A PureScript client and server can share an HTTP contract by defining each endpoint once as a route type, then using that type to constrain both server handlers and client fetch calls. This catches many disagreements about methods, paths, request bodies, and declared responses at compile time—but it does not make every possible HTTP failure or application bug a type error.
What a shared route type represents
TeamTavern is a team-finding site for online games. Its project announcement describes a browser app built with Halogen and a Node API server, with both written in PureScript in one codebase and built using one spago build. The project is open under AGPL-3.0, and its routing library, Jarilo, belongs to the maintainer’s purescript-bklaric collection. See the project announcement and implementation article for the author’s explanation.
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The idea resembles Haskell’s Servant: an endpoint is described as type-level data, not as a runtime route value. That declaration can combine an HTTP method, path, optional request body, and the response cases the endpoint is allowed to produce. The server and client import the same declaration rather than maintaining separate descriptions.
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In the StartSession example, a route declares POST /sessions with a JSON request body represented by a variant. Its permitted responses are 204 No Content, 400 with typed error content, or 500. The type makes those cases part of the endpoint contract, rather than informal expectations scattered across two code paths.
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A path with typed captures
The ViewPost example includes literal path segments plus named captures: a game handle typed as String and a post ID typed as Int. The route therefore describes not only the endpoint’s general shape but also what values must be supplied to address a particular resource.
How the server and client use the declaration
Server: route collection and handler record
Routes live in a route module alongside their request and response data types. On the server, the application assembles routes into a named collection and passes that collection to serve with a handler record. Type-class constraints derive the handler record’s fields, inputs, and possible response variants from the collection.
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For a captured path value, the handler receives a record field with the declared type. Jarilo parses an integer capture before calling the handler; a non-integer segment is rejected before application code runs. Request JSON is likewise decoded to the route’s declared request type. These checks protect the boundary between HTTP input and the handler, but they are not compile-time checks of a database row or other external runtime data.
Client: typed URL, body, and response handling
A client-side Fetch type class reads the same route declaration to select the method, construct a URL from typed path and query records, encode a request body, and decode declared responses. A ViewPost call must supply both the game handle and integer post ID: omitting the ID or passing it as a string is a compile error.
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This arrangement makes the endpoint declaration the shared source of API shape while leaving client and server behavior in their respective implementations. It reduces the chance that one side silently drifts from the other; it does not eliminate the need to write the handler, call the endpoint, or test the page’s behavior.
Which API changes become compile-time failures?
The TeamTavern article reports 47 routes in its example, a project-specific count rather than a benchmark. Its change examples show the kinds of mismatch that route types can surface during a build:
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- Rename a response field, and handler or client code still reading the old field no longer type-checks.
- Add a path capture, and existing client calls must supply the newly required path field.
- Remove a response case, and handlers or clients that refer to that case fail to type-check.
- Add a route to the server’s route collection, and the server needs a corresponding handler before it builds.
- Return a response that the route does not declare, and the handler’s response type rejects it.
The guarantee is about consistency with the declared API shape. It is not proof that the server can successfully produce a response in every situation, that its business logic is correct, or that a user-facing page works as intended.
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Adding a response and exhaustive handling
Adding a new response does not necessarily force every existing client call site to change. A site using onMatch with a default branch can keep compiling without naming the new case. A call site using exhaustive match must spell out every response case. Exhaustive handling gives stronger feedback when the contract grows, at the cost of more explicit code.
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What route types do not guarantee
As Branimir Klarić, the article’s author, put it on DEV Community on September 30, 2026: “The route type describes what my server sends. It does not describe everything a browser can receive.” That distinction matters because infrastructure, deployment timing, and runtime data can produce outcomes outside the declared contract.
- Proxy limits: TeamTavern’s example uses a reverse proxy that rejects request bodies over 64 KB with
413. That status is not in the route’s declared responses, so it is not a typed response case. - Stale clients during a deploy: A client left running briefly can talk to a newer server whose response differs from what that client declares. The client treats an undeclared status as a
FetchError. - Unexpected-status reporting: The client reports unexpected responses and unanticipated common error statuses, but expected status labels are plain strings. A misspelled label can compile and only become apparent in logs if that response occurs.
- Database result shapes: SQL rows are decoded at runtime. If a query returns the wrong shape, the failure occurs when the query runs, not during compilation.
- Application correctness: A type-correct handler may still fail to produce its declared output in practice, and matching API shapes do not establish that the page behaves correctly. Those behaviors still need testing.
When this approach is useful—and what it costs
Route types are most useful when a client and server are maintained together and API changes commonly require coordinated edits. The compile-time feedback can expose missed call-site updates while the change is being made. The trade-off is that the route declarations, handler types, and response handling must be designed and maintained with enough care to express the contract clearly.
TeamTavern also derives a method-and-path RequestLine for failure reporting. The derived string omits query strings, which may contain one-time email codes. That is an example of using a route declaration for safe operational context as well as dispatch.
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The author describes several possible derivations: OpenAPI documentation, endpoint lists for tests, TypeScript declarations, or a mock server. These are potential uses, not features TeamTavern currently ships. The project does not generate OpenAPI because its own pages are its only client.
Generating accurate body schemas is a harder part of that work: records, arrays, optional values, and variants need to correspond to the JSON codec’s actual representation. A route type can be a strong foundation for generated artifacts, but the generated output is only useful if its schema matches the wire format clients really exchange.
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