Hello World

No tutorial would be complete without a Hello World example. Below is the source code for Hello World in Regent. The source for this and other tutorials can also be found in the GitHub repository.

import "regent"

local c = terralib.includec("stdio.h")

task hello_world()
  c.printf("Hello World!\n")
end

task main()
  hello_world()
end
regentlib.start(main)

Preamble

Regent is an embedded language. The outermost level of the source code is a Lua script. Two languages are embedded inside this context: Terra and Regent itself. Each language provides special keywords (e.g. task) which trigger execution of the compiler. First, though, Regent needs to register its language definition so that these keywords available to the program.

In order to load this language definition, every Regent program starts with the following line.

import "regent"

(If you forget this line, you will typically see some obscure parser errors, as Lua attempts to interpret Regent keywords as Lua variable names.)

Lua Code

The script executes top-to-bottom in Lua. From the perspective of Regent, this is happening at compile time, similar to the execution of templates in C++. However, Lua is a full-featured programming language, making powerful metaprogramming possible.

For example, the line below parses the C header file stdio.h and stores the result into a Lua variable. (So printf can be accessed as c.printf.) This makes it easy to interact with arbitrary C code.

local c = terralib.includec("stdio.h")

Hello World Task

Next execution hits the definition of the hello world task itself. Conceptually, execution of the task definition functions to invoke the Regent compiler on the source code of the task. That is, after the following code executes, there will be a new Lua global variable hello_world which points to a task object (corresponding to the source code below).

The task itself won’t run until we star the Legion runtime. Remember: the script executes at compile time. But this feature makes it possible to perform powerful metaprogramming on Regent code.

task hello_world()
  c.printf("Hello World!\n")
end

Main Task

After this, we define the main task. The main task, like main in many languages, takes no arguments and produces no result. (Unlike many languages, the name main is just a convention in Regent.)

The main task will invoke hello_world to produce the output message.

task main()
  hello_world()
end

Starting the Runtime

Finally, at the end of the file, we invoke the Legion runtime. This kicks off execution of the main task (and subsequent execution of the rest of the program).

The start call does not return. Furthermore, the Lua execution environment used to compile the Regent program is also unavailable after this function is called. So complete any Lua programming prior to calling this function.

regentlib.start(main)