added a new list exercise to 3.iv (#87)

Author: Liable5607

src/Solutions/DataTypes.idr

@@ -190,6 +190,10 @@ filterList f Nil       = Nil
 filterList f (x :: xs) =
   if f x then x :: filterList f xs else filterList f xs
 
+(++) : List a -> List a -> List a
+(++) Nil ys = ys
+(++) (x :: xs) ys = x :: (Solutions.DataTypes.(++) xs ys)
+
 headMaybe : List a -> Maybe a
 headMaybe Nil      = Nothing
 headMaybe (x :: _) = Just x

src/Tutorial/DataTypes.md

@@ -1212,6 +1212,14 @@ signature are treated as type parameters.
    total
    filterList : (a -> Bool) -> List a -> List a
 
+   -- re-implement list concatenation (++) such that e.g. (++) [1, 2] [3, 4] = [1, 2, 3, 4]
+   -- note that because this function conflicts with the standard
+   -- Prelude.List.(++), if you use it then you will need to prefix it with
+   -- the name of your module, like DataTypes.(++) or Ch3.(++). alternatively
+   -- you could simply call the function something unique like myListConcat or concat'
+   total
+   (++) : List a -> List a -> List a
+
    -- return the first value of a list, if it is non-empty
    total
    headMaybe : List a -> Maybe a