[Haskell-cafe] error to Fail Conversion

[Anwar Bari]

HI Cafe 
  I have problem with this function 
value :: Theorm-> [(String,Int)] -> Tem -> Int
value _   env (Var s)      = maybe (error "Unknown variable.") id (lookup s env)
value mdl env (OP s _ _ l) = maybe (error "Function not defined.") id (lookup 
(map (value mdl env) l) (S.toList $ maybe (error 

"Uknown function.") id (fmap snd (multiLookup s (operations mdl)))))

This function is working fine, but I want to change error function to fail 
function to be able use it in my GUI application. 

I can't change the output of value function to IO Int,any idea will help.
Thanks
Nour.