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 <theedge456@free.fr> 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
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