Introduction#

Input and Output in scheme is usually handled trough ports. A port is a data structure which is used to interact with the world outside Scheme. A Port isn’t limited to files but can be used to read/write to sockets.

In some ways, the port object is some kind of universal object that can not only manipulate file and sockets but any kind of read/write operation with the OS.

For example, one could implement a port that can write to a printer or even control a CNC machine from Scheme using a port.

Create an input port

An input port can be created in many ways, but usually the method starts with open-input-.

String port

You can use a string as a port using open-input-string. It will create a port that will be able to read from the string.

(define p
  (open-input-string "(a . (b . (c . ()))) 34"))

File port

You can open a file for reading with open-input-file.

(define p
  (open-input-file "path/to/file"))

Read from an input port

Reading from an input port can be done in many ways. We can use the read method used by the REPL. It will read and interpret space separated expressions.

Taking the example from the string port above. We can read from the port like this:

(define p
  (open-input-string "(a . (b . (c . ()))) 34"))
(read p) -> (a b c)
(read p) -> 34

We can read from a port as char using the special method read-char. This will return a single char from the port that we’re reading from.

(define p (open-input-string "hello"))
(read-char p) -> #\h