Reading Files in Scheme

Our first example demonstrates reading files in Scheme. Reading files is a common task in many programs. Let’s explore various ways to read files.

(import (scheme base)
        (scheme file)
        (scheme read)
        (scheme write))

;; Helper function to check for errors
(define (check result)
  (if (not result)
      (error "An error occurred")))

;; Main function
(define (main)
  ;; Read entire file contents into memory
  (let ((content (call-with-input-file "/tmp/dat"
                   (lambda (port)
                     (get-string-all port)))))
    (check content)
    (display content))

  ;; Open file for more controlled reading
  (call-with-input-file "/tmp/dat"
    (lambda (port)
      ;; Read some bytes from the beginning of the file
      (let* ((b1 (make-bytevector 5))
             (n1 (read-bytevector! b1 port)))
        (check n1)
        (display (format "~a bytes: ~a\n" n1 (utf8->string b1 0 n1))))

      ;; Seek to a known location and read from there
      (file-position port 6)
      (let* ((b2 (make-bytevector 2))
             (n2 (read-bytevector! b2 port)))
        (check n2)
        (display (format "~a bytes @ 6: ~a\n" n2 (utf8->string b2 0 n2))))

      ;; Seek relative to current position
      (file-position port 4 'current)

      ;; Seek relative to end of file
      (file-position port -10 'end)

      ;; Read at least a certain number of bytes
      (file-position port 6)
      (let* ((b3 (make-bytevector 2))
             (n3 (read-bytevector! b3 port 0 2 #t)))
        (check n3)
        (display (format "~a bytes @ 6: ~a\n" n3 (utf8->string b3 0 n3))))

      ;; Rewind to the beginning of the file
      (file-position port 0)

      ;; Use a buffered reader (not directly available in Scheme, but we can simulate it)
      (let ((line (read-line port)))
        (check line)
        (display (format "5 bytes: ~a\n" (substring line 0 5)))))))

;; Run the main function
(main)

To run this program:

$ echo "hello" > /tmp/dat
$ echo "scheme" >> /tmp/dat
$ scheme reading-files.scm
hello
scheme
5 bytes: hello
2 bytes @ 6: sc
2 bytes @ 6: sc
5 bytes: hello

This Scheme code demonstrates various file reading operations:

Reading an entire file into memory.
Opening a file and reading specific portions.
Seeking to different positions in the file.
Reading a minimum number of bytes.
Simulating a buffered read.

Note that Scheme doesn’t have built-in buffered I/O like some other languages, so we’ve simulated it using basic file operations. Also, error handling in Scheme is typically done using conditions and exceptions, but for simplicity, we’ve used a basic error check function.

The concepts of file reading are similar across languages, but the specific APIs and functions may differ. This example shows how to accomplish these tasks in Scheme.

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Reading Files in Scheme