Alternatively you could refactor your language to have binary operations on one hand and unary ones on the other. You'd write:

data BinOp = PlusOp | MultOp deriving (Show, Eq)
data UnOp  = ConstOp deriving (Show, Eq)

data Expr  = Bin BinOp Expr  Expr
           | Un  UnOp  Expr
         deriving (Show,