@param filt The filter, produced by hts_filter_init
@param data Arbitrary caller data, passed into sym_func
@param sym_func Callback function to lookup variables.
@param res Filled out with the result of the filter evaluation
@return Returns 0 on success, -1 on failure
sym_func and data may be NULL if the caller does not need its own data
pointer or if it has no variables to lookup.
The type of the returned result may be numeric of string, as defined by
the is_str member. It can also be explicitly defined to be true even
for a null value. This may be used to check for the existence of
something, irrespective of whether that something evaluates to zero.
@param filt The filter, produced by hts_filter_init @param data Arbitrary caller data, passed into sym_func @param sym_func Callback function to lookup variables. @param res Filled out with the result of the filter evaluation @return Returns 0 on success, -1 on failure
sym_func and data may be NULL if the caller does not need its own data pointer or if it has no variables to lookup.
The type of the returned result may be numeric of string, as defined by the is_str member. It can also be explicitly defined to be true even for a null value. This may be used to check for the existence of something, irrespective of whether that something evaluates to zero.