Prev Up Next Scheme contains some other data types. One is the procedure. We have already seen many procedures, eg, display, +, cons. In reality, these are variables holding the procedure values, which are themselves not visible as are numbers or characters:

=> <procedure>

The procedures we have seen thus far are primitive procedures, with standard global variables holding them. Users can create additional procedure values.

Yet another data type is the port. A port is the conduit through which input and output is performed. Ports are usually associated with files and consoles.

In our ``Hello, World!'' program, we used the procedure display to write a string to the console. display can take two arguments, one the value to be displayed, and the other the output port it should be displayed on.

In our program, display's second argument was implicit. The default output port used is the standard output port. We can get the current standard output port via the procedure-call (current-output-port). We could have been more explicit and written

(display "Hello, World!" (current-output-port))

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