# Part I. Introduction

Tepl is a library that eases the development of GtkSourceView-based text editors and IDEs. Tepl is the acronym for “Text editor product line”.

See the Tepl web page.

Tepl was previously named Gtef (GTK+ text editor framework). The project has been renamed in June 2017 to have a more beautiful name. The end of Tepl is pronounced like in “apple”.

The Tepl library follows a product line approach (see the book Feature-Oriented Software Product Lines: Concepts and Implementation). Instead of creating one general-purpose text editor or IDE with a plugin system, the idea is to create several specialized text editors. For example specialized for one programming language, or one development platform, or a small group of related languages. By being specialized, an application is potentially better at what it does. It follows more closely the UNIX philosophy to “do only one thing but do it well”. And it better follows the GNOME philosophy: writing applications that Just Works; when a user opens the text editor for the first time, it should work out-of-the-box, without the need to find, install and configure plugins. Of course it still makes sense to develop one general-purpose text editor for the languages not yet covered by specialized text editors.

Most of the features of the library are available as a toolkit, but the interesting part of Tepl is that it contains also a framework (for now without a plugin system). The goal of the framework is to provide higher-level APIs, to be able to create a new text editor easily. An application wanting to use Tepl is not forced to use the framework in its entirety, it is possible to use just the toolkit parts. The library is implemented this way to achieve maximum re-usability: the framework is less re-usable because it makes some assumptions about the general architecture of the application, but since the features are also available with a lower-level API – as a toolkit – all text editors and IDEs based on GtkSourceView (or even other types of applications) should be able to benefit from the features implemented in Tepl.

Maybe some of the Tepl features will be moved to GtkSourceView when it's considered more stable. So Tepl can also be seen as an incubator for some GtkSourceView features.

## Boundary between GtkSourceView and Tepl

For the framework part of Tepl (not the toolkit parts), there is a somewhat clear boundary between GtkSourceView and Tepl: the top-level object in GtkSourceView is the GtkSourceView widget (representing only one view, or one file), while the GtkSourceView widget is at the bottom of the containment hierarchy in the Tepl framework (it is a “somewhat” clear boundary because there is also TeplBuffer, a subclass of GtkSourceBuffer).

The top-level object in Tepl is TeplApplication, representing the whole application which can contain several windows, with each window containing one or several GtkSourceView widgets (typically with a Tabbed Document Interface).

So the GtkSourceView library is not aware that there can be several files opened in the application, while the Tepl framework has a complete view of the application with regards to the opened files/tabs. It permits to implement higher-level APIs.

## Iterative API design and stability guarantees

Tepl uses the same versioning scheme as GNOME. API/ABI stability is guaranteed. For example Tepl 17.2 is backward-compatible with Tepl 17.0 (those versions don't necessarily exist, it's just an example). But the peculiarity of Tepl is that new major versions are released more often, at most every 6 months if needed. Different major versions can be installed in parallel, like GTK 2 and GTK 3.

New major versions are released more often because Tepl contains more experimental APIs than GtkSourceView, sometimes even unfinished features. API design is hard, it needs an iterative process. Instead of being stuck for years with a non-optimal API, the Tepl developers want to be able to break the API at any time, to iteratively improve it. Sometimes we see possible improvements several years later.

So applications depending on an old (stable) vesion of Tepl will still compile and run fine, and the Tepl developers can develop the library with more freedom.

The API breaks are well documented in this section.

We think that it is the best development model for a library like Tepl. We have thought about other solutions: (a) providing an unstable API, forcing all applications to be ported to the new APIs when a new version of Tepl is released, and (b) have a much longer development cycle (5 years for instance) to develop Tepl and port several applications to it – including gedit. We prefer to follow an agile methodology, which implies to have frequent releases, and to follow the GNOME release schedule with a new stable version every six months. So we don't like the solution (b). As for the solution (a), it should be self-evident that it would cause problems for downstream distributors. There is another solution: (c) bundling, applications statically link the Tepl library with the app binary. But we know that some downstream distributors don't like bundling. If you think of other solutions, comments are more than welcome. We prefer to release a new stable version every six months, and we prefer to implement and design the APIs iteratively.

## GTK and GtkSourceView dependencies

Tepl 4 depends on GTK 3 and GtkSourceView 4.

## pkg-config name

For Tepl 4, the pkg-config name is: `tepl-4`

To compile a program that uses Tepl 4, you can for example use the following command:

`$ gcc hello.c `pkg-config --cflags --libs tepl-4` -o hello`