Dhall is a programmable configuration language optimized for maintainability.
You can think of Dhall as: JSON + functions + types + imports
Note that while Dhall is programmable, Dhall is not Turing-complete. Many of Dhall's features take advantage of this restriction to provide stronger safety guarantees and more powerful tooling.
You can try the language live in your browser by visiting the official website:
The easiest way to get started experimenting with Dhall is to install the
dhall-to-json and/or dhall-to-yaml executables, which enable you to
generate JSON and YAML, respectively, on the command line. Platform- and
runtime-specific installation instructions can be found in the Dhall
wiki.
For other ways to integrate Dhall in your project, read:
For a short introduction, read:
To learn more about core language features, read:
For an even longer hands-on tutorial, read:
... and for an even longer tutorial, read:
Finally, we have a cheatsheet for a very condensed overview and quick lookup:
The Dhall configuration language has multiple implementations so that Dhall configuration files can be understood natively by several programming languages. You can find the latest list of the language bindings and their respective repositories here:
This repository contains language-independent functionality, such as:
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The grammar and formal semantics
Dhall is a formally-specified language standard, and language bindings follow the specification in order to ensure portability of Dhall configuration files across language bindings.
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This repository contains a test suite that language bindings can use to check compliance against the standard.
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The Prelude
One Dhall package named the Prelude is versioned with and distributed alongside the language standard. This package contains general-purpose utilities.
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Shared infrastructure for the Dhall ecosystem
Several services support Dhall developers and this repository contains a NixOps specification of that infrastructure that automatically deploys changes merged to that configuration.
The current version and versioning policy is detailed in the Versioning document, and you can see the latest changes in the Changelog.
The Dhall configuration language slowly evolves in response to user feedback and if you would like to participate in the language evolution process then you should read:
Programming languages are all about design tradeoffs and the Dhall language uses the following guiding principles (in order of descending priority) that help navigate those tradeoffs:
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Polish
The language should delight users. Error messages should be fantastic, execution should be snappy, documentation should be excellent, and everything should "just work".
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Simplicity
When in doubt, cut it out. Every configuration language needs bindings to multiple programming languages, and the more complex the configuration language the more difficult to create new bindings. Let the host language that you bind to compensate for any missing features from Dhall.
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Beginner-friendliness
Dhall needs to be a language that anybody can learn in a day and debug with little to no assistance from others. Otherwise people can't recommend Dhall to their team with confidence.
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Robustness
A configuration language needs to be rock solid. The last thing a person wants to debug is their configuration file. The language should never hang or crash. Ever.
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Consistency
There should only be one way to do something. Users should be able to instantly discern whether or not something is possible within the Dhall language or not.
The Dhall configuration language is also designed to negate many of the common objections to programmable configuration files, such as:
"Config files shouldn't be Turing complete"
Dhall is not Turing-complete. Evaluation always terminates, no exceptions
"Configuration languages become unreadable due to abstraction and indirection"
Every Dhall configuration file can be reduced to a normal form which eliminates all abstraction and indirection
"Users will go crazy with syntax and user-defined constructs"
Dhall is a very minimal programming language. For example: you cannot even compare strings for equality. The language also forbids many other common operations in order to force users to keep things simple
The language is named after a Dustman from the game Planescape: Torment who belongs to a faction obsessed with death (termination). The fountain pen in the logo is the modern analog of Dhall's quill.
The name rhymes with "tall"/"call"/"hall" (i.e. "dɔl" for a US speaker or "dɔːl" for a UK speaker using the International Phonetic Alphabet).
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