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Declarative Programming
Non- & Meta-logical features of Prolog
Geraint A. Wiggins
Professor of Computational Creativity
Department of Computer Science
Vrije Universiteit Brussel
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Input/Output (IO)
▶ Input/Output is a problem for declarative programming
languages
▶ Output: we cannot give a truth value to the action of
printing something
▶ Input: we could, maybe, view input as coming from a
changing Prolog database, but this still gives us problems
in understanding what the program means
▶ The problems get worse when we get on to flexible
computation rules, later!
▶ In fact, there are ways to tell a declarative story about
I/O, but we will not cover them here.
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IO Facilities in Prolog
▶ Two ways of thinking about IO:
▶ file-based;
▶ stream-based
▶ File-based IO is nice and simple, but is not
well-thought-out and can be confusing
▶ Stream-based IO is more complicated, but much more
reliable
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File-based IO in Prolog
▶ File-based IO is based on the idea of a “current file”
(which may be the terminal)
▶ You can have several files open at once, but you can only
read from or write to one at a time
▶ Each file has a pointer, which marks where the last
operation took place
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File-based IO in Prolog (2)
▶ We have nine basic predicates:
see/1 choose the current reading file (and open it
if necessary)
seen/0 close the current reading file
seeing/1 is this the current reading file?
read/1 read a term from the current reading file
tell/1 choose the current writing file (and open it if
necessary)
told/0 close the current writing file
telling/1 is this the current writing file?
write/1 write a term to the current writing file
nl/0 write a newline to the current writing file
▶ The terminal is a file called user.
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File-based IO in Prolog (3)
| ?- tell(example).
true
| ?- write(example(term)), write( ‘.’ ).
true
| ?- told.
true
| ?- see(example).
true
| ?- seeing(File).
File = example ?
true
| ?- read(Term).
Term = example(term) ?
true
| ?- seen, seeing(File).
File = user ?
true
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Stream-based IO in Prolog
▶ We associate each open file with an explicit pointer, with
which we refer to it subsequently.
▶ There are too many predicates to learn usefully – see the
manual for details
▶ The basic stream handling predicates are:
open/3 Arguments: a file specification (+);
a mode (read/write/append) (+); a
pointer (-)
close/1 Argument: a pointer (+)
current_input/1 Argument: a pointer (?)
set_input/1 Argument: a pointer (-)
read/2 Arguments: a pointer (+); a term (?)
write/2 Arguments: a pointer (+); a term (?)
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General IO in Prolog
▶ Most of the reading and writing predicates come in two
versions:
▶ to the current file/stream (eg write/1)
▶ to a named stream (eg write/2)
▶ The first argument is conventionally the stream pointer
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The format Predicate
▶ The most useful writing predicates are format/2 and
format/3
▶ format/2 has two arguments:
▶ a format specification (atom or string);
▶ a list of arguments
▶ For example, the predicate call
format( ‘v\n~w – ~w+\n’,
[term1, term2] ).
will print out to the terminal:
v
term1 – term2+
▶ See the manual for more details of format specifications
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Arithmetic Operators
▶ Recall that =/2 means “unify”
▶ We need arithmetic functions, even in logic programs
▶ Here are the basic arithmetic predicates
is/2 computes the value of the arithmetic
expression in its second argument (+) and
unifies it with the first argument (?)
>= > =:= =\= < =