[Haskell-cafe] Replace list item with list.

[Richard A. O'Keefe]

On 4/11/17 11:48 PM, mirone wrote:
> Hello everyone!
> If I have a list A : ["x", "y", "z"]
> and a list B: ["f", "g"].
> Here is my question:
> What's the simplest way to replace "z" in the list A by list B, and get
> ["x", "y", "f", "g"] ?

f _ _ = ["x","y","f","g"]

That's a joke, but it really isn't clear what the question means.
One possibility is

f a b = init a ++ b

f [1,2,3,4,5] [9,8,7] => [1,2,3,4,9,8,7]

If you mean that you want to replace "z" by ... wherever it occurs,
something like
-- (f needle straws haystack) replaces each occurrence of needle as
-- an element of haystack with the elements of straws.
f :: Eq t => t -> [t] -> ([t] -> [t])
f needle straws =
   concatMap (\straw -> if straw == needle then straws else [straw])

f "z" ["f","g"] ["x","z","y","z"] => ["x","f","g","y","f","g"]

Your question admits of too many generalisations for us to be really
helpful.