Dhall: not quite it

8 August 2020

Last month I dove into Dhall after discovering it more than a year ago but never having the excuse to really get stuck into it. Last month I got two different opportunities, and used it for both: an autoinstall template for Ubuntu 20.04, and a Kubernetes config template for very similar containers that each needed a pod and a service just to vary one or two settings.

While Dhall is good at what it does, despite many rough edges, as I progressed I realised it’s really not what I want.

What Dhall does well

Types. Dhall is fully typed in the Haskell fashion, and every Dhall document is an expression, which contains types or values, or trees of types and trees of values. Dhall does structural typing, so two types A = { an: Integer } and B = { an: Integer } are actually the same type.

Safety. Dhall is strictly non-Turing-complete, and most notably is guaranteed to complete (no halting problem here). Functions have no side effects, and while input can be done from files, the environment, and remote URLs (!), output is only through whatever structure is left at the end.

Reproducibility. Dhall includes concepts and tools that can enforce the integrity of imports, and verify that one expression is equivalent to another, such that you can refactor how that expression is constructed and authoritatively assert that your refactor is correct.

Library. As an established project, there are libraries that are built up for various projects, such as for Kubernetes manifests, Terraform, GitHub actions, OpenSSL, Ansible... additionally, the built-in function and keyword set is very small, so everything is accessible, inspectable, etc.

Where I found it lacking

Errors. Good erroring is hard, I’ll acknowledge. Dhall erroring isn’t terrible... but it’s often obscure and mislaid me many times. Dhall often stops at the first error, which might be a consequence or a symptom of the actual mistake, and gaining that visiblity is hard.

Familiarity. and layperson-friendliness. Is basically zero. Dhall errors require familiarity with Dhall errors: they’re not very approachable unless you’re already familiar with them. Dhall itself is foreign at times, and some of its syntax quirks are downright baffling (in one pet peeve, it bills itself as whitespace insensitive, but what it really means is that as long as whitespace is in the right place, it doesn’t care what that whitespace is... but a(b c) is still different to a (b c), to hilariously-hard-to-debug effects.) While I can use Ruby, Rust, and advanced Bash in work projects, I would never use Dhall because it would add more barriers than it adds value.

Inconsistency. For a language with a tiny built-in library, it’s quite surprising. Everything in Dhall is an expression... except some things that look like expressions but aren’t (like the merge keyword). The whitespace thing. Imports get an optional argument for a checksum, something that nothing else can do (no optional or default arguments, though the record pattern approximates some of it). Some things are keywords, some things are symbols, and some things are nothing at all, with little rhyme or reason. It makes hard to develop intuition.

Information loss. There’s a bug open for at least three years where the formatting tools of Dhall will silently erase all comments except those at the top of a file. Dhall is bad at respecting ordering. This is surprising for a configuration tool: while the consuming application might not care, order can be very important for humans. Some tools may even interpret ordering, for example overriding earlier identical keys in a JSON map, or keeping the first one, and re-ordering may actually change meaning.

Inference. Because Dhall does structural typing with named and anonymous members, and because it has no generics, there’s many situations where it knows the type of something, but will refuse to compile unless you explicit it, which can be very repetitive and/or require refactorings to put a name on a previously-anonymous type.

Inheritance or extensibility. While I like the lack of class-based inheritance in programming languages like Rust and instead embrace the wrapping and trait and composition types concepts, configuration is a different space. It’s not uncommon for a configuration schema to have a general shape for a stanza that is specialised in many different variants, and representing that in Dhall is painful, repetitive, or both.

Translation. Somewhat related or an alternative to the above. Dhall makes it easy to create type-friendly structures, but offers little to translate those structures back into what the actual consumer expects. This ranges from key/value translation, where a Dhall-idiomatic spelling would be StorageKind but the configuration requires storage-kind, to flattening, where you could express a structure as a Action<Specifics> where Action has a type and id, and Specifics is an enum/union for AddPartition or WriteFilesystem but the required structure has type and ids and all specific properties on the same level, to different translations for different outputs.

Postel’s Law. Or robustness principle. The one that goes “Be conservative in what you do, be liberal in what you accept from others.” Dhall is conservative in what it does, certainly, and also very strict in what it accepts. This would not be so much a problem if the tooling/erroring was better: JSON can also be said to be strict on the input, and tooling exists that will point to where the error is quite precisely; YAML can be said to be quite lax, and may silently do the wrong thing. Dhall, however, doesn’t improve one way or the other.