haskell - 我可以在 IO monad 中使用 printf 吗？

Question

stopLoss 函数会导致以下错误：

Could not deduce (Text.Printf.PrintfType (m a0))
arising from the ambiguity check for `stopLoss'
from the context (Monad m,
                Text.Printf.PrintfType (m b),
                Text.Printf.PrintfType (m a))
bound by the inferred type for `stopLoss':
  (Monad m, Text.Printf.PrintfType (m b), 
   Text.Printf.PrintfType (m a)) =>
   Float -> Float -> Float -> m b
Possible fix:
   add an instance declaration for (Text.Printf.PrintfType (m a0))

When checking that `stopLoss'
has the inferred type `forall (m :: * -> *) a b.
  (Monad m, Text.Printf.PrintfType (m b),
   Text.Printf.PrintfType (m a)) =>
   Float -> Float -> Float -> m b'
Probable cause: the inferred type is ambiguous

功能：

stopLoss qty pb lossRate = do
    let t = qty * pb * (1 + sxf)
    printf "Stop Loss at: %.2f\n" ((pb - (t * lossRate) / qty) :: Float)
    printf "Lost Money: %.2f\n"  ((t * lossRate) :: Float)

任何建议表示赞赏！

Answer 1

的类型printf是PrintfType r => String -> r。以下实例PrintfType可用：

IsChar c => PrintfType [c]   
PrintfType (IO a)    
(PrintfArg a, PrintfType r) => PrintfType (a -> r)

（最后一个只是为了让它printf表现得像多变量一样。）

这stopLoss是一个多态函数；IO ()无法自动推断类型，GHC 假设该函数适用于任何 monad。因此，GHC 抱怨PrintfType通用单子的实例不存在。

提供类似的类型签名Float -> Float -> Float -> IO ()应该会有所帮助。