在Haskell中,我如何递归地操作一个元组,并将一个字符预附加到元组中的第一个元素上?

此函数的类型为 function :: Num a => ([Char],a) -> ([Char],a)

(".'*",0) '.' 或 '*' 和更新 a

function ([], a) = ([], a)
function ((x:xs), a)
    | x == '.' = ('-':xs, a+200)
    | x =='*' = ('-':xs, a+400)
    | otherwise = function(xs, a) --how do I put the unchanged x before xs without processing it?

Main> function ("./-", 0)
("-/-",200)

或

Main> function ("-/-", 0)
("-/-",0)