Type for expression symbol lookups; name -> value.
Frees a hts_expr_val_t type.
@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
@param filt The filter pointer.
@param str The filter expression @return A pointer on success, NULL on failure
Note we cope with zero-but-true in order to implement a basic "exists(something)" check where "something" may even be zero.
Opaque hts_filter_t type. Definition in hts_expr.c