w the types of each of its
fields. Each closure instance has its own unique anonymous type: that is, even
if two closures have the same signature, their types are still considered
different. To define structs, enums, or function parameters that use closures,
we use generics and trait bounds, as we discussed in Chapter 10.
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<p>クロージャを保持する構造体を作成するために、クロージャの型を指定する必要があります。
構造体定義は、各フィールドの型を把握