看完ReaderT的源码，接着来分析WriterT的源码。

示例

先看一个超级简单的例子：

import Control.Monad
import Control.Monad.Trans.Writer

data LogEntry = LogEntry { msg :: String } deriving (Eq, Show)

calc :: Writer [LogEntry] Integer
calc = do
    tell [LogEntry "Start"]
    let x = sum [1 .. 10000000]
    tell [LogEntry (show x)]
    tell [LogEntry "done"]
    return x
    
test = execWriter calc
test2 = runWriter calc

test执行结果：

[LogEntry {msg = "Start"},LogEntry {msg = "50000005000000"},LogEntry {msg = "done"}]

ReaderT是Haskell里很常见的一种模式。比如读取环境配置：

所有权（ownership）系统是 Rust 最与众不同的特性，让Rust实现了既要保障内存安全又要无GC，运行时高性能的目标。