Snap, Postgres and Heist: Displaying Data from Queries

The github repo for this post is here.

Single AuthUser Splice

Assuming we have instances and initializations for the Postgres Auth backend and Postgresql-Simple Snaplet, we can do a few things. First, we need to write a Splice for the AuthUser data type. We will only use a couple fields to show how it works.

authUserSplice :: Monad m =>
                  AuthUser ->
                  Splices (HeistT n m Template)
authUserSplice authUser = do
  "userLogin" ## I.textSplice (userLogin authUser)
  "userLoginCount" ## I.textSplice (T.pack $ show $ userLoginCount authUser)

Using ##, we are binding the strings "userLogin" and "userLoginCount" to their respective values from the authUser object. userLogin authUser gives us a Text result, so "userLogin" is bound to a text splice. We also bind "userLoginCount" as a text splice by converting the int we get from using userLoginCount on authUser.

Now that we have our Splice written, let’s write our template.

<apply template="base">
  <h1><userLogin /></h1>
  <p>Number of Times Logged In: <userLoginCount /></p>
</apply>

Nothing major here. We have used the strings we bound before as tags to display the text from the authUser object.

Finally, we can write a Handler to make the database request and render the template using our splice.

getFromPostgres :: Handler App
(AuthManager App) () getFromPostgres = do result

Using a simple query to store the data from Postgres in result, we then render using our template (auth_user_splice.tpl) and apply the splice to the data in our result.

List of AuthUsers

To display a list of data, the process is very similar.

We can reuse the authUserSplice we just wrote and map it across the list of data we plan on passing in using mapSplices and runChildrenWith.

authUsersSplice :: [AuthUser] ->
I.Splice AppHandler authUsersSplice = I.mapSplices (I.runChildrenWith .
authUserSplice)

We will also write a new template to display the information. Included in this is the authUsers tag, which we are going to bind in our Handler.

<apply template="base">
  <dl>
    <authUsers>
      <dt><userLogin /></dt>
      <dd>Number of Times Logged In: <userLoginCount /></dd>
    </authUsers>
  </dl>
</apply>

We can then write our handler as such:

getManyFromPostgres :: Handler App
(AuthManager App) () getManyFromPostgres = do results

We are rendering our new template auth_users_splice.tpl with our mapped splices bound to "authUsers". The code inside of <authUsers> will be run for each result in our list.

End

That's it. Feel free to pull the code from github and leave any questions you have below.