Skip to main content

Hello

rascal-0.34.0

The Hello example shows where to put your code such that it can do something. In this case all we do is print Hello World.... After you know where to put code such that it can be executed, you can replace the slightly silly example with more useful things.

hello on command line, a.k.a. Rascal Shell​

The place to get Rascal code executed immediately is the Rascal Shell prompt. Here is how to get it started.

When you have it up and running, it shows the prompt rascal> that indicates that Rascal is ready for our input.

Next, we import the library module IO since we will require a function from that library. Rascal responds with the feedback ok so we know that all went well. Now we can call println and proudly observe our first Rascal output:

rascal>import IO;
ok
rascal>println("Hello world, this is my first Rascal program");
Hello world, this is my first Rascal program
ok

hello as function​

As the command above, a piece of code is not reusable. To be able to use some code again and again, the best way is to wrap it in a Function, and then call it:

rascal>import IO;
ok
rascal>void hello() {
>>>>>>>   println("Hello world, this is my first Rascal program");
>>>>>>>}
void (): function(|prompt:///|(0,76,<1,0>,<3,1>))

When you type in a command and continue on a new line the Rascal systems prompts you with >>>>>>> to indicate that more input is needed.

Don't get scared by the void (): function(|prompt:///|(0,76,<1,0>,<3,1>)) that you get back when typing in the hello function. The first void () part says the result is a function that takes no arguments and returns nothing, and the second part function(...) is a simplified print-out of the function-as-a-value that elides all of the details of how functions are represented in memory.

Finally, we call the hello function and we do this twice to make point:

rascal>hello()
Hello world, this is my first Rascal program
ok
rascal>hello()
Hello world, this is my first Rascal program
ok

hello in a module​

To have the hello function also for the future and to let it be used by others, we will place it in a file. A Rascal file is called a "module":

module demo::basic::Hello

import IO;

void hello() {
   println("Hello world, this is my first Rascal program!");
}

This module should be placed in <project dir>/src/demo/basic/Hello.rsc.

Using this Hello module is now simple:

import demo::basic::Hello;
hello();

The hello function is by default visible outside the Hello module. We could further specify this by writing public void hello() { ... }.

Restricting visibility to the module itself can be achieved by adding the keyword private to the definition of hello. When other modules import Hello;, they would not be able to invoke this private hello() function.

Benefits​

  • Println is a simple function that renders a string to the output stream
  • The String constants you pass to Println may contain entire string templates with automatic indentation

Pitfalls​

  • Don't forget to import the IO module.
rascal>println("I think I forgot to import the IO module...")
|prompt:///|(0,7,<1,0>,<1,7>): Undeclared variable: println
Advice: |https://www.rascal-mpl.org/docs/Rascal/Errors/CompileTimeErrors/UndeclaredVariable|
ok