-- Lexer15min.hs
--
-- Lexer Module for the 15 minute example.
--

module Lexer15min where

-- needed for isChar, isAlpha, isSpace, isDigit
import Data.Char


-- Token datatype.
-- Much like an enumerated type except the TokenInt and TokenId 
-- constructors also take a value.  Deriving Show so they can
-- be printed.
data Token
      = TokenCIRCLE
      | TokenTRIANGLE
      | TokenNUM Int
      | TokenSemi
      | UnknownChar Char
      deriving (Show,Eq)

-----------------------------------------------------------------
-- Function that performs lexical analysis.

-- The following line declares the type of the function.
-- The input parameter is a String type and the function returns
-- a list of Tokens.
lexer :: String -> [Token]

-- Below has examples of pattern matching.  The first pattern "[]"
-- is an empty list.  The later patterns match some character at the
-- beginning of a list.
-- See http://www.haskell.org/haskellwiki/How_to_work_on_lists.
lexer [] = []
lexer (';':cs) = TokenSemi : lexer cs

-- The below uses pattern matching to bind the variables c and cs,
-- then it uses guards to determine what recursion to perform.
-- See http://www.haskell.org/haskellwiki/Case#Guards
-- and http://stackoverflow.com/questions/4156727/what-is-the-difference-between-pattern-matching-and-guards.
-- Note that the guards are executed in order with only the first
-- one returning true being executed.
lexer (c:cs) 
      | isSpace c = lexer cs
      | isAlpha c = lexKW (c:cs)
      | isDigit c = lexNum (c:cs)
      | True = UnknownChar c : lexer cs

-- Function of processing integers.
-- Uses a where clause.
lexNum cs = TokenNUM (read num) : lexer rest
      where (num,rest) = span isDigit cs

-- Function for processing keywords.
-- The span function is built in to the Prelude.  Look it up.
-- This function implementation shows an example of the case expression,
-- which also uses pattern matching.
lexKW cs =
   case span isAlpha cs of
      ("Circle",rest)   -> TokenCIRCLE     : lexer rest
      ("Triangle",rest) -> TokenTRIANGLE   : lexer rest
      (id,rest)         -> error ("Lexer error: unknown keyword " ++ id)