telling apart files ending with a newline

[Shriram Krishnamurthi]

Suppose I have two files that are identical, except one ends in a newline and the other does not. If I use `read-line` to read the successive lines of this file, because it swallows the line separators, there is no way to tell them apart. E.g., these two strings

"a

b"

and

"a

b

"

read using `read-line` and `open-input-string` produce the same result.

This is unfortunate for a program SPDEGabrielle has induced me to write (-:.

Any reasonable ways to work around this that rely, as much as possible, on the OS-specific handling `read-line` already provides?

Shriram

[Ryan Culpepper]

If I understand the docs correctly, the OS-specific handling is in open-input-file, but it is not the default.

Here is an alternative to read-line that preserves line endings:

  #lang racket/base

  ;; my-read-line : InputPort -> String
  ;; Like read-line, but preserves line ending.
  ;; Fails if port contains specials.
  (define (my-read-line in [mode 'any])
    (define rx
      (case mode
        [(any) #rx"^[^\n\r]*(?:\r\n|\n|\r|$)"]
        [(linefeed) #rx"^[^\n]*(?:\n|$)"]
        ;; ...
        [else (error 'my-read-line "unsupported mode: ~e" mode)]))
    (define m (car (regexp-match rx in))) ;; rx accepts "", can't fail
    (if (equal? m #"") eof (bytes->string/utf-8 m)))

  (require rackunit racket/port)

  (check-equal?
   (port->list my-read-line (open-input-string "abc\ndef\r\ngh\n"))
   '("abc\n" "def\r\n" "gh\n"))

  (check-equal?
   (port->list my-read-line (open-input-string "abc\ndef\r\ngh"))
   '("abc\n" "def\r\n" "gh"))

  (check-equal?
   (port->list my-read-line (open-input-string "\n\r\n\n\r\r\n"))
   '("\n" "\r\n" "\n" "\r" "\r\n"))

  (check-equal?
   (port->list (lambda (in) (my-read-line in 'linefeed))
               (open-input-string "\n\r\n\n\r\r\n"))
   '("\n" "\r\n" "\n" "\r\r\n"))

Ryan

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