On 6/11/17 12:20 PM, Richard A. O'Keefe wrote:
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"] ? Your question admits of too many generalisations for us to be really helpful.
Here are some questions that might help. Let's suppose for the sake of argument that you want a function f needle straws {-B-} haystack {-A-} such that f "z" ["f","g"] ["x","y","z"] => ["x","y","f","g"] What should f n s h do when n does not occur in h? What should f n s h do when n is the first element of h? What should f n s h do when n occurs more than once in h? What should f n s h do when n occurs in s? What should f n s h do when h has 0, 1, 2, 4 or more elements? May f assume that h is in sorted order? May f use compare at all? May f use ==? What if we want to match more than one element; should we be looking for an *element* equal to "z" or a *prefix* equal to ["z"] in this case but possibly longer in others? With different answers to these questions, you might get f' :: Eq t => [t] -> [t] -> [t] -> [t] f' needles straws haystack | needles `isPrefixOf` haystack = straws ++ (length needles `drop` haystack) f' needles straws [] = [] f' needles straws (item:items) = item : f' needles straws items f' ["z"] ["f","g"] ["x","y","z","z"] => ["x","y","f","g","z"]