (), which is slightly different from the type we aliased in Application.hs.

loginUser is a function from the auth snaplet. It takes the username field and the password field from a form submission, a “remember” field, a failure function and a success function, in that order. The type signature looks like this:

:: ByteString -- name of username field
-> ByteString -- name of password field
-> Maybe ByteString -- name of remember field (`Nothing` means there isn't one)
-> (AuthFailure -> Handler b (AuthManager b) ()) -- failure function
-> Handler b (AuthManager b) () -- success function
-> Handler b (AuthManager b) () -- return value is a handler

So going back to handleLoginSubmit we use: "login" as the username field "password" as the password field Nothing as the remember field _ -> handleLogin err as our error function and redirect "/" as our success function.

err is defined as Just "Unknown user or password" which matches up with the type from handleLogin.

The handleLogin code will be covered in a Heist tutorial at another time, but suffice it to say that handleLogin is rendering the login form with the error that we’ve supplied it (which is “Unknown user or password”).

Our "/logout" route is pretty simple. Just call the snaplet-auth supplied function logout and redirect to "/"

"/new_user" does a similar thing, except it displays the empty form on GET request and handles a form submit on POST.

Next we’ll replace the backend, currently a json file, with postgres.