Postgres function for calculating measure in meters between two points

measure.sql

CREATE FUNCTION measure(_from POINT, _to POINT)
  RETURNS DOUBLE PRECISION AS $$
DECLARE
  d_lat DOUBLE PRECISION;
  d_lon DOUBLE PRECISION;
  _a    DOUBLE PRECISION;
  _c    DOUBLE PRECISION;
  _d    DOUBLE PRECISION;
BEGIN
  d_lat = (_to [0] - _from [0]) * pi() / 180;
  d_lon = (_to [1] - _from [1]) * pi() / 180;

  _a = sin(d_lat / 2) * sin(d_lat / 2) +
       cos(_from [0] * pi() / 180) * cos(_to [0] * pi() / 180) *
       sin(d_lon / 2) * sin(d_lon / 2);

  _c = 2 * atan2(sqrt(_a), sqrt(1 - _a));
  _d = 6378.137 * _c;

  RETURN _d * 1000;
END;
$$ LANGUAGE plpgsql;