[Haskell-beginners] question on evaluation

[Fabien R]

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