[mercury-users] Perl split, assign, retrieve in Mercury

[Ralph Becket]

> From: Terrence Brannon [mailto:tmbranno at oracle.com]
> Sent: 13 July 2001 19:41
> 
> A very common and easy thing to do in Perl is to read a file and split
> each line on a delimiter and assign each element of the line to a
> member of a hash and then obtain some member of the hash for
> processing of some sort. A typical Perl program for this task is
> included below. How would this be done in Mercury?
> 
> while (<DATA>) {
> 
>     @split{letters, numbers, junk} = split /:/;
>     print $split{numbers};
> }

I've been putting a small library together to support this sort of
Perl/Awk style file processing.

The code to read a line and split it according to a given separator
is

    % Read a line in and split it.  The line returned is empty (i.e. has
    % zero elements) on EOF.
    %
:- pred get_line(string, line, io__state, io__state).
:- mode get_line(in, out, di, uo) is det.

get_line(Separators, Line) -->
    io__read_line_as_string(Result),
    {   Result = ok(String),    Line = split_string(Separators, String)
    ;   Result = eof,           Line = make_empty_array
    ;   Result = error(E),      throw(E)
    }.

    % split_string(" ", " the quick brown fox!") =
    %   array([" the quick brown fox!", "the", "quick", "brown",
"fox!"])
    %
:- func split_string(string, string) = line.

split_string(Separators, Line) = array([Line | string__words(P, Line)])
:-
    P = ( pred(C::in) is semidet :- string__contains_char(Separators, C)
).

To handle your example you'd read in the file line by line with
get_line(":", Line) and print out Line ^ elem(2) each time.

- Ralph
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