transform_err

struct A {
	int a;
}
struct B {
	int b;
}
struct test01 {
	static auto opCall(R)(R t) if (isForwardRange!R) {
		return ParseResult!(R, A)(t, A(1));
	}
}

alias test02 = transform!(test01, x => B(x.a+1));
auto res = test02("asdf");
static assert(isParser!test01);
static assert(isParser!test02);
static assert(is(typeof(res).DataType == B));