Why does runParsecT requires the initial user state twice?

Question

I've defined the following custom parser:

newtype St = St Int
type TxsParser = ParsecT String St (State St)

Now to be able to run this parser, I have to use the runParserT function.

runParserT :: Stream s m t 
           => ParsecT s u m a 
           -> u 
           -> SourceName
           -> s 
           -> m (Either ParseError a)

Which instantiated to my custom parser reads:

runParserT :: ParsecT String St (State St) a 
           -> St 
           -> SourceName 
           -> String 
           -> State St (Either ParseError a)

But this means that if I want to evaluate the result of runParserT (which is a state monad) I have to supply another initial state (of type St in this case). For instance:

 evalState (runParserT myParser (St 0) fp input) (St 0)

While this works, it seems wrong that I have to repeat the state twice. Does this mean that mixing ParsecT and the State monads is not a good idea?

It's not a bad idea, it just may not be *necessary*. The two states are logically separate; one initializes the parser itself, the other initializes the `State` *produced* by the parser. — chepner, Mar 15 '18 at 15:06

score 6 · Answer 1 · answered Mar 15 '18 at 15:12

6

Correct: mixing ParsecT and StateT is not a good idea. The ParsecT monad carefully restores its state when it backtracks, an action that an enclosing (or enclosed) StateT transformer doesn't and can't know about or handle correctly.

answered Mar 15 '18 at 15:12

Daniel Wagner

145,880
9
220
380

Unless maybe you want to collect statistics on backtracking or something. – dfeuer Mar 15 '18 at 15:44
1

@dfeuer Neat idea! Seems pretty niche for a production parser, but I could totally imagine using that during development. – Daniel Wagner Mar 15 '18 at 18:15

score 6 · Accepted Answer · answered Mar 15 '18 at 15:23

ParsecT provides state. State provides state. Using both together gives you two independent states.

You have to initialize both, which is why you see it twice.

There's nothing wrong with using them together if you want two separate sources of state (e.g. one that backtracks and one that doesn't), but if you only wanted one, this is clearly not what you'd do.

Why does runParsecT requires the initial user state twice?

2 Answers2