Configuration

Topiary is configured using languages.ncl files. The .ncl extension relates to Nickel, a configuration language created by Tweag. There are up to four sources where Topiary checks for such a file.

Configuration sources

At build time the languages.ncl in the root of the Topiary repository is embedded into Topiary. This file is parsed at runtime. The purpose of this languages.ncl file is to provide sane defaults for users of Topiary (both the library and the CLI binary).

The next two are read by the Topiary binary at runtime and allow the user to configure Topiary to their needs. The first is intended to be user specific, and can thus be found in the configuration directory of the OS:

This file is not automatically created by Topiary.

The next source is intended to be a project-specific settings file for Topiary. When running Topiary in some directory, it will ascend the file tree until it finds a .topiary directory. It will then read any languages.ncl file present in that directory.

Finally, an explicit configuration file may be specified using the -C/--configuration command line argument (or the TOPIARY_CONFIG_FILE environment variable). This is intended for driving Topiary under very specific use-cases.

To summarise, Topiary consumes configuration from these sources in the following order (highest to lowest):

1. The explicit configuration file specified as a CLI argument.
2. The project-specific Topiary configuration.
3. The user configuration file in the OS's configuration directory.
4. The built-in configuration file.

Configuration merging

By default, Topiary only considers the configuration file with the highest priority. However, if the -M/--merge-configuration option is provided to the CLI, then all available configurations are merged together, as per the Nickel specification.

In which case, if one of the sources listed above attempts to define a language configuration already present in the built-in configuration, or if two configuration files have conflicting values, then Topiary will display a Nickel error.

To understand why, one can read the Nickel documentation on merging. However, the short answer is that a priority must be defined. The built-in configuration has everything defined with priority 0. Any priority above that will replace any other priority. For example, to override the entire Bash configuration, use the following Nickel file.

{
  languages = {
    # Alternatively, use `priority 1`, rather than `force`
    bash | force = {
      extensions = [ "sh" ],
      indent = "    ",
    },
  },
}

To override only the indentation, use the following Nickel file:

{
  languages = {
    bash = {
      indent | force = "    ",
    },
  },
}

Configuration options

The configuration file contains a record of languages. That is, it defines language identifiers and configures them accordingly.

For instance, the configuration for Nickel is defined as such:

nickel = {
  extensions = ["ncl"],
},

The language identifier is used by Topiary to associate the language entry with the respective query file, as well exposing it to the user through the --language CLI argument. This value should be written in lowercase.

File extensions

The list of extensions is mandatory for every language, but does not necessarily need to exist in every configuration file. It is sufficient if, for every language, there is a single configuration file that defines the list of extensions for that language.

Indentation

The optional field, indent, exists to define the indentation method for that language. Topiary defaults to two spaces " " if it cannot find the indent field in any configuration file for a specific language.

Specifying the grammar

Topiary fetches and builds the grammar for you, or a grammar can be provided by some other method. To have Topiary fetch the grammar for you, specify the grammar.source.git attribute of a language:

nickel = {
  extensions = ["ncl"],
  grammar.source.git = {
    git = "https://github.com/nickel-lang/tree-sitter-nickel",
    rev = "43433d8477b24cd13acaac20a66deda49b7e2547",
  },
},

To specify a prebuilt grammar, specify the grammar.source.path attribute, which must point to a compiled grammar file on your file system:

nickel = {
  extensions = ["ncl"],
  grammar.source.path = "/path/to/compiled/grammar/file.so",
},

Note
If you want to link to a grammar file that has already been compiled by Topiary itself, those look like ~/.cache/topiary/<LANGUAGE>/<GIT_HASH>.so (or the equivalent for your platform).

For usage in Nix, a prefetchLanguages.nix file provides utilities allowing to transform a Topiary configuration into one where languages have been pre-fetched and pre-compiled in Nix derivations. The only caveat is that, for each Git source, the configuration must contain a nixHash for that source. For instance:

nickel = {
  extensions = ["ncl"],
  grammar.source.git = {
    git = "https://github.com/nickel-lang/tree-sitter-nickel",
    rev = "43433d8477b24cd13acaac20a66deda49b7e2547",
    nixHash = "sha256-9Ei0uy+eGK9oiH7y2KIhB1E88SRzGnZinqECT3kYTVE=",
  },
},

The simplest way to obtain the hash is to use nix-prefetch-git (and look for the hash field in its output). The second simplest way is to compile, which will show something like:

evaluation warning: Language `nickel`: no nixHash provided - using dummy value
error: hash mismatch in fixed-output derivation '/nix/store/jgny7ll7plh7rfdnvdpgcb82kd51aiyx-tree-sitter-nickel-43433d8.drv':
         specified: sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=
            got:    sha256-9Ei0uy+eGK9oiH7y2KIhB1E88SRzGnZinqECT3kYTVE=
error: 1 dependencies of derivation '/nix/store/0q20rk8l4g0n5fzr0w45agxx0j9qy65v-nickel-grammar-43433d8477b24cd13acaac20a66deda49b7e2547.drv' failed to build
error: 1 dependencies of derivation '/nix/store/s5phxykjyzqay7gc33hc6f8kw4ndba25-languages-prefetched.json.drv' failed to build
error: 1 dependencies of derivation '/nix/store/5w15p3b3xfw5nd6mxz58ln09v10kvf8v-languages-prefetched.ncl.drv' failed to build
error: 1 dependencies of derivation '/nix/store/7zzyha67jw09kc37valp28bp5h6i7dka-topiary-0.6.0.drv' failed to build