Question

I have an ambiguous type variable error on the definition of “trial” below, I am wondering if there is anything that can be done to make this situation work? I want to really just deal with instances and not explicit data types (such as the MO1, MO2 included below).

module Tc102 where

class (Show a, Read a) => MyObj a where
    alpha :: a->String
    beta  :: a->Int

data MO1 = MO1 { a1 :: String, b1 :: Int } deriving (Show,Read)
data MO2 = MO2 { a2 :: String, b2 :: Int } deriving (Show,Read)

instance MyObj MO1 where
    alpha = a1
    beta = b1

instance MyObj MO2 where
    alpha = a2
    beta = b2


a = MO1 "a" 3
b = MO2 "b" 4

test :: MyObj a => a->String
test = alpha


showMe :: (MyObj a)=> a -> String
showMe = show

readMe :: (MyObj a) => String -> a
readMe = read

trial :: MyObj a => a -> String
trial = test . readMe . showMe

thanks in advance all! I fear however i might need to go to a helper function that would convert old ADT to the ‘latest versions…

Simon

EDIT To clarify, imagine that I first show to a file, then later reload the object. Then the function i have is more like

trial :: String -> Int
trial s = beta x
  where x = readMe s

Answer 1

You can’t read compile-time type from run-time file. You should explicitly implement run-time type-varying value.

As example:

data AnyMyObj = forall a . MyObj a => AnyMyObj a

test :: AnyMyObj -> String
test (AnyMyObj x) = alpha x

showMe :: AnyMyObj -> String
showMe (AnyMyObj x) = show x

readMe :: String -> AnyMyObj
readMe s = AnyMyObj (read s :: MO1) -- maybe something more complicated

trial :: AnyMyObj -> String
trial = test . readMe . showMe