Controller & Actions

Routing Basics

In your project routes are defined in the Web/Routes.hs. In addition to defining that route, it also has to be added in Web/FrontController.hs to be picked up by the routing system.

The simplest way to define a route is by using AutoRoute, which automatically maps each controller action to an URL. For a PostsController, the definition in Web/Routes.hs will look like this:

instance AutoRoute PostsController
type instance ModelControllerMap WebApplication Post = PostsController

Afterwards enable the routes for PostsController in Web/FrontController.hs like this:

instance FrontController WebApplication where
    controllers =
        [ -- ...
        , parseRoute @PostsController

Now you can open e.g. /Posts to access the PostsAction.

Changing the Start Page / Home Page

You can define a custom start page action using the startPage function like this:

instance FrontController WebApplication where
    controllers =
        [ startPage ProjectsAction
        -- Generator Marker

In a new IHP project, you usually have a startPage WelcomeAction defined. Make sure to remove this line. Otherwise, you will still see the default IHP welcome page.

URL Generation

Use pathTo to generate a path to a given action:

pathTo ShowPostAction { postId = "adddfb12-da34-44ef-a743-797e54ce3786" }
-- /ShowPost?postId=adddfb12-da34-44ef-a743-797e54ce3786

To generate a full URL, use urlTo:

urlTo NewUserAction
-- http://localhost:8000/NewUser


Let’s say our PostsController is defined in Web/Types.hs like this:

data PostsController
    = PostsAction
    | NewPostAction
    | ShowPostAction { postId :: !(Id Post) }
    | CreatePostAction
    | EditPostAction { postId :: !(Id Post) }
    | UpdatePostAction { postId :: !(Id Post) }
    | DeletePostAction { postId :: !(Id Post) }

Using instance AutoRoute PostsController will give us the following routing:

GET /Posts                          => PostsAction
GET /NewPost                        => NewPostAction
GET /ShowPost?postId={postId}       => ShowPostAction { postId }
POST /CreatePost                    => CreatePostAction
GET /EditPost?postId={postId}       => EditPostAction { postId }
POST /UpdatePost?postId={postId}    => UpdatePostAction { postId }
PATCH /UpdatePost?postId={postId}   => UpdatePostAction { postId }
DELETE /DeletePost?postId={postId}  => DeletePostAction { postId }

The URLs are very close to the actual action which is called. Action parameters are taken automatically from the request query. This design helps you to always know which action is called when requesting an URL.

AutoRoute & Beautiful URLs

Lots of modern browsers don’t even show the full URL bar anymore (e.g. Safari and most mobile browsers). Therefore AutoRoute doesn’t aim to generate the “most” beautiful URLs out of the box. It’s rather optimized for the needs of developers. If you need beautiful URLs for SEO reasons, instead of using AutoRoute you can use the more manual APIs of IHP Routing. See the section “Beautiful URLs“ for details.

Multiple Parameters

An action constructor can have multiple parameters:

data PostsController = EditPostAction { postId :: !(Id Post), userId :: !(Id User) }

This will generate a routing like:

GET /EditPost?postId={postId}&userId={userId} => EditPostAction { postId, userId }

Parameter Types

AutoRoute by default only works with UUID arguments (and Id types like Id Post). If you have a controller like data HelloWorldController = HelloAction { name :: Text } where you have a text parameter, you have to configure AutoRoute like this:

instance AutoRoute HelloWorldController where
    parseArgument = parseTextArgument

This way the name argument is passed as Text instead of UUID.

This also works with integer types:

instance AutoRoute HelloWorldController where
    parseArgument = parseIntArgument

This will support a controller like data HelloWorldController = HelloAction { page :: Int }.

Right now AutoRoute supports only a single type for all given parameters. E.g. an action which takes a UUID and a Text is not supported with AutoRoute right now:

data HelloController = HelloAction { userId :: !(Id User), name :: Text }
instance AutoRoute HelloController -- This will fail at runtime

This is a technical problem we hope to fix in the future. Until then consider using param for the Text parameter.

Request Methods

When an action is named a certain way, AutoRoute will pick a certain request method for the route. E.g. for a DeletePostAction it will only allow requests with the request method DELETE because the action name starts with Delete. Here is an overview of all naming patterns and their corresponding request method:

Delete_Action => DELETE
Update_Action => POST, PATCH
Create_Action => POST
Show_Action   => GET, HEAD
otherwise     => GET, POST, HEAD

If you need more strong rules, consider using the other routing APIs available or overriding the allowedMethodsForAction like this:

instance AutoRoute HelloWorldController where
    allowedMethodsForAction "HelloAction" = [ GET ]

Application Prefix

When using multiple applications in your IHP project, e.g. having an admin back-end, AutoRoute will prefix the action URLs with the application name. E.g. a controller HelloWorldController defined in Admin/Types.hs will be automatically prefixed with /admin and generate URLs such as /admin/HelloAction.

This prefixing has special handling for the Web module so that all controllers in the default Web module don’t have a prefix.

