How can I read the contents of a file into a list in Lisp?

Question

I want to read in the contents of a file into a list. Some of my attempts so far have been -

(defun get-file (filename)
  (let ((x (open filename)))
    (when x
      (loop for line = (read-line x nil)
     while line do (list line)))
    (close x)))

(defun get-file (filename)
  (let ((x (open filename :if-does-not-exist nil)) (contents (list nil)))
    (when x
      (loop for line = (read-line x nil)
     while line do (cons contents line)))
    (close x) contents))

(defun get-file (filename)
  (let ((x (open filename :if-does-not-exist nil)) (contents nil))
    (when x
      (loop for line = (read-line x nil)
     while line do (append contents line)))
    (close x) contents))

None of these worked. Can anyone tell me a way? Or even better - how to put all of the contents into an array?

Answer 1

How about

(defun get-file (filename)
  (with-open-file (stream filename)
    (loop for line = (read-line stream nil)
          while line
          collect line)))

Answer 2

Where are the problems?

(defun get-file (filename)
  (let ((x (open filename)))
    (when x
      (loop for line = (read-line x nil)
            while line
            do (list line)))    ; <-- you are not collecting, just doing
    (close x)))                 ; <- the function returns the value of CLOSE

(defun get-file (filename)
  (let ((x (open filename :if-does-not-exist nil))
        (contents (list nil)))
    (when x
      (loop for line = (read-line x nil)
            while line
            do (cons contents line)))  ; <-- again, the cons goes nowhere
    (close x) contents))               ; <-- CONTENTS has never been changed

(defun get-file (filename)
  (let ((x (open filename :if-does-not-exist nil))
        (contents nil))
    (when x
      (loop for line = (read-line x nil)
            while line
            do (append contents line)))  ; <- again the result goes nowhere
    (close x) contents))                 ; <-- CONTENTS has never been changed

DO will just execute something for side effects.

COLLECT will collect the result and the LOOP then will return a list of collected values upon exit.

As already mentioned, use WITH-OPEN-FILE instead of OPEN/CLOSE. WITH-OPEN-FILE will close the file upon leaving the dynamic scope. Not just from a normal exit, but also upon error conditions using UNWIND-PROTECT.

If you want to read the contents of a file, you can use READ-SEQUENCE. With the usual problems. For example when you read an ASCII file as text into a string, the string may be shorter than the file. For example Common Lisp will represent internally CRLF with a single character, on platforms where CRLF is newline. Another example: in Unicode supporting implementations the UTF-8 code in the file may be replaced with a single character.