Types

On page 2 a todo's title was any Text, so the app took an empty title, or one a megabyte long. On this page you give the title rules, add a status that can only be "open" or "done", and an optional due date. The compiler then refuses a bad value in your code before the app runs, and the app refuses one in a request before your action sees it.

A type with rules

In TypeScript you might write type Title = string and check its length with a validation library at the edge. In Jadpo the rules are part of the type, and every value of the type follows them, wherever it came from.

domains/todos/types/todo_title.jadpo:

/// What a todo says: at least one character, at most 200.type TodoTitle = Text {    min_length: 1    max_length: 200}

TodoTitle is text with two rules: at least one character, and at most 200. You declare every type the same way: type, its name, =, the type it builds on, and a block. Here the block holds rules. For the other types on this page it holds fields or names instead. Text takes rules on its length; numbers take min and max.

A type also says what a value means. A todo's title and a person's name are both text, but they are different types, and Jadpo never turns one into the other without being asked. So a function that wants a TodoTitle can't be handed someone's name by mistake.

A value from a fixed list

A todo is either open or done. That's an enum: a type whose values are the names it lists.

domains/todos/types/todo_status.jadpo:

/// Whether a todo is still to do.type TodoStatus = Enum {    open    done}

You write a value as the type's name, a dot and the name: TodoStatus.open. In JSON it is the name as a string, "open". Unlike a TypeScript string union, a TodoStatus is never mixed up with plain text.

A value that may be missing

Jadpo has no null. A type ending in ? may hold a value, or none, and the compiler makes you deal with both. A todo's due date is optional:

domains/todos/types/new_todo.jadpo, changed. title is now a TodoTitle, and due_at is new.

use todos/types { TodoTitle }/// What a caller sends to add a todo.type NewTodo = Input {    title: TodoTitle    due_at: Instant? {        default: none    }}

Instant is a built-in type for a moment in time. In JSON it is text in the ISO 8601 form, such as "2026-11-02T09:00:00Z". Instant? is "an instant, or none". default: none means a request may leave due_at out, and then it is none. Without a default, a request would have to send JSON null for due_at every time.

The response type gets the same fields, and the status in place of done:

domains/todos/types/todo_view.jadpo, changed. done is replaced by status, title is a TodoTitle, and due_at is new.

use todos/types { TodoStatus, TodoTitle }/// A todo, as the app sends it back.type TodoView = Output {    title: TodoTitle    status: TodoStatus    due_at: Instant?}

The other built-in types are Bool, Int for whole numbers, Decimal for exact decimal numbers, Text, Bytes and Uuid. Beyond those, Jadpo has about 250 standard types for common things, such as Email, Url, Money and Duration, each with its rules already written. Use one when it fits, rather than declaring your own.

A function

The action now has more to build, so give it a helper. A function is like an action, but it only works things out: it can't touch the database, the current time (page 5) or anything outside the app. So the same arguments always give the same result, and it's easy to test.

domains/todos/actions/add_todo.jadpo, changed. Replace the whole file: the body of add_todo moves into the new function new_view.

use todos/types { NewTodo, TodoStatus, TodoView }/// A new todo, as the app sends it back: always open.function new_view(input: NewTodo) -> TodoView {    return TodoView {        title: input.title        status: TodoStatus.open        due_at: input.due_at    }}/// Adds a todo. For now it only sends the todo back.action add_todo(input: NewTodo) -> TodoView {    return new_view(input)}

A function that only one action uses can sit in that action's file, as new_view does here. add_todo calls it by name.

Refused while compiling

Try breaking a rule, then put it back. In new_view, change title: input.title to title: "", and check:

error: `""` isn't a valid `TodoView.title` --> domains/todos/actions/add_todo.jadpo:6:16  |6 |         title: ""  |                ^^ breaks `min_length: 1`  reason: `TodoTitle` declares `min_length: 1`, and the value has 0 characters. A literal is checked against every rule while compiling.  next: Use a value that `TodoView.title` allows (agent)  rule: type.invalid_literal  help: https://jadpo.dev/docs/errors/type.invalid_literalcheck failed: 1 error, 9 files

A value written in the code is checked against the type's rules before the app ever runs. A misspelt enum value is caught the same way. Change TodoStatus.open to TodoStatus.finished:

error: `TodoStatus` has no variant `finished` --> domains/todos/actions/add_todo.jadpo:7:28  |7 |         status: TodoStatus.finished  |                            ^^^^^^^^ not a variant  reason: `TodoStatus`'s variants are `open` and `done`.  next: Use one of `TodoStatus`'s variants (agent)  rule: type.unknown_enum_variant  help: https://jadpo.dev/docs/errors/type.unknown_enum_variantcheck failed: 1 error, 9 files

The error calls open and done variants: that is Jadpo's word for the values an enum lists.

Put both lines back as they were before you go on, or the next steps fail.

From here on, every time you change a route or a type a route uses, jadpo check reports the API record as out of date, and the fix is always jadpo api update. Later pages just say "update the record".

Update the API record

Check the project. The types changed what clients send and get back, so the API record is out of date:

error: `api/main.jadpo_record` is out of date --> api/main.jadpo_record:15:1   |15 | input NewTodo   | ^^^^^^^^^^^^^ the source says otherwise  reason: The record is a promise to API `main`'s clients, and the routes now say otherwise: changed `input NewTodo`, added `enum TodoStatus` and changed `output TodoView`.  next: Run `jadpo api update`, and commit the record with the code (agent)  rule: record.out_of_date  help: https://jadpo.dev/docs/errors/record.out_of_date

Run jadpo api update. If your project is in git, git diff api/ shows what changed, which is what a reviewer sees:

 input NewTodo-    title: Text+    due_at: Instant? default none+    title: Text { min_length: 1, max_length: 200 }++enum TodoStatus+    variant done+    variant open output TodoView-    done: Bool-    title: Text+    due_at: Instant?+    status: TodoStatus+    title: Text { min_length: 1, max_length: 200 }

jadpo check now says checked: 9 files.

Refused at the door

Run jadpo dev, and add a todo with a due date:

curl -i -X POST http://localhost:3000/todos \  -H 'content-type: application/json' \  -d '{"title": "Buy milk", "due_at": "2099-11-02T09:00:00Z"}'

HTTP/1.1 200 OKcontent-type: application/jsonx-request-id: req_f84f1aab5f40fb5acontent-length: 68date: Thu, 29 Oct 2026 21:54:15 GMT{"title":"Buy milk","status":"open","due_at":"2099-11-02T09:00:00Z"}

Now an empty title, and then a due date that isn't a date:

curl -X POST http://localhost:3000/todos \  -H 'content-type: application/json' \  -d '{"title": ""}'curl -X POST http://localhost:3000/todos \  -H 'content-type: application/json' \  -d '{"title": "Buy milk", "due_at": "next tuesday"}'

{"error":{"code":"invalid_input","message":"Request input is invalid.","request_id":"req_a7dafd2160674c97","details":{"problems":[{"field":"title","rule":"min_length"}]}}}{"error":{"code":"invalid_input","message":"Request input is invalid.","request_id":"req_d6c5b05a9b3540ad","details":{"problems":[{"field":"due_at","rule":"format"}]}}}

Each problem names the field and the rule it broke. format means the text isn't in the form the type needs: here, not an ISO 8601 date. You wrote the rule once, on TodoTitle, and it holds in the code, in requests and in the API description.

What you have

The same route, with a title that has rules, a status and an optional due date. Next, on page 4, the todos are stored, so they're still there after the request ends.

Read more in the reference: Named types, Enums and Values that may be missing.