type check in printf function

[feiwang3311]

should check the format string and the types of the given Defs

[feiwang3311]

-- Parse the format string (list of characters) into an AST.
-- Example: "%d,%s" → (FInt (FOther ',' (FString FEnd)))
format : List Char -> Format
format ('%' :: 'd' :: cs ) = FInt ( format cs )
format ('%' :: 's' :: cs ) = FString ( format cs )
format ( c :: cs ) = FOther c ( format cs )
format [] = FEnd

https://gist.github.com/chrisdone/672efcd784528b7d0b7e17ad9c115292