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Simple bidirectional parser/printers for your routing data types.

Strongly-typed languages let you define your routes as a data type, ensuring invalid routes fail to compile. But the browser represents locations as strings, so you have to write functions to decode strings into your routing data type and functions to write a route to a string.

Unfortunately, writing separate functions to parse and print your routing data type is error-prone and boilerplate-heavy. It’s easy to update a parser and forget to update the accompanying printer, even though almost all routing definitions should round-trip (parsing then printing returns the original string value).

routing-duplex takes an approach that solves both problems. This library lets you define a codec, or a means to both decode and encode a particular data type, for your routes. Write this codec once and it will handle parsing and printing the same representation for you.

A Brief Example

Let’s build a codec for a simple app with two routes: the homepage and user profiles (identified by usernames).

  1. Write a data type to represent our two routes, deriving Generic.
  2. Build a codec using generics and combinators from Routing.Duplex
import Data.Generic.Rep (class Generic)
import Routing.Duplex (RouteDuplex', path, root, segment, string)
import Routing.Duplex.Generic as G

data Route = Home | Profile String

derive instance genericRoute :: Generic Route _

route :: RouteDuplex' Route
route = root $ G.sum
  { "Home": G.noArgs
  , "Profile": path "profile" (string segment)

With our codec in place we can parse browser locations (strings) into our routing data type:

parse route "/"
> Right Home

parse route "/profile/jake-delhomme"
> Right (Profile "jake-delhomme")

And we can also serialize our routing data type into browser locations:

print route $ Profile "jake-delhomme"
> "/profile/jake-delhomme"

routing-duplex works by letting you define a codec which represents how to encode and decode your routing data type. You can define your routing data however you see fit, and then provide it to the library’s codec type, RouteDuplex:

data RouteDuplex i o = RouteDuplex (i -> RoutePrinter) (RouteParser o)

Intuitively you can think of this as a data type that describes how you can parse a browser location into a route or print a route as a browser location. This type is used for low-level combinators and records and allows you to print / parse a different input and output. However, a proper codec for a routing data type would be bidirectional: it should parse and print the same type, your Route. For that reason, you will generally use the RouteDuplex’ type instead:

type RouteDuplex' a = RouteDuplex a a

This library exports a number of helper functions and combinators for constructing this codec with minimal boilerplate, mostly concentrated in two modules:

  • Routing.Duplex exports the RouteDuplex type, the print and parse functions, and combinators that represent constants (“/post“), segments (/:id), parameters (?foo=), prefixes and suffixes, optional values, and more.
  • Routing.Duplex.Generic exports helpers for deriving code via your data type’s Generic instance, most notably the sum function for describing a route as a sum type and the product and noArgs functions for working with product types.
  • Routing.Duplex.Generic.Syntax exports some symbols that can be used to write terse codecs similar to those found in string-based routers.

We’ll explore several practical examples of this library in practice while building a real-world routing data type and codec.

Example: Writing a codec for a sum type

Let’s begin developing a more complex set of routes. We’ll design routes for a small blogging site made up of users and their posts, as well as feed showing new posts from across the site. We can represent these routes in a small data type:

data Route
  = Root
  | Profile String
  | Post String Int
  | Feed

derive instance genericRoute :: Generic Route _

Next, we can define our codec. First, we’ll use the root combinator to match on an opening slash:

-- Will match '/', and then anything further specified by the codec
-- it is passed as an argument.
root :: forall a b. RouteDuplex a b -> RouteDuplex a b

Next, we need to represent the ability to parse static and dynamic segments from a path. Paths are always separated by slashes (like user/5/favorites), where each string between slashes is considered a segment (like ”user", ”5”, and ”favorites". Sometimes we need to match a segment exactly, like matching the segment ”user", but at other times we want to capture the value of a segment that could have many different values. Here, we have a dynamic segment representing a user ID that we’d like to capture.

We can use the path, segment, and param helper functions to capture these static and dynamic segments.

-- Allows you to match a static segment. For example, to match the
-- /feed path, use `path "feed" ...` where `...` represents further
-- segments.
path :: forall a b. String -> RouteDuplex a b -> RouteDuplex a b

-- Allows you to capture a variable segment. For example, the path
-- /user/5 has one static segment and one variable segment. This always
-- reads and writes a string, but when used with other combinators, it
-- can transform that string into a type of your choice. For example:
-- `path "user" (int segment)` parses to `User Int`.
segment :: RouteDuplex' String

-- Allows you to capture a query parameter. For example, the path
-- /user?foo=bar has one query parameter and could be represented
-- with `param "foo"`.
param :: String -> RouteDuplex' String

While the path you work with is always a String, you can transform that input into your routing data type by using parser combinators from the library. We’ve just seen an example of one of them, int, but there are several more, including:

string :: RouteDuplex' String -> RouteDuplex' String
boolean :: RouteDuplex' String -> RouteDuplex' Boolean
int :: RouteDuplex' String -> RouteDuplex' Int
optional :: forall a b. RouteDuplex a b -> RouteDuplex (Maybe a) (Maybe b)

You can easily implement your own combinators using as, the function used to construct each of the built-in combinators. We’ll see an example of that later on!

