Is it possible to force a type to a class in Haskell?
module T where
import Debug.Trace

dosum :: (Num a) => [a] -> a
dosum xs = dosum' 0 xs
    where
        dosum' n [] = n
        dosum' n (x:xs) = trace (show n) $ dosum' (n+x) xs

Could not deduce (Show a) arising from a use of dosum'
from the context (Num a)

unsafeShow :: a -> String

class (Num a) => Number a
instance (Num a) => Number a

-- original function, should add NOINLINE to make sure your rule gets a chance to fire.
{-# NOINLINE dosum #-}
dosum :: (Num a) => [a] -> a

-- a version with added debugging
dosumShow :: (Num a, Show a) => [a] -> a

{-# RULES "sub/dosum" forall x. dosum x = dosumShow x #-}