Advanced: Custom parseArgument

It’s possible to use a custom data type as a routing parameter with AutoRoute. Now might be a good point to switch to a custom routing implementation (described later in this Guide) instead of hacking this into AutoRouting. If this warning can’t stop you, go ahead.

A parseArgument function has the signature:

parseArgument :: forall d. Data d => ByteString -> ByteString -> d

The first bytestring argument is the field name of the action argument we are dealing with. The second argument is the value of the query string:


The last argument d cannot be implemented in a type safe way. This is implemented by calling unsafeCoerce on our result value before returning it. The result of this is later used with fromConstrM. Therefore misusing parseArgument can result in a runtime crash. Again, consider not using this API.

Given we have a custom argument type in the format ID-{numeric} like ID-0, ID-1, etc, we can define a custom parseCustomIdArgument like this:

import qualified Data.Attoparsec.ByteString.Char8 as Attoparsec
import qualified Data.ByteString.Char8 as ByteString

parseCustomIdArgument :: forall d. Data d => ByteString -> ByteString -> d
parseCustomIdArgument field value =
    |> ByteString.stripPrefix "ID-"
    |> fromMaybe (error "Failed to parse custom id, ID- missing")
    |> Attoparsec.parseOnly (Attoparsec.decimal <* Attoparsec.endOfInput)
    |> \case
        Right value -> unsafeCoerce value
        Left _ -> error "AutoRoute: Failed to parse custom id, numeric part invalid"

Custom Routing

Sometimes you have special needs for your routing. For this case, IHP provides a lower-level routing API on which AutoRoute is built.

Let’s say we have a controller like this:

data PostsController = ShowAllMyPostsAction

We want requests to /posts to map to ShowAllMyPostsAction. For that we need to add a CanRoute instance:

instance CanRoute PostsController where
    parseRoute' = string "/posts" <* endOfInput >> pure ShowAllMyPostsAction

The parseRoute' function is a parser that reads an URL and returns an action of type PostsController. The router uses attoparsec. See below for examples on how to use this for building beautiful URLs.

Next to the routing itself, we also need to implement the URL generation:

instance HasPath PostsController where
    pathTo ShowAllMyPostsAction = "/posts"

Beautiful URLs

Let’s say we want to give our blog post application a beautiful URL structure for SEO reasons. Our controller is defined as:

data PostsController
    = ShowPostAction { postId :: !(Id Post) }

We want our URLs to look like this:


Additionally we also want to accept permalinks with the id like this:


To accept URLs like this, we first need to make some changes to our data structure. We have to make the postId optional. Additionally, we need to have a parameter for the URL slug:

data PostsController
    = ShowPostAction { postId :: !(Maybe (Id Post)), slug :: !(Maybe Text) }

This will also require us to make changes to our action implementation:

action ShowPostAction { postId, slug } = do
    post <- case slug of
            Just slug -> query @Post |> filterWhere (#slug, slug) |> fetchOne
            Nothing   -> fetchOne postId
    -- ...

This expects the posts table to have a field slug :: Text.

Now we define our CanRoute instance like this:

instance CanRoute PostsController where
    parseRoute' = do
        string "/posts/"
        let postById = do id <- parseId; endOfInput; pure ShowPostAction { postId = Just id, slug = Nothing }
        let postBySlug = do slug <- remainingText; pure ShowPostAction { postId = Nothing, slug = Just slug }
        postById <|> postBySlug

Additionally we also have to implement the HasPath instance:

instance HasPath PostsController where
    pathTo ShowPostAction { postId = Just id, slug = Nothing } = "/posts/" <> tshow id
    pathTo ShowPostAction { postId = Nothing, slug = Just slug } = "/posts/" <> slug

Real-World Example

Here is a real world example of a custom routing implementation for a custom Apple Web Service interface implemented at digitally induced:

instance CanRoute RegistrationsController where
    parseRoute' = do
        appleDeviceId <- string "AppleWebService/v1/devices/" *> parseText <* "/registrations/"
        passType <- parseText

        let create = do
            string "/"
            memberId  <- parseId
            pure CreateRegistrationAction { .. }
        let show = do
            pure ShowRegistrationAction { .. }

        choice [ create, show ]

instance HasPath RegistrationsController where
    pathTo CreateRegistrationAction { appleDeviceId, memberId } = "/AppleWebService/v1/devices/" <> appleDeviceId <> "/registrations/" <> passType <> "/" <> tshow memberId
    pathTo ShowRegistrationAction { appleDeviceId } = "/AppleWebService/v1/devices/" <> appleDeviceId <> "/registrations/" <> passType

Method Override Middleware

HTML forms don’t support special HTTP methods like DELETE. To work around this issue, IHP has a middleware which transforms e.g. a POST request with a form field _method set to DELETE to a DELETE request.

Custom 404 Page

You can override the default IHP 404 Not Found error page by creating a new file at static/404.html. Then IHP will render that HTML file instead of displaying the default IHP not found page.