Lisp - Reading Data From Streams

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Understanding how to reading data from streams is very important in order to work with I/O in LISP. In this chapter, we'll explore various methods to read data from streams in various ways.

Core Input Functions

read() function

  • read() is a primary function to read data or LISP objects from streams.

  • read() function parses the input stream, interprets the characters read as LISP form and returns the corresponding LISP object.

  • This function can handle multiple type of data as inputs like symbols, numbers, lists and strings.

  • By default, read() method reads from *standard-input*. We can specify other streams as well using optional stream parameter.

read-char() function

  • read-char() is a specialized function to read a single character from an input stream.

  • This function is useful in performing char-by-char processing.

read-line() function

  • read-char() is a specialized function to read a line of text from an input stream upto new line character. New line character is included in the returned string.

  • This function returns the new line as a string.

read-byte() function

  • read-byte() is a specialized function to read binary data from a stream.

  • This function reads a single byte from a stream and returns an integer.

peek-char() function

  • peek-char() functions allows us to inspect next available character in the stream without actually consuming it.

  • This function is useful when we need to check what is coming as next in the stream.

Key Considerations

Standard Input

A special variable *standard-input* represents the default input stream, generally connected to the keyboard. read functions use *standard-input* by default but we can provide other stream as input as well.

EOF (End of File) handling

While reading a stream, we should handle EOF to mark end of stream. Most of the input functions accepts optional parameters as eof-error-p and eof-value in order to control how to handle EOF is handled.

  • eof-error-p − returns true if error occured when EOF is encountered.

  • eof-value − allows to return a value instead of error when EOF is encountered.

Error Handling

During I/O operations, error can popup in any form like invalid syntax, or error during I/O etc. Error should be handled properly in streams operation.

Example - Reading a File

In following example, we're reading a file test.txt lying in current folder with following content.

Welcome to LISP
It is an easy to learn language.

main.lisp

; open test.txt in read mode
(let ((input-stream (open "test.txt" :direction :input)))
  (when input-stream ; if file is readable
    (loop for line = (read-line input-stream nil) ; read each line using read-line function
          while line ; if line is present
          do (format t "Read line: ~a~%" line)) ; print the line
    (close input-stream))) ; close the stream

Output

When you execute the code, it returns the following result −

Read line: Welcome to LISP
Read line: It is an easy to learn language.

Explanation

  • open function is used to open test.txt in read mode.

  • read-line function is used to read file line by line until nil is returned.

  • close function is used to close the file after reading it completely.

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