data Pos = NoPos | Pos Int Int | Eof 
           deriving (Eq,Ord,Show,Read ) 

lexical :: String -> [(Pos,Str ing )] 

lexical i = lex' 1 1 i 
  where lex' r c [] = [] 
        lex' r c xxs@('\n': xs) = lex' (r+1) 1 xs 
        lex' r c xxs@(x:xs) = 
             if isSpace x then lex' r (c+1) xs 
        else if isDigit x then token r c (span isDigit xxs) 
        else if isAlpha x then token r c (span isAlpha xxs) 
        else (Pos r c,[x]) : lex' r (c+1) xs 

token r c (t,xs) = (Pos r c,t) : lex' r (c+length t) xs 

This is a lexical analyser that transforms a list of characters into a
list of position-tokens pairs. Positions contain row and column numbers of
the token, unless it is the end of file which has its own constructor. The
NoPos and Eof positions are not used by the lexical analyser itself, the
parsers use them if they reach an unexpected end of the tok en list or do
not know where the error is.  The input string is transformed from a list
of characters to a a list of strings, where each string is a lexical tok
en. The tokens are paired with their positions to make it possible to
report positions of errors in the input.