Hello World in Opa

Published on 16 July 2018 (Updated: 09 December 2023)

Welcome to the Hello World in Opa page! Here, you'll find the source code for this program as well as a description of how the program works.

Current Solution

function page() {
	<pre>Hello, World!</pre>

	{~page, title: "SPEPL"}

Hello World in Opa was written by:

This article was written by:

If you see anything you'd like to change or update, please consider contributing.

Note: The solution shown above is the current solution in the Sample Programs repository as of Dec 09 2023 12:43:52. The solution was first committed on Jul 16 2018 16:34:59. The documentation was last updated on May 15 2023 15:51:23. As a result, documentation below may be outdated.

How to Implement the Solution

As we mentioned, Opa code contains both front-end and server-side code.

Notice how we define a function that returns raw HTML (a la JSX) and also set up the server, all in the same file. The Server.start would be analogous to a main() function. In this case, it's parameters take the server's settings (default http server in this case) and a component to render.

Also, we do not define imports of any kind. Everything is picked up off the standard library.

How to Run the Solution

Typically at this point, we would cover a couple methods for running the solution. For instance, we might share a link to an online editor. If that isn't available, sometimes we'll even offer a way to run the solution locally on your PC. And, you can do that with Opa, but we've opted instead to run Opa using Docker.

But First, Lemme Explain Docker

To give a small overview of Docker and containers, imagine a virtual machine that can boot up in less than a second, and the cost of provisioning, screwing up and starting from scratch, is negligible. (Disclaimer: it's totally not like that, but let's go with it.)

I absolutely despise installing stuff, using it once or twice, and then forgetting about it. I rarely remember to uninstall such things, and you can never be too sure that it hasn't spread it's 1kB-sized temp/config files all over. That's why I use containers.

Would you like to see what happens when you create a thousand random files in / ? Or how the system screws up when deleting /var ? Or even try out a fork-bomb (Disclaimer 2: It's sorta dangerous in a container too)?

Well now you can! You can spin up a container with a fresh Ubuntu in it, toy around with it, and have it deleted upon exit.

Containers are like VMs, except they share the kernel, network stack, and more, with your own OS. They also can be constraint via cgroups (hard/soft CPU/memory limits, and more). The root filesystem they use is mapped to folders in your own machine, and whatever harm you do to them, does nothing on your system.

So, you can "easily" install Opa (or any other language/thing) in a container, play around, stop it, and when you delete the container, your OS is still as clean as before.

How to Run Opa in Docker

Finally, we can actually dig into the execution of Hello World in Opa.

Image Setup

For my next trick, I built a Dockerfile. It contains the definition of all which needs to be installed and set up in order to run Opa, on Ubuntu.

A Dockerfile is the definition/blueprint for an image, and an image is the blueprint for a container. Applications run in containers.

This is mostly not reversible: While you can commit a container into an image, you cannot extract the Dockerfile from an image.

So, if you want to build your own, local image, you can build it off the Dockerfile, like so:

~/devel/SPEPL/archive/o/opa master
❯ ls
Dockerfile hello-world.opa README.md
~/devel/SPEPL/archive/o/opa master
❯ docker build --tag opalang:1.1.1 .
Sending build context to Docker daemon 5.12kB
Step 1/6 : FROM ubuntu
---> 113a43faa138
Step 6/6 : RUN sudo npm install -g opabsl.opp intl-messageformat intl
---> Using cache
---> 626e6038445b
Successfully built 626e6038445b
Successfully tagged opalang:1.1.1

Now you have a working image. Congrats.

Docker has a "hub", conveniently called Docker Hub, where users can upload their public images for others to use. So, instead of building your own image, you could have just used mine:

~/devel/SPEPL/archive/o/opa master
❯ docker pull ajbetteridge/opa-1.1.1-build4308:opa-1.1.1-build4308
Using tag: opa-1.1.1-build4308
opa-1.1.1-build4308: Pulling from ajbetteridge/opa-1.1.1-build4308
Status: Downloaded newer image for ajbetteridge/opa-1.1.1-build4308:opa-1.1.1-build4308

So, finally, we can run hello-world.opa

How to Run it for Real This Time

Having the .opa file in the current directory, run: docker run -it --rm --volume $PWD:/data/ ajbetteridge/opa-1.1.1-build4308:opa-1.1.1-build4308 opa /data/hello-world.opa

~/devel/SPEPL/archive/o/opa master 31s
❯ ls
Dockerfile  hello-world.opa  README.md
~/devel/SPEPL/archive/o/opa master
❯ docker run -it --rm --volume $PWD:/data/ ajbetteridge/opa-1.1.1-build4308:opa-1.1.1-build4308 opa /data/hello-world.opa
~/devel/SPEPL/archive/o/opa master*
❯ ls
Dockerfile  hello-world.js  hello-world.opa  package.json  README.md

What we just did was run my image (if you want to use your own, replace "ajbetteridge/opa-1.1.1-build4308" for what your provided as –tag in your docker build), mapping the current directory ($PWD) to /data inside the container.

This allows the Opa container to pick up the .opa file and compile it, inside the container. It gave no output, but notice that there's a new hello-world.js

As this .js has some npm dependencies, we can also run it inside a container, using the same image:

~/devel/SPEPL/archive/o/opa master* 15m 22s
❯ docker run -it --rm -v $PWD:/data/ --publish 8080:8080 ajbetteridge/opa-1.1.1-build4308:opa-1.1.1-build4308 sh -c '/data/hello-world.js'
Http serving on http://380924f6ba29:8080

Now we also --published the 8080 port. This maps your own 8080 port, to the container's 8080. Having this up, if you fire up your browser and navigate to http://localhost:8080, you'll see "Hello, World!", printed using Opa.