5分で完全理解するGoのiota

by uji

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5分で完全理解するGoのiota  Fully understand Go's iota in 5 minutes.    uji 

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Name  uji  Twitter  @uji_rb  Orgs    Kyoto.go  Profile 

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Go Conference 2022 Spring  https://gocon.jp/2022spring  archive: https://youtu.be/GZCr-Dbi0YM?t=6205  slide: https://t.co/3DGnnOslLG 

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What is iota? 

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Goにおける定数生成器  Go's Constant generator.    定数に連番の整数を割り当てる際に便利  Useful for assigning sequential integers to constants.  What is iota? 

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iotaを使わない連番定数の宣言  Declaration of sequential constants without iota. 

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iotaを使った連番定数の宣言  Declaration of sequential constants with iota.  全て = iota で表現できる  All can be expressed using = iota 

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iotaを使った連番定数の宣言  Declaration of sequential constants with iota.  さらに定義を省略できる  Definitions can be more abbreviated. 

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iota Specification 

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● 定義は定数, コンパイル時に値が決まる  Definition is constant, value determined at compile time.  ● ConstDecl中のConstSpecのindex値が入る  The index value of ConstSpec in ConstDecl is entered.          カウントアップしているわけではない  iota Specification  ConstDecl  ConstSpec 

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iota Specification  Approved = ? 

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Approved = 2  iota Specification  index: 0  index: 1  index: 2 

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iota Specification  Approved = ?  https://go.dev/play/p/ZxykC3awxTk 

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iota Specification  Approved = 2 + 2 = 4  https://go.dev/play/p/ZxykC3awxTk  index: 0  index: 1  index: 2 

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Tips 

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iota + 1              ゼロ値の変数が意図せず分類されてしまうのを防ぐ  Prevent unintentional classification of zero-valued variables  Tips 

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0の定数をUnknownなどの命名にするやり方もある  Name 0 constants Unknown, etc.  Tips 

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1 << iota            複数の状態を一つ値で表現したいときに効果的  This is effective when you want to express multiple states with a single value.  Tips 

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https://go.dev/play/p/3Aub3AACmco  Tips  ビット和(OR)で  CreateとReadが  許可された権限を表現    ORs can express the permissions allowed to Create and Read. 

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Tips  ビット積(AND)で権限チェック    AND can be used to check authority.  https://go.dev/play/p/3Aub3AACmco 

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「数値に意味がある」定数 Constants with meaningful numerical values  e.g.  ・Exit Code  ・DBに定義された値 Value defined in DB  コードの差分に数値の変更が表れないため意図しない数値の変更に気付きにくい  Difficult to notice unintended numerical changes because numerical changes are not represented in the code differences.  Iotaを使わない方が良い場面  Situations where iota should not be used. 

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Goチームは一般的な列挙型は解決したい課題に  対して仕様が複雑すぎると考えている  The Go team believes that general enums are too complex a specification for the problem they want to solve.    Go2に向けて今も議論されている  Still being discussed for Go2.  https://github.com/golang/go/issues/28987  Why not Enum? 

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● iotaはGoの連番定数生成の仕組み  iota is Go's sequential constant generation mechanism  ● iotaはconst宣言のindex値として扱われる  iota is treated as an index value of const declaration  ● iota + 1でゼロ値による意図しない挙動を防げる  `iota +1` to prevent unintended behavior due to zero values.  ● 1 << iotaで単一の値で複雑な状態も表現できる  Complex states can be represented with a single value in `1 << iota`    Summary 