打印状态值进行调试

发布于 2024-12-29 00:50:34 字数 781 浏览 2 评论 0原文

我们如何打印当前状态值以进行调试？例如，在http://www.haskell.org/haskellwiki/State_Monad<的concrete-example-1的代码中/a> ，如何在读取每个输入字符后打印当前状态值？

module StateGame where

import Control.Monad.State

type GameValue = Int
type GameState = (Bool, Int)

playGame :: String -> State GameState GameValue
playGame []     = do
    (_, score) <- get
    return score

playGame (x:xs) = do
    (on, score) <- get
    case x of
       'a' | on -> put (on, score + 1)
       'b' | on -> put (on, score - 1)
       'c'      -> put (not on, score)
       _        -> put (on, score)
    playGame xs

startState = (False, 0)

main = print $ evalState (playGame "abcaaacbbcabbab") startState

原文

How can we print current state value, for debugging purposes? E.g., in code from concrete-example-1 of http://www.haskell.org/haskellwiki/State_Monad , how can we print the current state value after each input character is read?

module StateGame where

import Control.Monad.State

type GameValue = Int
type GameState = (Bool, Int)

playGame :: String -> State GameState GameValue
playGame []     = do
    (_, score) <- get
    return score

playGame (x:xs) = do
    (on, score) <- get
    case x of
       'a' | on -> put (on, score + 1)
       'b' | on -> put (on, score - 1)
       'c'      -> put (not on, score)
       _        -> put (on, score)
    playGame xs

startState = (False, 0)

main = print $ evalState (playGame "abcaaacbbcabbab") startState

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夕嗳→ 2025-01-05 00:50:34

要进行快速而肮脏的调试，请使用 Debug.Trace 中的 trace。我经常发现翻转参数顺序并定义很有用，

infixl 0 `debug`

debug :: a -> String -> a
debug = flip trace

然后您可以将调试添加到行尾，并且更容易注释掉（并在最后删除）。

对于更有原则的日志记录，请将 State 与 Writer 结合起来，并tell 记录消息，或者使用 StateT s IO if您直接想要记录到文件或 stderr。

For quick and dirty debugging, use trace from Debug.Trace. I often find it useful to flip the argument order and define

infixl 0 `debug`

debug :: a -> String -> a
debug = flip trace

Then you can tack the debugging to the end of the line and it's easier to comment out (and at the end remove).

For more principled logging, combine the State with a Writer and tell the logging messages or use StateT s IO if you directly want to log to a file or stderr.

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感悟人生的甜 2025-01-05 00:50:34

正如 nm 指出的那样，有 Debug.Trace，但自己编写一些东西很容易。不过，我强烈建议仅将其用于调试，并在实际代码中将其删除。这是一个示例：

import System.IO.Unsafe

output a b = seq (unsafePerformIO (print a))  b

(output "test" 23) * 25
-- "test"
-- 527

这里 output 接受一个要打印的参数和一个返回值，其行为类似于 const，只是有副作用。需要 seq 来强制对 print 进行评估，否则惰性将阻止打印任何内容。

As n.m. points out there is Debug.Trace, but it's easy to write something yourself. However I strongly recommend to use this only for debugging, and to remove it for real world code. Here is an example:

import System.IO.Unsafe

output a b = seq (unsafePerformIO (print a))  b

(output "test" 23) * 25
-- "test"
-- 527

Here output takes an argument to print out, and a return value, behaving like a const, just with a side effect. seq is needed to force the evaluation of print, else laziness will prevent to print anything.

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