Re: How to read a line of text?

[Ron Stodden]

Thanks, Alex. but your code has problems:

1.     It will not deal with an empty stream.  IOW the test for (not) 
end of stream must immediately follow the repeat.  gprolog does not 
support a 'not' operator.

2.     My experiments show that list decomposition is limited to the 
head item only.    IOW [Item|Remainder] works, but 
[Item1|Item2|Remainder] will not produce your expected result.

3.     Your case is limited to the processing of single lines.   For 
example, using your technique one could not output a  list of all the 
lines in a file because all data is lost at the fail of a repeat fail 
combination.   That would require the use of persistent storage, ie a 
database or global variables.

4.     The requirement to specify words as code lits is insufferably 
tedious.

I will have a look at your http://apax.net/swpets/plg_p3.html

The best so far, expressed in swi-prolog, is:

read_list('locals.sort',[],Locals),
 ...

read_list(File,Build,List):-
 open(File,read,Handle), !,
 read_list_2(Handle,Build,List),
 close(Handle).
read_list(File,_,_):-
 format('open ~a failure',[File]),
 fail.

read_list_2(Handle,Build,[Line|List]):-
 not(at_end_of_stream(Handle)),
 read_line_to_codes(Handle,Codes),
 atom_codes(Line,Codes), !,
 read_list_2(Handle,Build,List).
read_list_2(_,List,List).


Alexander V. Diemand wrote:

> Hi,
>
> I somehow missed the discussion but want to throw my 2c in:
>
> the trick is to avoid working with "atoms" when reading files but to
> use lists of characters. memory space for atoms is limited, lists
> consume stack space (dynamic, released).
>
> Since ages I am using a small toolbox in pure Prolog code. It has
> recently been published:
> http://apax.net/swpets/plg_p3.html
>
> The predicate you might be interested in is read_txtline/2. It returns
> a list of characters read from a (file) stream.
>
> example:
>
> open('file.txt',read,Str), repeat, read_txtline(Str,Line),
> parseline(Line), at_end_of_stream(Str), close(Str).
>
> % matches  "From:"
> parseline([70,114,111,109,58|R]) :-
>    do_some_thing.
> % matches "To:"
> parseline([84,111,58|R]) :-
>    do_some_thing_else.
> % fallback
> parseline(_).
>
>
> hope that helps
>
> Alex.

--
Ron. [Melbourne, Australia]
"If you keep a green bough in your heart, the singing bird will come"
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