Question

I have problem to extract extra information from my parsing.

I have my own data structure to parse, and that works fine. I wrote the parser for my data structure as Parse MyDataStructure which parse all the information about MyDataStructure.

The problem is that in the string I’m parsing, mixed with MyDataStructure, there is also some information about what should I do with MyDataStructure which is of course not part of MyDataStructure, i.e. I cannot store this information inside MyDataStructure.

Now the problem is that I don’t know how to store this information, since in Haskell I cannot change some global variable to store information, and the return value of my parser is already MyDataStructure.

Is there a way I can somehow store this new information, without changing MyDataStructure, i.e. including field to store the extra information (but the extra information are not part of MyDataStructure so I would really like avoiding doing that)?

I hope I have been clear enough.

Answer 1

As @9000 says, you could use a tuple. If you find yourself needing to pass it through a number of functions, using the State Monad might make things easier.