Category: Tech

Developers sometimes choose not to use the latest available language features that would be appropriate to tackle a problem for fear of alienating users and collaborators. This is a bad habit and we should stop doing that. Part of the solution are transpilers. What are transpilers, where are they used and what is the benefit? Why should we consider transpiling all our code?

I cut this piece from an upcoming article that is way too long anyway. I made this new article by reusing and reshaping existing text for a new audience and frame. You are reading a new text that first was built as a part of another text. – Yes. This was transpiling: Rephrasing an input, including externally supplied, derivative or implicit facts about it to an output that generally expresses the same. Excuse me, what? Let me go into some details.

Transpiling: Saying the same but different

In software, transpilers are also known as source to source compilers. They take in a program written in one language and write out a roughly equivalent program for another language. The other language may be another version or dialect of the input language or something entirely different. Don’t be too critical about the words: transpilers are just like all other compilers. Source code is machine-intelligible, otherwise it could not be compiled. Machine code is intelligible by humans, at least in principle.

Preprocessors are transpilers

A preprocessor is essentially a transpiler even if it does not interpret the program itself. The C language preprocessor is a mighty tool. It allows you to write placeholders that will be exchanged for code before the actual C compiler touches it. These placeholders may even have parameters or make the program include more code only if needed. Concatenating many source files into one and minifying these by stripping unnecessary whitespace can also be seen as a primitive form of transpiling.

Coding Style Fixers are transpilers

Automatic tools that edit your source code are transpilers. They might only exchange tabs for four space characters or make sure your curly braces are always in the same place or they may do much more involved stuff. For example php-cs-fixer transforms your technically correct code written in plain PHP into technically correct code in standards-conforming plain PHP. One such standard is PSR-2, later deprecated in favor of PSR-12 and PER-1 – these are all maintained by the PHP FIG. Software projects may define their own standards and configure tools to transpile existing code to conform to their evolving standards.

Compilers are transpilers

A compiler is a transpiler. It takes in the source code and builds a machine-executable artifact, the binary code. It might also build a byte code for some execution platform like Java’s JVM. It might build code for a relatively primitive intermediate language like CIL or a machine specific Assembly Language. Another compiler or an interpreter will be able to work with that to either run the software or turn it into a further optimized format. These transformations are potentially lossy.

Decompilers are transpilers

Earlier in life I used tools like SoftICE that would translate back from binary machine instructions to Assembly Language so that I could understand what exactly the machine is doing and make it do some unorthodox things. Compiling back from Machine Code to the machine-specific Assembly Language is technically possible and lossless but the result is not pretty.

Lost in translation

When humans rewrite a text for another target audience, they will remove remarks that are unintelligible or irrelevant to the new audience. They may also add things that were previously understood without saying or generally known in the former audience. Transpilers do the same. When they transpile for machine consumption, they remove what the machine has no interest in: Whitespace, comments, etc. They can also replace higher concept expressions by detailed instructions in lower concepts. Imagine I compiled a program from assembly language into binary machine code and then decompiled back to assembly language. Is it still the same program? Yes and no. It is still able to compile back into the same machine program. It does not look like the program I originally wrote. Any macro with a meaninful name was replaced by the equivalent step by step instructions without any guidance what their intention is. Any comments I wrote to help me or others navigate and reason about the code are lost in translation. The same is true anytime when we translate from a more expressive higher concept to a lower concept. Any implicit defaults I did not express now show up as deliberate and explicit artifacts or the other way around, depending on tool settings.

Lost in Translation but with Humans

You may know that from machine translated text. Put any non trivial text into a machine translator, translate it from English to Russian to Chinese to German and then back to English. In the best case, it still expresses the core concept. In the worst case it is complete garbage and misleading.

Another such thing are Controlled Languages like Simple English, Français fondamental, Leichte Sprache, etc. They use a reduced syntax with less options and variations and a smaller selection of words. Some like Aviation English or Seaspeak also try to reduce chance for fatal ambiguity or mishearing.

These reduced languages are supposedly helpful for those who cannot read very well, are still learning the language or have a learning disability. They may also enable speakers of a closely related foreign language to understand a text and they generally cater to machine translation. For those who easily navigate the full blown syntax and vocabulary and can cope with ambiguity and pun, simplified language can be repetitive, boring and an unnecessary burden to actual communication. Choosing a phrase or well known roughly fitting word over a less used but more precise word is an intellectual effort. Reading a known specific word can be easier on the brain than constructing a meaning from a group of more common words. Speaking to an expert in a language deliberately evading technical terms may have an unintended subtext. Speaking to a layman in lawyerese or technobabble might not only make it hard for them to understand you but also hard for them to like you. Readers will leave if I make this section any longer.

Useful application

Now that everybody is bored enough, let’s see why it is useful and how good it is.

Upgrading Code to newer language versions

You can use a transpiler to upgrade code to a newer version of the language. Why would you want that? Languages evolve. New features are added that allow you to write less and mean the same. Old expressions become ambiguous by new syntax and features. Keywords can be reserved that previously weren’t. Old features become deprecated and will finally stop working in later versions. A transpiler can rewrite your code in a way that it will run in the current and next version of a language. It can also move meta information from comments or annotations into actual language code.

