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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