[Haskell-beginners] Pound character use in code?

[M.v.Gulik]

Hi

Trying to learn a bit about Haskell as general (script) coder (batch,
autoit, python).
And run into some code I don't get. (could not find the right online
doc that explained it.)

[Code]
length'                  :: [a] -> Int
length' l                =  len l 0#
	where
		len :: [a] -> Int# -> Int
		len []     a# = I# a#
		len (_:xs) a# = len xs (a# +# 1#)

[/Code]
source: http://www.haskell.org/ghc/docs/latest/html/libraries/base/src/GHC-List.html#length

1) What is the purpose of the used pound/"#" character here. (Looks
like some type-casting, but that's just a wild guess from me here.
(I'm not a C~ or Java coder))

2) kinda the same for the "I#" in the "len [] ..." line. As the length
function takes only one return result. (note: its a capital i and not
a lowercase L.)

3) Why is it giving a compile error on the "where" line. Error:
"Haskell\baby.hs:<lineNr>:9: parse error on input `where'" (while the
adjusted code below compiles fine. Tab(4s) indented.)

[Code]
length'                  :: [a] -> Int
length' l                =  len l 0
	where
		len :: [a] -> Int -> Int
		len []     a = a
		len (_:xs) a = len xs (a + 1)
[/Code]

Using: WinGHCi 1.0.6 on Windows Xp. (Haskell Platform 2012.2.0.0
windows install)

TIA
MvGulik