mikewarot on Rethinking Prolog

I tried Turbo Prolog[1] back in the 1980s. I could never figure out how to get it to do file I/O at that time, so I got frustrated and gave up. Even now, it looks fairly hopeless as a language.

Is there a modern example of the language that can do file I/O? To me, if you can't do I/O, there's no point to a language. It's why Standard Pascal lost so much ground to Turbo Pascal with it's excellent I/O support.

[1] https://ia802902.us.archive.org/1/items/bitsavers_borlandtur...

mikewarot 3y

I finally got a hello world program that works like I expect it would, here's hello.pl

  main :- write('This is a sample Prolog program'),
      write('\nThis program is written into hello.pl file').
  
  ?- main.
  ?- halt.

That shouldn't have taken 35 years. ;-)

graymatters 3y

Tells more about you than about Prolog.

mikewarot 3y

Ok lay it on me

YeGoblynQueenne 3y

Why couldn't you do file I/O? Of course there's file I/O in Prolog, and I'd be really surprised if there wasn't in Turbo Prolog

Here's I/O in SWI-Prolog:

https://www.swi-prolog.org/pldoc/man?section=IO

I find it hard to parse your comment in good faith. You seem to say that Prolog is hopeless as a language because you couldn't find out how to do file I/O in Turbo Prolog in the '80s, when file I/O is both entirely possible and very simple to do in Prolog. It's not the fault of the language if you couldn't figure it out 40 years ago.

Edit: the Turbo Prolog manual you linked to lists the file_str/2 predicate for file I/O on page 73. Did you try that?.

mikewarot 3y

I installed SWI prolog, and I can not figure out how to run the example code I found on the internet.

Here's hello.pl:

  main :- write('This is a sample Prolog program'),
      write(' This program is written into hello.pl file').

Trying to run hello.pl

  Welcome to SWI-Prolog (threaded, 64 bits, version 9.0.4)
  SWI-Prolog comes with ABSOLUTELY NO WARRANTY. This is free software.
  Please run ?- license. for legal details.
  .
  For online help and background, visit https://www.swi-prolog.org
  For built-in help, use ?- help(Topic). or ?- apropos(Word).
  .
  1 ?- [hello.pl].
  ERROR: Type error: `dict' expected, found `hello' (an atom)
  ERROR: In:
  ERROR:   [13] throw(error(type_error(dict,hello),_15432))
  ERROR:   [11] '.'(hello,pl,_15466) at c:/program files/swipl/boot/dicts.pl:57
  ERROR:   [10] '<meta-call>'(user:user: ...) <foreign>
  ERROR:    [9] toplevel_call(user:user: ...) at c:/program files/swipl/boot/toplevel.pl:1173
  ERROR:
  ERROR: Note: some frames are missing due to last-call optimization.
  ERROR: Re-run your program in debug mode (:- debug.) to get more detail.
  2 ?-

Google seems to be no help at this point, it's taken about 2 hours to get to this point. This is typical of the frustration I've experienced in my past attempts to understand this language.

You can express a bunch of facts and rules, great... but you can't actually write code to act on them, apparently. 8(

-----------

Edit -- Success has been achieved

  C:\Program Files\swipl>bin\swipl -s hello.pl -g main -g halt
  This is a sample Prolog program This program is written into hello.pl file
  C:\Program Files\swipl>

----------

Even better... here's hello2.pl

  main :- write('This is a sample Prolog program'),
      write(' This program is written into hello.pl file').
  .
  ?- main.
  ?- halt.

and it works as expected

  C:\Program Files\swipl>bin\swipl hello2.pl
  This is a sample Prolog program This program is written into hello.pl file
  C:\Program Files\swipl>

Avshalom 3y

it is just

  [hello].
  %or
  ['hello.pl'].
  %or
  ["hello.pl"].

the '.' has syntactic meaning outside of quotes.

now to demonstrate file i/o:

we make a file hello.pl that has

  :- use_module(library(readutil)).

  readmyfile(Filename) :-
    open(Filename,read, Stream),
    read_line_to_string(Stream,Input),
    write("The line is:"), nl,
    write(Input), nl,
    close(Stream),
    %or slurp the whole thing
    read_file_to_string(Filename,WholeFile,[encoding(text)]),
    %lets use the formatted output predicate
    format("The file ~s has the following contents ~n~s ~n",[Filename,WholeFile]).

then launch swi and

  ?- [hello].
  true.

  ?- readmyfile('hello.pl').
  The line is:
  :- use_module(library(readutil)).
  The file hello.pl has the following contents 
  ...
  ...
  ...
  true.

YeGoblynQueenne 3y

> ?- [hello.pl].

Can I ask, why did you type that?

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