Project "Contraintes" Prolog Web Pages: IO in standard Prolog.
A major change in the Prolog language that is aprt of the standard
was the introduction of a completely new set of predicates for input and output.
In the standard these replace the see and tell predicates of
Edinburgh Prolog.
This page summarises the new predicates. A more detailed description of the I/O predicates is available with these pages.
Streams
The new I/O predicates introduce the notion of a stream. Streams are associated by a call to the open/4 predicate. sources
(sinks) from (to) which IO is done. Streams can be identified either
by a stream-term which is an implementation dependent ground term
created as a result of a call to open/4 or by use of an alias which is assigned by the call to open/4.
An input/output mode is an atom which defines in a call of open/4 the operations that may be performed on a source/sink. The required modes, which have the obvious meanings are:
Two streams are predefined and open during the execution of any goal:
- the standard input stream, and
- the standard output stream.
At any time there is a current input stream and a current output stream,
by default these are initially the standard input stream and the standard output stream respectively. Predicates are provided for changing the current input or output stream.
The standard makes a distinction between text streams and binary streams,
with distinct predicates for
For somke implementations it may be possible to set the position on a
source/sink. Whether this is possbile for a given stream is determined by the
call to open/4 and whether the appropriate stream option is set. A standard conforming program shall make no assumptions about the form of a stream position term except thst it is ground and uniquely identifies a
position in the stream.
At the end of a stream a specific value shall be returned to indicate that the end of the stream has been reached. The terms indicating end-of-stream and
past-end-of-stream shall be implemenetation defined.
When a strem is opened using open/4 the fourth argument specifies a list of options for the stream so created. The following options are requied by the standard.
- type(T)
- Specifies whether the stream is a text stream or a binary stream: T may be:
- text -- a text stream
- binary -- a binary stream
The default is text.
- reposition(Boolean)
- If Boolean is true then it shall be
possible to reposition the stream, else if it is false it is implementation defined whether it is possible to reposition.
- alias(A)
- Specifies that the atom A shall be an
alias for the stream.
- eof_action(Action)
- This determines the effect of attempting to input form a stream whose stream position is past-end-of-stream. The required values for Action are:
- error -- A permission error shall be raised;
- eof_code -- The result of input shall be as if the stream
position were end-of-stream;
- reset -- The stream position shall be rest to end-of-stream
and another attempt made to input form the source to
which the stream is attached.
It is implementation defined which Action is the default.
A close option modifies the effect of close/2
if an error condition occurs while attempting to close a stream.
The required options are:
- force(false) -- The default,
if there is an error the stream is not clsed.
- force(true) -- If a Resource Error or
a System Error
is satisfied, there shall be no error; instead the stream is closed and the goal
succeeds.
The properties of a stream can be discovered using the predicate stream_property(Stream, Property). The stream properties shall include:
- file_name(F) -- When the the stream is connected to a
source/sink which is a file, F shall be an implementation
defined term which identifies this file.
- mode(M) -- M is unified with the
input/output mode which was specified when the source/sink was opened.
- input -- the stream is connected to a source.
- output -- the stream is conencted to a sink.
- alias(A) -- If the stream has an alias then A shall be that alias.
- position(P) -- If the stream has a a reposition property, P shall be the current stream position,
- end_of_stream(E) If the stream position is end-of-stream then E is unified with at else if the position is past-end-of stream it is unified with past, else it is unified with
not;
- eof_action(A) -- A is unified with the eof_action for the stream;
- reposition(Boolean) If repositioning is possible
for the stream then boolean is unified with true else it is unified with false
- type(T) -- T is unified with text or binary depending on the type of the stream.
A read-options list is a list of options which affects the behaviour of read_term/3.
The read options shall include
- variables(Vars) -- After inputting a term
Vars is a list of the varialbes in the term that was input, in left-to-right order.
- variable_names(VN_List) -- After a term is input
VN_List is unified with a list of elements each of which has the form
A = V and V is a named variable of the term,
and
A is an atom whose name is the characters of V.
- singletons(VS_List) -- when a term is inout,
VS_List shall be unified with a list of elements of the form
A = V where V is a named variable of the term,
which occurs only once in the term and A is an atom whose name is the characters of V.
A write-options list affects write_term/3.
These shall include:
- quoted(Boolean) -- If Boolean is true each atom and functor is quoted if this would be necessary for the term to be
correctly input by read_term.
- ignore_ops(Boolean) -- If Boolean is true
each compound term is output in functional notation.
- numbervars(Bool) -- If Boolean is true then a term of the form '$VAR(N)' where N is an integer is output
as a variable name consiting of a capital letter possible followed byu a letter.
The scheme works as follows
| "$VAR(0)' | is written as | A |
| "$VAR(1)' | is written as | B |
"$VAR(25)' | is written as | Z |
| "$VAR(26)' | is written as | A1 |
"$VAR(27)' | is written as | B1 |
| "$VAR(52)' | is written as | A2 |
Many prologs contain a predicate numbervars/3 that causes variables
appearing in terms to be converted to this special form.
Live examples of the write options.
Return to Home Page of the INRIA ISO Prolog Web
Author: J.P.E. Hodgson
Saint Joseph's University
Philadelphia PA 19131
USA
Last Changed: 1 janvier 2003