Hi, I am trying to get the function showMinProp to return String, but I always get an error of parse error (possibly incorrect indentation)
Who can help with this? any idea?
Thank u in advance!

data Prop
= Var String
| Negation Prop
| BinOp Op Prop Prop

data Op = And | Or | Implies | Equiv
               derived Eq

showOp :: Op -> String
showOp And      = "&"
showOp Or       = "|"
showOp Implies = "=>"
showOp Equiv    = "<=>"

instance Show Op where show = showOp

precList = [(And,4),(Or,3),(Implies,2),(Equiv,1)]
showProp :: Prop -> String
showProp (Var s) = s
showProp (Negation p) = "~" ++ showProp p
showProp (BinOp op p q) = paren (showProp a p ++ space (showOp op) ++ showProp a q)

showMinProp :: Int -> Prop -> String
showMinProp preNo (BinOp op p q) =
       case op of
         And -> let a = 4
         Or -> let a = 3
         Implies -> let a = 2
         Equiv   -> let a = 1
       if (a > preNo)
         then (showMinProp a p ++ space (showOp op) ++ showMinProp a q)
         else paren (showMinProp a p ++ space (showOp op) ++ showMinProp a q))

space s = " " ++ s ++ " "
paren s = "(" ++ s ++ ")"

instance Show Prop where show = showProp