What Jadpo is, and setting up

Jadpo is a programming language for web backends: the server side of an app, which keeps its data and answers its HTTP requests. Coding agents write most Jadpo, and people review it. This page says what that means in practice, then gets the jadpo command running on your machine.

What Jadpo does for you

In a TypeScript backend, the rules of your app are spread across many places. A validation schema says what a request may hold. An ORM model says what a table holds. Middleware decides who is signed in, and a check in each handler decides what they may see. Nothing makes sure these agree, so a reviewer has to read every one.

In Jadpo you write each rule once, in the place the language gives it. Then one compiler checks the whole app against all of them. It checks the types and the stored data. It checks the HTTP routes, who can sign in, and who may see and change each record. It also checks the settings, calls to other services, background work and tests. So if a route would send a secret field to a client, or skip sign-in, the app doesn't compile.

That is also why Jadpo suits coding agents. An agent can't forget a permission check the compiler adds for it, and a reviewer reads one declaration instead of tracing every handler.

How it differs from TypeScript

Some habits don't carry over. The compiler catches most of them and tells you the Jadpo way:

• No null. A value that may be missing has a type ending in ?, and holds none when it is.
• No exceptions. Something that can go wrong is declared, like a return type, and the compiler checks every caller deals with it.
• No async, classes, methods, your own generics, while, break or floating point. for is the only loop, and decimal numbers are exact.
• No hand-written SQL. You declare what to read and write, and the compiler writes the SQL.
• No truthiness, no semicolons, and no !, && or ||: you write not, and and or.

Jadpo has no front end. A web page or a phone app calls it over HTTP, the same as any other API.

Install it

Pick one of these. Each puts jadpo on your PATH, and you install it once, not once per project.

On macOS or Linux:

curl --proto '=https' --tlsv1.2 -LsSf https://github.com/jadpo-dev/jadpo/releases/latest/download/jadpo-cli-installer.sh | sh

On Windows, in PowerShell:

powershell -c "irm https://github.com/jadpo-dev/jadpo/releases/latest/download/jadpo-cli-installer.ps1 | iex"

With Homebrew:

brew install jadpo-dev/tap/jadpo

With npm:

npm install -g @jadpo/jadpo

Open a new terminal, and check that jadpo runs:

jadpo --version

jadpo X.Y.Z

Each project names the Jadpo release line it is written for, and whichever jadpo you run fetches and runs that release for it, so projects on different releases work side by side.

Install PostgreSQL

From page 4 on, the app stores its data in PostgreSQL, so install PostgreSQL now if you don't have it, from postgresql.org or your package manager. You don't create a database or set a password: when you run or test the app, Jadpo starts a throwaway database of its own. It finds PostgreSQL's programs (initdb, pg_ctl and postgres) on your PATH, or where the usual installers put them.

Start a project

jadpo new starts a project, which is the todo app the rest of these pages build. Make it wherever you keep your work, for instance in your home folder:

cd ~jadpo new todo-app

Created todo-app    jadpo  X.Y    route  POST /hello    files  14 writtennext: cd todo-app && jadpo dev  serves it on http://localhost:3000

It made a folder, todo-app, holding a small starter app: one route, an HTTP endpoint that answers POST /hello, with a test. Go into the folder, and ask jadpo to check the project:

cd todo-appjadpo check

checked: 8 files in 0.7 s

jadpo check reads the whole project and reports every problem it finds. It doesn't compile the app into a program or run it, so it's quick, and you run it after every change. It does write a few reports, such as a description of your API, into a build/ folder inside the project, which you don't commit. Then run the project's tests:

jadpo test

tested: 1 passed, 1 route test

The test sent the starter route a request and checked its answer. On the next page you look at what jadpo new wrote, and replace the starter with the todo app's first route.

When something is wrong

jadpo never warns: everything it reports is an error, and the app doesn't run until you fix it. Every error has the same parts:

• where the problem is, with the line of code;
• reason:, why it's a problem;
• next:, what to do about it. (human) at the end of an error's next: line tells your coding agent to stop and ask you which fix you want, because the choice is yours to make (who can see data, what a setting holds). (agent) means your agent can make the fix itself;
• rule:, the error's name, and help:, the address of its page.

Every error has a page on this site: https://jadpo.dev/docs/errors/ followed by the rule, such as https://jadpo.dev/docs/errors/record.out_of_date, which the help: line prints for you to open. You'll meet real errors on the next page, and fix them.

Using it with a coding agent

An agent reads the primer, one page about the language, before it writes Jadpo, and the help page of each error it meets. To get an agent started, ask it to read the primer and then follow these pages with you.

Next: A project and your first route.