Extra features

Attributes and extensions

The language can be extended with attributes, enabling a variety of extensions. An attribute is of the form #[key.of.extension] or #[key.of.extension = VALUE], where VALUE can be of the form "STRING", or an expression in catala syntax. An attribute is always bound to the element directly following it: an extension could for example make use of them to retrieve labels to the input fields of a scope in order to generate a web form:

declaration scope SomeComputation:
  #[form.label = "Enter the number of children satisfying the condition XXX"]
  input children_of_age content integer

Built-in attributes

Some attributes affect the Catala compiler itself with built-in support.

Debug print

By adding #[debug.print] in front of an expression in a Catala program, the value of that expression will be printed upon computation by the interpreter, when run with the --debug option. It is otherwise ignored by the other backends.

It is also possible to print the value along with a specific tag using #[debug.print = "some debug tag"].

Note that, in some cases, due to how the compiler works, debug prints could appear duplicated or not at all, especially if optimisations are enabled (with the -O flag). If that happens, try to move the attribute to the root of the definition.

Implementing external modules

External modules need to precisely match the interface expected by the Catala code.

To help with this, the catala command supports a --gen-external flag that will generate a template implementation in the given backend:

$ catala ocaml --gen-external foo.catala_en
┌─[RESULT]─
│ Generated template external implementations:
│   "foo.template.ml"
│   "foo.template.mli"
└─

Rename the files (removing the .template part), and edit them to replace the Impossible error placeholders by your implementation.

• The type definitions, interface and module registration parts should be left unchanged
• Proceed in the same way for every backend your project will be compiled to. An OCaml implementation is required to run the Catala interpreter on the project, and strongly recommended.

Upon compilation, the Clerk build-system will automatically pick up the provided implementation.

In case of changes to the Catala interface (or upon update of the Catala compiler), you should re-run the --gen-external command and resolve discrepancies between your implementation and the new .template file.