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---
title: Monad
localeTitle: 单子
# Monad Laws
数据类型必须满足3个法则才能被视为monad
# 也许莫纳德
```haskell
justHead :: Maybe Char
justHead = do
(x:xs) <- Just ""
return x
```
# 列出Monad
回归与纯粹的应用相同
实例Monad \[\]在哪里
return x = \[x\]
xs >> = f = concat（map f xs）
失败\_ = \[\]