###
### Parser.pm
###

## Chapter 8 section 3

package Parser;
use Stream ':all';
use base Exporter;
@EXPORT_OK = qw(parser nothing End_of_Input lookfor
                alternate concatenate star list_of 
                operator T 
                error action test);
%EXPORT_TAGS = ('all' => \@EXPORT_OK);

sub parser (&);   # Advance declaration - see below


## Chapter 8 section 3.1

sub nothing {
  my $input = shift;
  return (undef, $input);
}
sub End_of_Input {
  my $input = shift;
  defined($input) ? () : (undef, undef);
}


## Chapter 8 section 3.1

sub lookfor {
  my $wanted = shift;
  my $value = shift || sub { $_[0][1] };
  my $u = shift;

  $wanted = [$wanted] unless ref $wanted;
  my $parser = parser {
    my $input = shift;
    return unless defined $input;
    my $next = head($input);
    for my $i (0 .. $#$wanted) {
      next unless defined $wanted->[$i];
      return unless $wanted->[$i] eq $next->[$i];
    }
    my $wanted_value = $value->($next, $u);
    return ($wanted_value, tail($input));
  };

  return $parser;
}


## Chapter 8 section 3.1

sub parser (&) { $_[0] }


## Chapter 8 section 3.2

sub concatenate {
  my @p = @_;
  return \&nothing if @p == 0;
  return $p[0]  if @p == 1;

  my $parser = parser {
    my $input = shift;
    my $v;
    my @values;
    for (@p) {
      ($v, $input) = $_->($input) or return;
      push @values, $v;
    }
   return (\@values, $input);
  }
}


## Chapter 8 section 3.2

sub alternate {
  my @p = @_;
  return parser { return () } if @p == 0;
  return $p[0]                if @p == 1;
  my $parser = parser {
    my $input = shift;
    my ($v, $newinput);
    for (@p) {
      if (($v, $newinput) = $_->($input)) {
        return ($v, $newinput);
      }
    }
    return;
  };
}


## Chapter 8 section 3.3

sub star {
  my $p = shift;
  my $p_star;
  $p_star = alternate(concatenate($p, parser { $p_star->(@_) }),
                      \&nothing);
}


## Chapter 8 section 3.3

sub list_of {
  my ($element, $separator) = @_;
  $separator = lookfor('COMMA') unless defined $separator;

  return concatenate($element,
                     star($separator, $element));
}

1;


## Chapter 8 section 4

sub T {
  my ($parser, $transform) = @_;
  return parser {
    my $input = shift;
    if (my ($value, $newinput) = $parser->($input)) {
      $value = $transform->(@$value);
      return ($value, $newinput);
    } else {
      return;
    }
  };
}


## Chapter 8 section 4.3

sub null_list {
  my $input = shift;
  return ([], $input);
}

sub star {
  my $p = shift;
  my $p_star;
  $p_star = alternate(T(concatenate($p, parser { $p_star->(@_) }),
                        sub { my ($first, $rest) = @_;
                              [$first, @$rest];
                            }),
                      \&null_list);
}


## Chapter 8 section 4.4

sub operator {
  my ($subpart, @ops) = @_;
  my (@alternatives);
  for my $operator (@ops) {
    my ($op, $opfunc) = @$operator;
    push @alternatives,
      T(concatenate($op,
                    $subpart),
        sub {
          my $subpart_value = $_[1];
          sub { $opfunc->($_[0], $subpart_value) }
        });
  }
  my $result = 
    T(concatenate($subpart,
                  star(alternate(@alternatives))),
      sub { my ($total, $funcs) = @_;
            for my $f (@$funcs) {
              $total = $f->($total);
            }
            $total;
          });
}


## Chapter 8 section 4.7.1

sub error {
  my ($checker, $continuation) = @_;
  my $p;
  $p = parser {
    my $input = shift;

    while (defined($input)) {
      if (my (undef, $result) = $checker->($input)) {
        $input = $result;
        last;
      } else {
        drop($input);
      }
    }

    return unless defined $input;

    return $continuation->($input);
  };
  $N{$p} = "errhandler($N{$continuation} -> $N{$checker})";
  return $p;
}


## Chapter 8 section 6

sub action {
  my $action = shift;
  return parser {
    my $input = shift;
    $action->($input);
    return (undef, $input);
  };
}


## Chapter 8 section 6

sub test {
  my $action = shift;
  return parser {
    my $input = shift;
    my $result = $action->($input);
    return $result ? (undef, $input) : ();
  };
}