Replace individual list elements in Haskell?

Question

I have a list of elements and I wish to update them:

from this: [\"Off\",\"Off\",\"Off\",\"Off\"]

to this: [\"Off\",\"Off\",\"On\",\"Off\

Answer 1

Here is a one liner that works perfectly

replace pos newVal list = take pos list ++ newVal : drop (pos+1) list

I doesn't seem efficient to do this kind of things in haskell.

Answer 2

I think you should consider using a data structure other than List. For example, if you just want to have a state of four on/off switches then:

data State = St { sw1, sw2, sw3, sw4 :: Bool }

For a dynamic number of switches then consider a mapping from switch name to Bool.

Answer 3

Typically, you modify elements of a list by splitting the list, replacing an element, and joining it back together.

To split a list at an index, we have:

 splitAt :: Int -> [a] -> ([a], [a]) 

which you can use to break up a list, like so:

 > splitAt 2 ["Off","Off","Off","Off"] 
 (["Off","Off"],["Off","Off"])

now you just need to pop the head element of the snd component of the list. This is easily done with pattern matching:

 > let (x,_:ys) = splitAt 2 ["Off","Off","Off","Off"]
 > x
 ["Off","Off"]
 > ys
 ["Off"]

you can now join the list back together, with an "On":

 > x ++ "On" : ys
 ["Off","Off","On","Off"]

I'll leave it to you to put those pieces together into a single function.

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As a style note, I'd suggest using a new custom data type, instead of String for your toggles:

 data Toggle = On | Off deriving Show