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Types that a request can be made into

6 May 2025·2 min read

A type in most web stacks is a promise about data that was never checked. A record that is checked at the door, with its defaults filled in, is a type you can trust.

In a typed web stack, the body of a request is usually cast to a type, not checked against it. The type says age: number, the client sends "forty", and the program finds out three functions later. The type was a hope.

A record is checked when it is made

type Address = { city: str, zip: str? }type User = {    name: str,    email: str,    role: "admin" | "member" = "member",    address: Address?,    tags: list<str> = [],}let ada = User({ name: "Ada", email: "ada@example.com" })// { name: "Ada", email: "ada@example.com", role: "member", tags: [] }

User(...) checks every field and fills in the defaults. A value that is not a User is an error, so a User you hold has been checked.

And it is a pattern

route POST /users { json: User } = save(json)route POST /users = respond(422, "not a user")

The same type turns a request body into a checked value, at the route, before any code runs. Fields the type does not name are left out, so a client cannot slip role: "admin" into a value that never asked for it.

Small on purpose

Records are maps with a name and a shape: no classes, no methods, no inheritance. They print as JSON, go into the database as they are and cross to the browser unchanged. The goal is a type that matches what a web program actually passes around.

Since then: records, their defaults and request patterns made from them are part of Quillo. See Records.