Question

The stopLoss function causes the following error:

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

The function:

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)

Any suggestion is appreciated!

Answer 1

The type of printf is PrintfType r => String -> r. The following instances of PrintfType are available:

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

(The last one is just to make printf behave like if it is polyvariadic.)

Here stopLoss is a polymorphic function; the type IO () can’t be inferred automatically, and GHC is assuming that the function works for any monad. So, GHC is complaining that the instance of PrintfType for a generic monad does not exist.

Providing a type signature like Float -> Float -> Float -> IO () should help.