    // Before
    /**
     * @readonly
     * @access private
     * @var FluffProviderInterface Tool that adds bovine output
     */
    var $fluffProvider;
    /**
     * Constructs an example
     * 
     * @access public
     *
     * @param IllustrableTopicInterface A topic to explain by example
     * @param bool $padWithFluff        Whether to make it longer than needed
     * @param int  $targetLength        How long to make the article
     *
     * @return string The Article
     */
    function constructExample($topic, $padWithFluff=true, $targetLength=3000)

Now that’s what I call a contrived example. Code might look like this if it was originally written in PHP 4 and later enhanced over the years, only using new expressiveness where needed. While it technically runs, it is not how we would possibly write it today.

    // After
    /**
     * @var FluffProviderInterface Tool that adds bovine output
     */
    private readonly FluffProviderInterface $fluffProvider;
    /**
     * Constructs an example
     * 
     * @param IllustrableTopicInterface A topic to explain by example
     * @param bool $padWithFluff        Whether to make it longer than needed
     * @param int  $targetLength        How long to make the article
     *
     * @return string The Article
     */
    public function constructExample(
        IllustrableTopicInterface $topic,
        bool $padWithFluff=true,
        int $targetLength=3000
    ): string

That could be the output of a transpiler. It takes meta information from controlled language in the comments and uses the advanced grammar of the improved PHP language to express them.
In other words, the upgraded code has turned instructions for the human or for external tools into instructions that the language can actually enforce at runtime. Before it helped you understand what to put in and what to expect out. Now it forbids you from putting in the wrong things and errors if the code tries to give back anything but text.

It may drop comments that are already expressed in the actual code. Some project standards suggest to drop @param and @return altogether to make the code more consise to read. I am a little conservative on this topic. A documentation block may be removed if it does not contain any guidance beyond the code. There is no need to rephrase “this is the constructor” or “The parameter of type integer with name $targetLength tells you how long it should be”. But sometimes things deserve explaining and sometimes the type annotations exceed what actual the language expresses. Intersection types are PHP 8.1+. PHP 8.2 can express “return this class or false but not true” while before the language only allowed “This class or a boolean (either true or false)”. Annotations can be read by tools to work with your code. As demonstrated, a transpiler can use them to rewrite your code to a more robust form. Static analyzers can detect type mismatch that can lead to all sorts of bugs and misbehaviours. Documentation generators can strip away the actual code and transform the comments and structural information into something you can easily navigate and reason about. Code including high concept and documentation is first and foremost for humans. Adapting it for machines often means dumbing it down.

Downgrading to an older platform or language version

Code can be transformed in the other direction, stripping or replacing advanced expressiveness to make the older runtime understand the code. This is very popular with Frontend Developers: Both Javascript and CSS are usually no longer shipped the way they are written. A variety of type safe and advanced languages exist that are not even intended to be run in their source form but compiled down to a more or less modern standard of JavaScript, then minified to the smallest valid representation. Possibly variable and function identifiers are changed to avoid them colliding between unrelated software loaded into the same browser. In other languages, we are used to develop against a target baseline and only use the features it provides, plus annotations for concepts it does not support. We choose the baseline by deciding on the lowest platform we want to or have to support. This is jolly insane and I mean it in a nice way.

Imagine we create a book for small children. We will first create a compelling story, lovely characters and possibly some educational tangent using our words and our thoughts, the level of abstraction we are fluent in and the tools we can handle. We finally take care to adapt wording, level of detail and difficult concepts to fit the desired product.
We don’t write to the agent, the publisher or the printing house in baby english. So why should we use anything less than our own development environment supports? It is not healthy. Outside very special situations or for the joy of it, we generally don’t work with one hand tied to the back, using antiquated tools and following outmoded standards.

If we cater to the lowest assumable set of capabilities at development time, we limit ourselves in a costly way. We cannot benefit from the latest and most convenient, i.e. effortless and reliable set of tools. We are slower than we could be, we will make more mistakes and it will exhaust us more than needed.

Provided our production pipeline from the development laptop or container to the CI are able to work with the latest tools, we can use them.

Deliver using a transpiler

The source branch should always target your development baseline, tools as modern as you can come by. Delivery artifacts, i.e. released versions, should deviate from the source distribution anyway:

• Why should you ship build time dependencies with a release?
• Why should you ship CI recipes or linter configurations with a release?
• Depending on circumstances, shipping the unit tests might be useful or waste.
• You would not normally ship your .git directory, would you?

Adding a transpiler step is just another item, just another reason. Transpiling to your lowest supported baseline is not really different from zipping a file, editing a version string or running a test suite to abort faulty builds before they ship. But still, it is not perfect. The shipped code will run on the oldest supported environment but it will miss many runtime benefits of newer versions. This is especially true if your library is a build time dependency of another project. In the best scenario, a build for a fairly recent but reasonable platform expectation exists and another build for an well-chosen older target exists. Both need to run through the test suite and ideally the older build will pass the test suite both when actually run on the old platform and when run on an upgraded platform. There are some details, edge cases and precautions needed to make this feasible and reliable. This will be detailed in an upcoming article which just shrank by a good portion.