Catala surface representation

This representation is the first in the compilation chain (see Architecture). Its purpose is to host the output of the Catala parser, before any transformations have been made.

The module describing the abstract syntax tree is:

• Surface.Ast Abstract syntax tree built by the Catala parser

This representation can also be weaved into literate programming outputs using the literate programming modules.

Lexing

Relevant modules:

• Surface.Lexer Concise syntax with English abbreviated keywords.
• Surface.Lexer_fr
• Surface.Lexer_en

The lexing in the Catala compiler is done using sedlex, the modern OCaml lexer that offers full support for UTF-8. This support enables users of non-English languages to use their favorite diacritics and symbols in their code.

While the parser of Catala is unique, three different lexers can be used to produce the parser tokens.

• Surface.Lexer corresponds to a concise and programming-language-like syntax for Catala. Examples of this syntax can be found in the test suite of the compiler.
• Surface.Lexer_en is the adaptation of Surface.Lexer with verbose English keywords matching legal concepts.
• Surface.Lexer_fr is the adaptation of Surface.Lexer with verbose French keywords matching legal concepts.

Parsing

Relevant modules:

• Surface.Parser
• Surface.Parser_driver Wrapping module around parser and lexer that offers the parse_source_file API
• Surface.Parser_errors Interface of the module auto-generated based on "parser.messages".

The Catala compiler uses Menhir to perform its parsing.

Surface.Parser is the main file where the parser tokens and the grammar is declared. It is automatically translated into its parsing automata equivalent by Menhir.

In order to provide decent syntax error messages, the Catala compiler uses the novel error handling provided by Menhir and detailed in Section 11 of the Menhir manual.

A parser.messages source file has been manually annotated with custom error message for every potential erroneous state of the parser, and Menhir automatically generated the Surface.Parser_errors module containing the function linking the erroneous parser states to the custom error message.

To wrap it up, Surface.Parser_driver glues all the parsing and lexing together to perform the translation from source code to abstract syntax tree, with meaningful error messages.

Name resolution and translation

Relevant modules:

• Surface.Name_resolution Builds a context that allows for mapping each name to a precise uid, taking lexical scopes into account
• Surface.Desugaring Translation from Surface.Ast to Desugaring.Ast.

The desugaring consists of translating Surface.Ast to Desugared.Ast of the desugared representation. The translation is implemented in Surface.Desugaring, but it relies on a helper module to perform the name resolution: Surface.Name_resolution. Indeed, in Surface.Ast, the variables identifiers are just string, whereas in Desugared.Ast they have been turned into well-categorized types with an unique identifier like Scopelang.Ast.ScopeName.t.