At this point, we have the tools we need to:

  • Handle static segments of a path, like "/user"
  • Handle variable segments of a path, like /:username or /:postid
  • Handle query parameters, like ?foo=bar
  • Transform string segments into other types, like using the int combinator to turn a post ID into a String

However, we still need two more tools to construct our codec. First, we need to be able to specify codecs for every case in our routing sum type: this can be done with the sum function from the Routing.Duplex.Generic module. Second, we need to be able to specify that a type should match zero or more of these dynamic segments. For routes that have no arguments, we’ll use noArgs; for routes with one argument, we’ll just provide a codec; and for routes with multiple arguments, we’ll combine codecs with product.

Let’s see all of this in action!

First, we’ll use the root combinator to match a leading slash. Then, we’ll use the sum function to specify that we’re matching a sum type (our Route data type):

route = root $ sum
  { ... }

sum exposes a nice record syntax so that we can specify codecs for each constructor of the type; if you forget to handle a constructor, you’ll get a compiler error. We need to write a few codecs:

  • The Root constructor takes no arguments and should match when the path is empty. We can represent that with a simple noArgs.
  • The Profile constructor takes one argument, a string Username, and should match a path that begins with ”user“. We can represent that using the path and segment functions, along with the string combinator.
  • The Post constructor takes two arguments: a string Username and an integer PostId. We’ll need to use the product function to put two dynamic segments together.
  • The Feed constructor should only match a string constant in the path, ”feed”. We can represent that with the path function and noArgs.
route = root $ sum
  { "Root": noArgs
  , "Profile": path "user" (string segment)
  , "Post":
        (path "user" (string segment))
        (path "post" (int segment))
  , "Feed": path "feed" noArgs

It can be a little awkward using product for complex routes, so there’s also an operator version, (/), which is more convenient to use infix. We can also omit the string combinator because all segments are strings by default. With this in mind, let’s revise our ”Post” case:

  , "Post": path "user" segment / path "post" (int segment)

In fact, when we’re matching string constants, we can omit the call to path and just provide the string directly:

  , "Post": "user" / segment / "post" / int segment

Example: Working with optional or required query params

Users need to be able to search their feeds. This information will come via query parameters, which will be optional. We haven’t dealt with optional segments or query params so far, but they’re easy to add.

First, let’s adjust our route type so that it can accommodate query parameters. Query parameters have a key:value pairing, so it’s typical to represent them with a record type.

data Route
  = Root
  | Profile String
  | Post String Int
  | Feed { search :: Maybe String }

We’ll have to update our codec so that Feed takes a record as an argument. We can do this manually with the record function and its := operator, which lets you assign a key in the record to a particular codec. Record keys are type-level strings, so we’ll need to use Proxy to create them.

Intuitively, we can read the below codec as “Match ”feed” and then, if it exists, a query parameter with the key “search”, storing its value at the key “search” in the output record.” This time, we'll use the optional combinator to represent an optional value:

route = root $ sum
  { ...
  , "Feed": path "feed" (record # _search := optional (param "search"))
    _search = Proxy :: Proxy "search"

This explicit record creation can be done any time you have a record in your route type. However, using a record for query parameters is common enough that this library exports a helper function, params, which lets you just provide a record of codecs where the record keys are treated as the query param keys, too. There's also an operator version of params, (?). We could rewrite our above codec using this helper function:

  { ...
  , "Feed": path "feed" $ params { search: optional <<< string }
  -- alternately
  , "Feed": "feed" / params { search: optional <<< string }
  -- alternately
  , "Feed": "feed" ? { search: optional <<< string }

At this point, our codec is looking much cleaner:

route = root $ sum
  { "Root": noArgs
  , "Profile": "user" / segment
  , "Post": "user" / segment / "post" / int segment
  , "Feed": "feed" ? { search: optional <<< string }

Example: Defining a new codec for custom data

Unfortunately, our route data type is not as type-safe as we’d like it to be. We aren’t really parsing just string and ints — we’re dealing with Usernames and PostIds. In addition, we’ve had a last-minute request to allow users to choose how to sort posts in their feed. We’ll need a custom data type for that, too.

Our codec can easily handle our custom data types. We just have to make our own combinator that describes how to transform to and from a string. In fact, the primitive combinators we saw before (int, boolean, string, optional, etc.) are all built using a helper function, as, which we can leverage as well.

