Modeling Web API Results with Pattern Matching

By Christian Findlay 10 Jan 2025 3 min read

An API operation can return a union that lists its domain outcomes:

type CreateUserResult =
    Created { id: int }
    | ValidationFailed { message: string }
    | DuplicateEmail { email: string }
    | DatabaseUnavailable

One function maps each outcome to an HTTP response:

fn json(status, body) = HttpResponse {
    status: status,
    headers: "Content-Type: application/json",
    contentType: "application/json",
    streamFd: -1,
    isComplete: true,
    partialBody: body
}

fn toResponse(result) = match result {
    Created { id } => json(201, "{\"id\": ${id}}")
    ValidationFailed { message } => json(400, "{\"error\": \"${message}\"}")
    DuplicateEmail { email } => json(409, "{\"error\": \"${email} is already registered\"}")
    DatabaseUnavailable => json(503, "{\"error\": \"try again later\"}")
}

The checker rejects a match that omits a variant, so adding a variant points at every match that needs a new case.

HTTP, parsing and FFI code can still fail; this keeps the modeled outcomes in ordinary data and their HTTP mapping in one function.

See the HTTP specification and pattern-matching specification for the implemented contracts.