ECLiPSeWiki | Prolog / IsoIo

I/O built-ins and EOF

According to ISO 13211-1, input streams have an end_of_stream property, which can take the values not, at and past.

It also defines the stream option eof_action(...) in section 7.10.2.11. The name is misleading, as it actually specifies the behaviour when the stream is past (rather than at) eof:

error: raise permission_error(input,past_end_of_stream,Stream)
eof_code: behave as if at eof
reset: re-check the source for new input that may cause it not to be at eof any longer

The default is implementation-defined, possibly different for different streams. In ECLiPSe, error (files, string streams) or reset (tty, pipes, sockets, queues and null).

ECLiPSe actually closes the stream after reporting the past_end_of_stream error.

predicate	not eof	at eof	past eof [error]	past eof [eof_code]	past eof [reset]
get_code/2	last char	-1	error	-1	new char
read/2	last term	end_of_file	error	end_of_file	new term
read_string/4	last string	fails	error	fails	new string
read_string/5	string,-1	n/a	error	"",-1	new string, new sep
read_string/3	last string	?	error	?	new string

It seems to make sense for read_string/5 to leave the stream in past-eof position when -1 was returned as the separator.

For read_string/3, if the stream contains less characters than requested, should it leave the stream at or past eof? As no separator is consumed in this case, it should be at. Reading from a stream in at-eof position could return "" or fail (as read_string/4 does).

EDIT 2019: It is more convenient for read_string/3,5 to once return "",-1 at eof.