-- Note: the actual function is more polymorphic, but here I've specialized
-- some types to how you will almost always use them in practice.
  :: forall a
   . (a -> String)
  -> (String -> Either String a)
  -> RouteDuplex' String
  -> RouteDuplex a

The as function allows you to produce your own combinator. In short, you are responsible for providing a function from your custom type to a String, and one from a string segment to either an error or your custom type. That leaves one argument free, which will be some existing codec that you are augmenting to produce your custom type. For example, consider the partially-applied as for our type vs. the int combinator:

as' :: forall a. RouteDuplex' String -> RouteDuplex' a
int ::           RouteDuplex' String -> RouteDuplex' Int

Let’s use this to revise our routing data type. First, let’s define a way to represent sorting, and functions that convert to and from strings for the data type.

data Sort = Asc | Desc

derive instance genericSort :: Generic Sort _

sortToString :: Sort -> String
sortToString = case _ of
  Asc -> "asc"
  Desc -> "desc"

sortFromString :: String -> Either String Sort
sortFromString = case _ of
  "asc" -> Right Asc
  "desc" -> Right Desc
  val -> Left $ "Not a sort: " <> val

With these functions in place, it’s trivial to write a new combinator for our sorting data type:

sort :: RouteDuplex' String -> RouteDuplex' Sort
sort = as sortToString sortFromString

Let’s put this to use! We’ll add Sort as a new query parameter, and we’ll use our new combinator to update our route codec, too.

data Route
  = Root
  | Profile String
  | Post String Int
  | Feed { search :: Maybe String, sorting :: Maybe Sort }

derive instance genericRoute :: Generic Route _

route :: RouteDuplex' Route
route = root $ sum
  { ...
  , "Feed": "feed" ? { search: optional, sorting: optional <<< sort }

Next, lets make our Route data type better by providing newtypes to uniquely identify a string as a Username or an int as a PostId.

newtype Username = Username String
derive instance newtypeUsername :: Newtype Username _

newtype PostId = PostId Int
derive instance newtypePostId :: Newtype PostId _

data Route
  = Root
  | Profile Username
  | Post Username PostId
  | Feed { search :: Maybe String, sorting :: Maybe Sort }

We could write new combinators as we did for the Sort data type, but we don’t really have brand-new data types here. We have newtypes around a String and Int, which already have combinators available, so we ought to re-use them. This re-use is trivial if we use the _Newtype iso from purescript-profunctor-lenses along with the existing combinators to create our two new codecs:

uname :: RouteDuplex' Username
uname = _Newtype segment

-- re-use the `int` combinator
postId :: RouteDuplex' PostId
postId = _Newtype (int segment)

Finally, we can replace our earlier segments with these new codecs.

route :: RouteDuplex' Route
route = root $ sum
  { "Root": noArgs
  , "Profile": "user" / uname
  , "Post": "user" / uname / "post" / postId
  , "Feed": "feed" ? { search: optional, sorting: optional <<< sort }

Example: Composing codecs to represent CRUD operations

We’ve seen the RouteDuplex’ a type all over the place, whether to represent a small codec for integers or strings or a larger one for our complex sum type. We can create codecs of any size and compose them into larger structures. Let’s walk through an example by extending our routing data type to accommodate create, read, and update operations for posts in our system.

First, we’ll define a data type to represent creating, reading, and updating a resource dependent on some kind of identifier, a:

data CRU a
  = Create
  | Read a
  | Update a

derive instance genericCRU :: Generic (CRU a) _

Next, we’ll again use the sum function to write a codec for this sum type. We don’t know how to handle a, so we’ll accept a codec to handle it as an argument. We’d like to handle three cases:

  • / should represent creation
  • /:id should represent reading
  • /edit/:id should represent updating

Exactly the same way we wrote a codec for our Route type we can write one for our new CRU type:

cru :: forall a. RouteDuplex' a -> RouteDuplex' (CRU a)
cru inner = sum
  { "Create": noArgs
  , "Read": inner
  , "Update": "edit" / inner

Even better, we can use our new data type as part of our Route type to describe a resource that follows this URL structure:

data Route
  = ...
  | Post Username (CRU PostId)

And re-use our codec to produce the larger Route codec:

route = root $ sum
  { ...
  , "Post": "user" / uname / "post" / cru postId

Example: Running our codec with purescript-routing

We've developed a capable parser and printer for our route data type. To be useful, though, we'll want to use our parser along with a library that handles hash-based or pushState routing for us. The most common choice is the purescript-routing library. If you aren't familiar with how the library works, consider skimming the official guide.

We'll use the library to handle hashes and pushState, but rather than use their parser combinators, we'll provide our own, custom parser -- our codec.

First, we'll choose the matchesWith function that fits our use case:

From here, we'll assume hash-based routing. Next, we'll take a look at the type signature of matchesWith. In a nutshell, this function expects a custom parser and a function that will accept a (possible) previous route and the route that just matched and perform some effects with them. It returns an effect that can be used to remove the event listener this will create.

matchesWith :: forall f a. Foldable f => (String -> f a) -> (Maybe a -> a -> Effect Unit) -> Effect (Effect Unit)

Our custom parser will be the parse function from Routing.Duplex given our codec:

parse :: forall i o. RouteDuplex i o -> String -> Either Parser.RouteError o

Filling in the types with our Route data type, we get:

matchesWith :: (String -> Either RouteError Route) -> (Maybe Route -> Route -> Effect Unit) -> Effect (Effect Unit)

To perform your routing effects, provide your custom callback function:

canceller <- matchesWith (parse route) \old new -> do
  ... your routing effects, called every time the route changes ...