-- scanner for a grammar with the following set of terminal symbols
--
--     T = { "use" , "dcl" , ';' , '(' , ')' , var }
-- 
--  where var is a peudo-terminal symbol defined by the regular expressionn
--    
--    var = [a-zA-Z]+



data Token
       = TokenUSE
       | TokenDCL
       | TokenSemiColon 
       | TokenOB
       | TokenCB
       | TokenVar String
       deriving Show

lexer :: String -> [Token]
lexer [] = []
lexer (c:cs) 
      | isSpace c = lexer cs
      | isAlpha c = lexSTR (c:cs)

lexer (';':cs) = TokenSemiColon : lexer cs
lexer ('(':cs) = TokenOB : lexer cs
lexer (')':cs) = TokenCB : lexer cs


lexSTR cs =
   case span isAlpha cs of
      ("use",rest)    -> TokenUSE : lexer rest
      ("dcl",rest)    -> TokenDCL : lexer rest
      (var,rest)      -> TokenVar var : lexer rest



-- runParser = parser . lexer