Here's why you're getting the longer repetition first: buildLenList "bang" = buildLenList (_:"ang") = [1 + length "ang"] ++ buildLenList "ang" = [4] ++ buildLenList "ang" (...) = [4, 3, 2, 1] And the reason calling myReverse on that doesn't give you [1, 2, 3, 4] is that you're reversing a different list every time: blowup "bang" = blowup ('b':"ang") = myRepeat 'b' (head (myReverse (buildLenList ('b':"ang"))) ++ blowup "ang" = myRepeat 'b' (head (myReverse [4, 3, 2, 1]) ++ blowup "ang" = myRepeat 'b' (head [1, 2, 3, 4] ++ blowup "ang" = myRepeat 'b' 1 ++ blowup "ang" = "b" ++ blowup "ang" = "b" ++ blowup ('a':"ng") = "b" ++ myRepeat 'a' (head (myReverse (buildLenList ('a':"ng"))) ++ blowup "ng" = "b" ++ myRepeat 'a' (head (myReverse [3, 2, 1]) ++ blowup "ng" = "b" ++ myRepeat 'a' (head [1, 2, 3] ++ blowup "ng" (...) Since you only asked for help with how it evaluates, I'll withhold further spoilers. :) However, I strongly recommend getting more comfortable using the (:) operator for construction instead of just pattern matching; in a couple of your functions it would be a better tool than (++). Also, "at-patterns" are great: blowup l@(x:xs) = myRepeat x (head (buildLenList l)) ++ blowup xs On Sat, Jan 9, 2016 at 7:21 AM, Fabien R wrote:

Hello,

I want to define a function blowup that takes "bang" as input and returns "baannngggg". I come up with these functions;

myReverse :: [a] -> [a] myReverse [] = [] myReverse (x:xs) = myReverse xs ++ [x]

buildLenList :: String -> [Int] buildLenList "" = [] buildLenList (_:xs) = [1 + length xs ] ++ buildLenList xs

myRepeat :: Char -> Int -> String myRepeat x 0 = [] myRepeat x n = [x] ++ myRepeat x (n - 1)

blowup :: String -> String blowup [] = [] blowup (x:xs) = myRepeat x (head ( (buildLenList (x:xs)))) ++ blowup xs

With this code, blowup "bang" returns "bbbbaaanng".

So I thought to insert myReverse between head and buildLenList but in that case, the result is only "bang".

It seems that the evaluation of buildLenList is not working as I thought. I tried to debug that using ghci debugger but failed (still learning). Can someone explain how the evaluation is done here ?

-- Fabien _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners