Length-limited line reading

safe-io.rkt

@@ -0,0 +1,26 @@
+#lang racket/base
+
+(provide read-line-limited)
+
+;; Uses Internet (CRLF) convention.
+(define (read-line-limited port limit)
+  (let collect-chars ((acc '())
+		      (remaining limit))
+    (let ((ch (read-char port)))
+      (cond
+       ((eof-object? ch) (if (null? acc)
+			     ch
+			     (list->string (reverse acc))))
+       ((eqv? ch #\return) (let ((ch (read-char port)))
+			     (if (eqv? ch #\linefeed)
+				 (list->string (reverse acc))
+				 (error 'read-line-limited
+					"Invalid character ~v after #\\return"
+					ch))))
+       ((eqv? ch #\newline)
+	;; Is this a good idea?
+	(error 'read-line-limited "Bare #\\linefeed encountered"))
+       ((positive? remaining) (collect-chars (cons ch acc) (- remaining 1)))
+       (else (error 'read-line-limited
+		    "Line too long (more than ~v bytes before CRLF)"
+		    limit))))))

test-safe-io.rkt

@@ -0,0 +1,23 @@
+#lang racket/base
+
+(require "safe-io.rkt")
+(require rackunit)
+
+(define (s str)
+  (open-input-string str))
+
+(check-equal? (read-line-limited (s "") 5) eof)
+(check-equal? (read-line-limited (s "abc") 5) "abc")
+(check-equal? (read-line-limited (s "abc\r\ndef") 5) "abc")
+(check-equal? (read-line-limited (s "abcxy\r\ndef") 5) "abcxy")
+
+(check-exn #rx"read-line-limited: Invalid character #<eof> after #\\\\return"
+	   (lambda () (read-line-limited (s "abc\r") 5)))
+(check-exn #rx"read-line-limited: Invalid character #\\\\d after #\\\\return"
+	   (lambda () (read-line-limited (s "abc\rdef") 5)))
+
+(check-exn #rx"Bare #\\\\linefeed encountered"
+	   (lambda () (read-line-limited (s "abc\ndef") 5)))
+
+(check-exn #rx"Line too long \\(more than 5 bytes before CRLF\\)"
+	   (lambda () (read-line-limited (s "abcxyz\r\ndef") 5)))