Exception: Prelude.head: empty list

Question

Here's my attempt at an expand function:

expand :: FileContents -> FileContents -> FileContents  
expand textfile infofile  
  = concat (combine (fst (split separators textfile)) (expand' (snd (split separators textfile))   infofile "")   )  
    where  
      expand' [] _ stack = []  
      expand' (t:ts) info stack  
        |(head t) == '$' = (head (change t info )):expand' ts info stack  
        |otherwise = t: expand' ts info stack  
          where  
            change x y  
              = lookUp x (getKeywordDefs (snd (split ['\n'] y)))  

When I do:

expand "Keywords (e.g. $x, $y, $z...) may appear anwhere, e.g. <$here>." "$x $a\n$y $b\n$z $c\n$here $this-is-one"

I get

 Exception: Prelude.head: empty list

but I wanted

"Keywords (e.g. $a, $b, $c...) may appear anwhere, e.g. <$this-is-one>."

Because I should have

("Keywords (e.g. $x, $y, $z...) may appear anwhere, e.g. <$here>.",
   "$x $a\n$y $b\n$z $c\n$here $this-is-one")
    ==> "Keywords (e.g. $a, $b, $c...) may appear anwhere, e.g. <$this-is-one>."

ADDITION i defined split myself ,that is:

split :: [Char] -> String -> (String, [String])  
split separators ( x : wordsrest)  
      | elem x separators  = (x : listseparators,"" : word : listwords)  
      | otherwise          = (listseparators, ((x : word) : listwords))  
        where  
          (listseparators,    word : listwords) = split separators wordsrest  
split _ _ = ("" , [""])  

for example if I input

split " .," "A comma, then some words."

then i'll get

(" , .",["A","comma","","then","some","words",""])

Answer 1

As you can see in the doc, split does not keep the separators in the chunks it outputs so they may very well be empty.

As a consequence, having (head t) == '$' in a guard is unsafe and can lead to exceptions being raised.