[Haskell-cafe] Simple Interpreter with State Monad

Jake jake.waksbaum at gmail.com
Thu Apr 7 14:29:13 UTC 2016

I'm currently in a graphics class where, in order to provide a standard
interface to all of our graphics libraries, we have to process small
scripts that look like this:

0 0 0 1 1 1
0 0 10
0 0 3

I successfully wrote a parser with attoparsec that parses the file into a
list of Commands. Now I'm trying to process that list to produce an IO
action, and I thought the State monad would be useful to keep track of the
persistent state of the shapes to draw and the transformations on them.

I'm confused about how exactly to do this though. What should the type of
my State be? Right now I have an execute function that is
execute :: Command -> State ParseState (IO ())
where ParseState is a tuple of stuff. ParseState also includes an IO ()
because I wanted to be able to create multiple pictures one after another,
and I couldn't figure out how to access the previous result value from
State to add on to it in the next one.

So can anyone offer advice on my specific situation, or maybe a simpler
example of how to go about writing an interpreter with the State monad?

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