2.1 Environment variables

Okay, let's be honest, the program we wrote in the first section was pretty boring. Let's try something that isn't hard-coded to just say "Hello world." Let's make the program actually greet us by name.

The way we will do this is by reading a value from the operating system which should contain our username. Here is the program:

  println$ "Hello " + Env::getenv "USER";

We notice the following:

• getenv is a function that will retrieve a value from the system's environment variables. getenv takes one input (the name of the variable to lookup) and uses that to query the operating system. Also, it can't fail. If the variable doesn't exist on the system it will just return an empty string. (We will get to smarter methods for handling invalid input later.)
• Env is the class (you can think of it as a namespace or module, if you like) that contains the getenv procedure. Classes make life nice because they allow us to group associated behavior together. The :: operator allows us to "reach into" a class to get to the contents inside. Felix borrows functionality and notation from C++, however things differ including classes, which are rather different from those in C++.
• The infix operator + you see is string concatenation. It takes two strings as inputs and returns one "joined" string as an output. So, in our example above, if Env::getenv "USER" returns "Jim" then concatenation would produce a new string containing "Hello Jim".