Are you a Ruby developer, especially one working with Rails generators, and have you ever stumbled upon files ending with a .tt extension, wondering what cryptic syntax they employ? You’re not alone! Many developers are initially confused by these files, often mistaking them for a proprietary “Thor template” language. Let’s demystify them.

The crucial takeaway is this: .tt files are not written in a unique “Thor syntax.” Instead, they are plain and simple Embedded Ruby (ERB) files. The “tt” in their extension actually stands for “Thor template,” but this refers to their usage within Thor commands, not to a special syntax. Thor is a powerful toolkit widely used by Rails generators for building command-line interfaces and manipulating files, hence its association with these templates.

So, when you encounter a .tt file, think ERB. It means you’ll find familiar Ruby code embedded within text, ready to be processed.

One common source of confusion, however, can be the appearance of unusual tags like <%% and %%> within some .tt files. If these are just ERB, what gives? This peculiar syntax is a clever solution to a specific problem: generating ERB files from other ERB files.

In Rails generators, .tt files often serve as templates to create other files, including new ERB templates. To ensure that a literal <% appears in the final generated ERB file, it must be “escaped” in the generator template. That’s where <%% comes in. It’s a lesser-known but valid ERB construct that tells the parser, “Don’t process this as a Ruby code block; treat it as a literal <%.” Similarly, %%> escapes a literal %>.

While canonical documentation for this specific ERB escaping might be elusive in current Ruby docs, older versions confirm that <%% and %%> are indeed designed to be replaced by <% and %>, respectively. This ingenious mechanism allows for the dynamic creation of ERB-based code and views within your Rails applications.

In essence, .tt files are powerful ERB templates leveraged by the Thor toolkit in Rails to streamline code generation. Their occasional use of <%% and %%> is merely an advanced ERB feature for robust, multi-stage template generation.

(For those working outside the Ruby ecosystem, it’s worth noting that .tt files can also refer to T4 Text Templates in Visual Studio, which use different syntax like <#. These are entirely unrelated to Ruby’s .tt files.)