Code Review: Shadowing

[cd155]

In the isValidParen, it has a variable name the first xs. Inside of let expression, there is a local function isValidParenHelper, it has the second xs.

I have two following questions:

1. In this situation, is the first xs shadowed by the second xs?
2. I intend to change the x:xs to y:ys to avoid confusion. I just wonder what will you do and what is a good practice?

isValidParen :: String -> Bool
isValidParen [] = something
isValidParen xs = 
    let 
        isValidParenHelper :: String -> String -> Bool
        isValidParenHelper [] s = something
        isValidParenHelper (x:xs) s
            | condition1 = isValidParenHelper xs (tail s)
            | otherwise = isValidParenHelper xs (x:s)
    in 
        isValidParenHelper xs []

[JacquesCarette]

1. yes
2. if you don't want the helper function to have access to the top xs, some consider it good practice to shadow it. I'm not so sure.

Personally, I see no reason to make isValidParenHelper a local function. I would pull it out into the surrounding module, but then not export it.

[cd155]

That's an interesting idea, shadow xs when I don't want the helper function to access the top xs. Certainly, it makes sense. Shadowing it will erase the previous xs, but will it decrease the readability?

One difference would be the isValidParenHelper is only accessible by isValidParen. If I put it on the surrounding module and even hide it from other modules, any functions in this module can still access it.

[JacquesCarette]

Shadowing does reduce "shallow readability", absolutely.