- GENTLE 97 was published in 1997 together with the first edition of this manual (Oldenbourg Verlag, Munich and Vienna, 1997)
- GENTLE 21 is distributed since 2001 by Metarga and is continuously maintained according to the requirements of its users
This manual covers the common functionality of both editions.
However, GENTLE 21 provides additional features such as a powerful parsing strategy that overcomes the limitations of Yacc, iteration statements that simplify the traversal of data structures, a compact notation to embed target text into code generation rules, and more. These extensions are described in a companion manual.
Gentle provides a uniform framework for specifying the components of a compiler.
The Gentle language was designed around a specific paradigm: recursive definition and structural induction. Input and internal data structures are defined by listing alternative ways to construct items from given constituents. Then the properties of these items are described by giving rules for the possible alternatives. These rules recursively follow the structure of items by processing their constituents. Experience has shown that this is the underlying paradigm of virtually all translation tasks.
The same concepts apply to analysis, transformation, and synthesis. They can be used to describe the backend of a compiler as a simple unparsing scheme such as sufficies for most source-to-source translations. They can also be used to specify a cost-augmented mapping to a low-level language which is used for optimal rule selection.
The rule-based approach follows the principle of locality: complex interactions are avoided and a system can be understood by understanding small pieces in isolation. Individual constructs are described independently.
The paradigm leads to a data-oriented methodology: the structure of data is mirrored by the structure of algorithms. This methodology proves to be a useful guideline in compiler projects.