[Haskell-cafe] Very simple parser
ariep at xs4all.nl
Mon Jul 2 17:46:14 EDT 2007
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?
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?
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