Debugging some Haskell functions, guards

Question

Once again I'm hitting roadblocks with this code... I posted my earlier code which was much different earlier, so now that its changing, so are the problems! so I have a function

 convertToHTML :: String -> String
 convertToHTML [] = [] --prevents calling head on empty line
 convertToHTML x
          | doubleHash x     == True     = "<h3>" ++ drop 2 x ++ "</h3>"
          | head x           == '#'      = "<h1>" ++ tail x ++ "</h1>"
          | x                == "---"    = "<hr/>"
          | otherwise                    = x

Now, basically what is happening is, my helper function doubleHash x which is supposed to read a line, and if that line begins with ## slap a h3 tag on the whole line and remove the ##. So the first guard, I believe does just that. So, I'm thinking there is a problem with the doubleHash function. So here is the doubleHash helper function

 doubleHash ('#' : '#' : []) = True
 doubleHash _ = False

so using cons, just saying ## would return true. Not sure what is wrong here, but when convertToHTML gets called to run doubleHash x, it doesnt apply the notion of doubleHash == True, so slap a H3 tag on this line! instead it goes right to head x and applies the H1 tag to both lines... ex: text

--> #This should be a H1 tagged line

--> ##This should be a H3 tagged line

however both are getting slapped with H1 tags.

Answer 1

'#' : '#' : [] is equivalent to "##" . In other words, you're checking if the entire string is equal to "##" . You probably want to check if the string starts with "##" . In Data.List there's the convenient function isPrefixOf which tests for just that.

> isPrefixOf "##" "## a string"
True
> isPrefixOf "##" "# another string"
False

Answer 2

Your doubleHash will return True only if the whole string is "##".
This one will do whatever you need:

 doubleHash ('#' : '#' : _) = True
 doubleHash _ = False