vala:tldr:806e4

This command is used to compile a Vala source file with the specified filename and link it with the Gtk+ library.

Let's break down the command:

• vala: This is the Vala compiler. It compiles Vala source code into C code.
• ${filename-vala}: This is a variable that represents the name of the Vala source file. The syntax ${variable-default} is used to specify a default value for the variable in case it is not defined. So, if filename-vala is defined (e.g., as main.vala), it will use that value, otherwise it will use vala as the filename.
• --pkg ${gtk+-3-0}: This option is used to specify the Gtk+ library package to be linked with the compiled code. ${gtk+-3-0} is a variable that represents the name of the Gtk+ library package. Again, if gtk+-3-0 is defined, it will use that value; otherwise, it will not link any Gtk+ package.

Overall, the command compiles a Vala source file with the provided or default filename and links it with the specified Gtk+ library package.