我正在设计一个基本上使用StateT并只是更新状态的小型游戏。以下是简化版本：
{-# LANGUAGE TemplateHaskell #-}
import Control.Lens
import Control.Monad
import Control.Monad.IO.Class
import Control.Monad.State
import Control.Monad.State.Class
import System.Random
data PlayerState = PlayerState {
_psName :: String,
_psScore :: Int
} deriving (Show)
makeClassy ''PlayerState
data Game = Game {
_turns :: Int,
_players :: [PlayerState]
} deriving (Show)
makeClassy ''Game
randomGameInit :: IO Game
randomGameInit = do
players <- replicateM 5 $ PlayerState <$> (replicateM 4 $ randomRIO ('a', 'z')) <*> randomRIO (1,10)
return $ Game 0 players
update :: (MonadState …test1 = hspec $ do
describe "blabla" $ do
it "should be equl" $ verbose $
\input-> ...
在上面的代码中，当测试失败时，它会打印失败的input. 但我实际上对可以从 计算出的另一个值感兴趣input。我可以要求QuickCheck打印其他值吗？
我按照 Simon Marlow 的关于并行 Haskell 的书（第 1 章）使用rpar/ rseq。
下面是代码（解决Squid Game桥模拟）：
{-# LANGUAGE FlexibleContexts #-}
import Control.DeepSeq (force)
import Control.Exception (evaluate)
import Control.Parallel.Strategies
import Data.Array.IO
( IOUArray,
getAssocs,
newListArray,
readArray,
writeArray,
)
import Data.Functor ((<&>))
import System.Environment (getArgs)
import System.Random (randomRIO)
game ::
Int -> -- number of steps
Int -> -- number of glass at each step
Int -> -- number of players
IO Int -- return the number of survivors
game totalStep totalGlass = go …在以下函数定义中：
app :: Application
app _ respond = do
putStrLn "I've done some IO here"
respond $ responseLBS
status200
[("Content-Type", "text/plain")]
"Hello, Web!"
您好，app函数似乎没有参数。为什么在上面的示例中有两个参数？
I use read "123" :: Int to read in Int from String. This works great for a simple case such as "123"; But it fails when the input is "12.34".
What do people usually do in such cases?
Edit: I also would like to know if there is a simple way to parse numbers with thousand separator such as "12,345"
我计划使用inline-c包装C函数：
lxw_workbook *workbook = workbook_new("filename.xlsx");
我需要捕获返回的Ptr，以便以后在其他函数中使用。
问题：Ptr指向lxw_workbook，这是一个自定义结构。我是否需要为此Haskell类型编写一个存储实例？因为如果我不直接使用该结构，而仅使用Ptr，可以跳过此步骤吗？
I have a data type definition:
data Point = Point {x :: Int, h :: Int} | EmptyPoint
In my property test, I would like to limit the test only on the Point constructor cases. For example point1 - point2 = Point 0 0. This presumes that the accessor x is defined which is not the case with EmptyPoint.
in other words: I don't want EmptyPoint to be generated.
Is there a way to do that?
在mtl库中,我看到了以下定义MonadWriter:
class (Monoid w, Monad m) => MonadWriter w m | m -> w where
#if __GLASGOW_HASKELL__ >= 707
{-# MINIMAL (writer | tell), listen, pass #-}
#endif
-- | @'writer' (a,w)@ embeds a simple writer action.
writer :: (a,w) -> m a
writer ~(a, w) = do
tell w
return a
-- | @'tell' w@ is an action that produces the output @w@.
tell :: w -> m ()
tell w = writer ((),w)
所以 …