getFlat

import std.conv : to;

Nullable!int value = 100.nullable;
auto mapped = value.getFlat!((v) => v.to!string.nullable);

static assert(is(typeof(mapped) == Nullable!string));
assert(!mapped.isNull);
assert(mapped.get == "100");

assert(Nullable!int.init.getFlat!((v) => v.to!string.nullable).isNull);
assert(Nullable!int.init.getFlat!((v) => Nullable!string.init).isNull);