Gregory Propf wrote:
As a programming exercise I'm trying to use the State monad to create a simple parser. It's for a very simple assembly language for a simple virtual machine. The state is a string of instructions. I want to be able to call something like getNextInstruction to pull out the next instruction and then update the state (string). I know I can do this non-monadically by just passing the string explicitly each time but I'd like to learn more about the State monad. I also know about Parsec and Happy and so forth but this is just an exercise so I want to do it this way. Any ideas? I can't seem to get anything to work. I've tried different things but I suspect I'm just missing something basic. Can someone post a simple prototype for this? Just assume the instructions are integers.
Did you look at the documentation for the State monad? http://haskell.org/ghc/docs/latest/html/libraries/mtl/Control-Monad-State-La... It also contains some examples. Getting the next instruction sounds like a job for 'get'; to remove it from the state you might use 'put' or 'modify'. If this doesn't help, can you show something you tried? Greetings, Arie