Reading-in Programs


KEY
A  program  is  made  up  of  a sequence of clauses, possibly interspersed with
directives to the interpreter.  The clauses of a procedure do not  have  to  be
immediately  consecutive,  but  remember  that  their  relative  order  may  be
important.

To input a program from a file file, give the directive:

            | ?- [file].

which will instruct the interpreter to read-in (or consult) the program.    The
file  specification  file must be a Prolog atom.  It may be any EMAS file name,
note that if this file name contains characters which are not normally  allowed
in  an  atom then it is necessary to surround the whole file specification with
single quotes (since quoted atoms can include any character); e.g.

            | ?- ['ecmi25.greeks'].

The specified file is then read in.  Clauses in the  file  are  stored  in  the
database  ready  to  be  executed,  while any directives are obeyed as they are
encountered.  When the end of the file is found, the  interpreter  displays  on
the terminal the time spent for read-in and the number of bytes occupied by the
program.

In general, this directive can be any list of filenames, such as:

            | ?- [myprogram,extras,testbits].

In this case all three files would be consulted. If a filename is preceded by a
minus sign, as in:

            | ?- [-testbits,-moreideas].

then   that   file  is  reconsulted.  The  difference  between  consulting  and
reconsulting is important, and works as follows:  if a file is  consulted  then
all  the  clauses  in  the  file  are simply added to Prolog's database. If you
consult the same file twice then you will get two copies of  all  the  clauses.
However,  if  a  file is reconsulted then the clauses for all the procedures in
the file will replace any existing clauses for those procedures, i.e  any  such
previously  existing  clauses  in  the  database get thrown away.  reconsult is
useful for telling Prolog about corrections in your  program  (see  the  'redo'
predicate in section 1.5).

Clauses may also be typed in directly at the terminal.  To enter clauses at the
terminal, you must give the directive:

            | ?- [user].

The  interpreter  is  now  in  a  state  where  it  expects input of clauses or
directives.   To return to interpreter top level, type ^Y (Control Y).  This is
equivalent to an end of file for the ersatz file 'user'.
However,  this  is  only  recommended  if  the  clauses  will  not  be   needed
permanently, and are few in number.  For significant bits of program you should
use  an  editor to produce an Emas file containing the text of the program. See
the next section for some useful Prolog predicates that help you to do this.