The way forward for Leaf and Tau

Which template engine ought to I select for Vapor? What is the distinction between Leaf and Tau? Let me clarify all the things.



To start with, if you do not know, the Leaf 4 launch accommodates some main syntactical modifications in comparison with the earlier model. Curly brackets have been changed with the brand new #finish prefix syntax.

It is a main step ahead, but additionally a breaking change, which means it’s important to improve your presently current Leaf 3 codebase, in any other case your templates will not work. The opposite main change is said to template inheritance. The brand new lengthen, export and import key phrases changed the earlier embed and set key phrases, once more.

So appropriate me if I am mistaken, however this enchancment is like taking a step backward, as a result of in Leaf 2, we had kind of the identical template inheritance API capabilities (import, export, lengthen, and embed). I suppose that this variation could be complicated for some individuals who have been already aware of the 2nd or the third model of Leaf. (I used to be shocked after I noticed these modifications.) 😐

I do not need to criticize this resolution, however these modifications do not needed deserve a significant model quantity. It isn’t an enormous enchancment from a developer (end-user) perspective. The one factor what modified with Leaf 4.0.0 is that we builders have to alter our codebase to make issues work once more, however we have gained nearly zero new options, that I would count on from a significant launch.

I used to be very unhappy about this and I can think about that this may make server facet Swift builders fairly upset. I’ve already missed various options from Leaf, that I would count on from a contemporary template engine. Simply check out mustache or handlebars, they’re after all older, nevertheless it’s price to say that they’ve some far more superior options, that you just nonetheless cannot do in Leaf. 😢

Now, in the course of the 4.0.0 improvement an attention-grabbing factor occurred with Leaf…


There was a brand new department referred to as Tau (means rebirth) created and maintained by @tdotclare. I jumped the practice and began to make use of this experimental department as a substitute of the principle / dev.

We additionally began to speak quite a bit with tdotclare, since there have been some points, particularly to start with, however lastly I’ve discovered somebody on Vapor’s discord who I might ask about Leaf. He helped me loads and I am extraordinarily grateful, as a result of with out him, the template layer of Feather CMS would not be such an incredible (modular, reusable, extensible) system. 🙏

Lengthy story quick, tdotclare applied a lot of nice new options for Leaf Tau, the entire system began to seem like as an actual template engine, not simply one thing that you might use to switch key-based variables in a really lengthy output string. (Pardon me, however that is the case with Leaf.) 🙈

We had a imaginative and prescient about the way forward for Tau, the place you might differentiate varieties, create new language parts and particular extensions (capabilities, strategies, blocks, contexts) for the duty you should resolve, in a protected and simple method. Tau already has all these goodies and much more.

You may outline variables and constants, and the brand new template hierarchy system appears to be extra logical and future proof than some other earlier variants of the template inheritance APIs. Lastly, after numerous hours of conversations, feedbacks, testing, and bug squashing, the Tau options have been merged to the principle department and launched underneath the 4.0.0-tau.1 tag.

If you wish to preserve utilizing Tau, you need to pin your Swift dependency variations to:
.bundle(url: "", .precise("4.0.0-tau.1")),
.bundle(url: "", .precise("1.0.0-tau.1.1")),

Sadly, the Vapor core workforce determined to revert these modifications shortly after the Tau launch and not using a detailed clarification. So far as I do know the principle motive was that the core workforce desires to maintain Leaf as a “easy” template engine. Do not get me mistaken, I am high-quality with that call, however then it was undoubtedly a mistake to formally launch Tau and shortly after take away it similar to that. 😕

The long run

So here is what I used to be fascinated with this complete matter in the previous few months.

Leaf 4.x

I’ve at all times felt like these new Tau options are method too good to easily drop them, simply to go along with Leaf. Plus what does the time period “easy” means? Can I count on something new in Leaf or is it going to be solely bug fixes solely any further? Is the long run brilliant for Leaf or ought to I merely drop it? 💧

Nicely, sure. I am not going to make use of Leaf once more.

In case you are planning to construct a brand new mission and also you want a dependable, quick, dynamic template engine with a simply very fundamental (restricted) function set, then you need to think about using Leaf. 🍃

Area Particular Languages

These days everyone seems to be into DSL’s (SwiftUI), so why not use a DSL library, reminiscent of HTMLKit or Plot as a substitute of Leaf? If the template engine will not permit me to do extra sooner or later then a DSL can be a method more sensible choice. It may very well be extra protected and performant, however after all we would sacrifice the dynamic nature (you at all times should recompile the complete server code after one thing modifications), however nonetheless, based mostly on the circumstances, I can solely consider this resolution as a greater various.

If you happen to do not need to dynamically replace templates and also you’re high-quality with the “static website generator” strategy, then I would counsel to make use of a Area Particular Language based mostly template engine.


It’s a dynamic template engine with a really wealthy function set, that you need to use to construct extra advanced templates. Since Tau is derived from Leaf it signifies that the syntax may be very comparable, so the official Leaf documentation covers just about all the things that it’s important to find out about it, plus I have already got some good tutorials on my website about utilizing the brand new options of Tau.

Based mostly on my expertise, I’ll keep on with “Tau”.

It is best to know, that the 4.0.0-tau.1 launch underneath the vapor/leaf repository may be very secure, so you need to use that tag for now if you wish to give an opportunity to Tau.

It’s price to say that Tau shall be re-released in a while in a separate repository underneath a model new title. Sadly, I can not give an actual schedule for the transition course of simply but. Hopefully this manner we are able to keep away from additional confusions and builders can decide the suitable software for the job.

The Vapor core workforce and the authors of Leaf are doing an incredible job. This submit will not be meant to hurt them in any method, however I simply needed to offer some actual clarification for everybody who was asking me concerning the present standing of Leaf and Tau. Thanks for understanding this. 🙏

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