The documentation for drain-input says that it returns a string of characters, implying that the result is equivalent to what you'd get from calling read-char some number of times. In fact it differs in a significant respect: whereas read-char decodes input octets according to the port's selected encoding, drain-input ignores the selected encoding and always decodes according to ISO-8859-1 (thus preserving the octet values in character form).
$ echo -n $'\1a\2b\3c' | guile-2.0 -c '(set-port-encoding! (current-input-port) "UCS-2BE") (write (port-encoding (current-input-port))) (newline) (write (map char->integer (let r ((l '\''())) (let ((c (read-char (current-input-port)))) (if (eof-object? c) (reverse l) (r (cons c l))))))) (newline)' "UCS-2BE" (353 610 867) $ echo -n $'\1a\2b\3c' | guile-2.0 -c '(set-port-encoding! (current-input-port) "UCS-2BE") (write (port-encoding (current-input-port))) (newline) (peek-char (current-input-port)) (write (map char->integer (string->list (drain-input (current-input-port))))) (newline)' "UCS-2BE" (1 97 2 98 3 99) The practical upshot is that the input returned by drain-input can't be used in the same way as regular input from read-char. It can still be used if the code doing the reading is totally aware of the encoding, so that it can perform the decoding manually, but this seems a failure of abstraction. The value returned by drain-input ought to be coherent with the abstraction level at which it is specified. I can see that there is a reason for drain-input to avoid performing decoding: the problem that occurs if the buffer ends in the middle of a character. If drain-input is to return decoded characters then presumably in this case it would have to read further octets beyond the buffer contents, in an unbuffered manner, until it reaches a character boundary. If this is too unpalatable, perhaps drain-input should be permitted only on ports configured for single-octet character encodings. If, on the other hand, it is decided to endorse the current non-decoding behaviour, then the break of abstraction needs to be documented. -zefram
