GHC 能够尾部调用优化 IO 操作吗？

发布于 2024-07-17 07:58:25 字数 275 浏览 7 评论 0原文

GHC 会默认对以下函数执行尾调用优化吗？唯一奇怪的是它递归地定义 IO 操作，但我不明白为什么这不能实现 TCO。

import Control.Concurrent.MVar

consume :: MVar a -> [a] -> IO ()
consume _ [] = return ()
consume store (x:xs) = do putMVar store x
                          consume store xs

原文

Will GHC perform tail-call optimization on the following function by default? The only weird thing about it is that it is recursively defining an IO action, but I don't see why this couldn't be TCO'd.

import Control.Concurrent.MVar

consume :: MVar a -> [a] -> IO ()
consume _ [] = return ()
consume store (x:xs) = do putMVar store x
                          consume store xs

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旧竹 2024-07-24 07:58:25

由于你的代码相当于

consume store (x:xs) = putMVar store >> consume store xs

调用实际上并没有发生在尾部位置。但是，如果您运行 ghc -O 并打开优化器，则 -ddump-simpl 选项将向您显示 GHC 中间代码的输出，并且它确实会优化为尾递归函数，它将编译成循环。

所以答案是 GHC 默认不会对此进行优化；您需要 -O 选项。

（使用 GHC 版本 6.10.1 进行的实验。）

Since your code is equivalent to

consume store (x:xs) = putMVar store >> consume store xs

the call does not actually occur in tail position. But if you run ghc -O and turn on the optimizer, the -ddump-simpl option will show you the output of GHC's intermediate code, and it does indeed optimize into a tail-recursive function, which will compile into a loop.

So the answer is GHC won't optimize this by default; you need the -O option.

(Experiments done with GHC version 6.10.1.)

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