How to conciliate / constraint types between two separated expressions

问题

I'm playing with haskell-distributed (Cloud Haskell) and I cannot make use of type constraint using returning type (must be as ... -> Process ()).

To simplify context (and give a reproducible example), let

-- Request / Response
data ReqRes a b = Req a | Res b deriving (Read, Show)

and one generic function mapping generic processes

compute :: (Read a, Read b, Show a, Show b) => (a -> b) -> IO ()
compute f = do
    req <- readLn
    case req of
      Res _ -> error "Req expected"
      Req a -> print $ Res $ f a

this cannot compile since req <- readLn and Res $ f a cannot deduce b type and a types respectively.

My solution is use a variant of const to "link / constraint" both expressions:

linkType :: a -> a -> a
linkType = flip const

compute' :: (Read a, Read b, Show a, Show b) => (a -> b) -> IO ()
compute' f = do
    req <-readLn
    case req of
      Res _ -> error "Req expected"
      Req a -> print $ linkType req $ Res $ f a

now compile and works fine

> compute' (2 *)
Req 12
Res 24

> compute' (show :: Int -> String)
Req 12
Res "12"

but I wish to know the correct way to do this kind of things.

Thank you!

来源：https://stackoverflow.com/questions/35927439/how-to-conciliate-constraint-types-between-two-separated-expressions