fmapOr

// source: https://doc.rust-lang.org/std/result/enum.Result.html#method.map_or
auto x = Result!string.ok("foo");
assert(x.fmapOr!(str => str.length)(42) == 3);

auto y = Result!string.err("bar");
assert(y.fmapOr!(str => str.length)(42) == 42);