mrkkrp/megaparsec

MonadAccum instance for ParsecT

Opened this issue · 5 comments

olafklinke commented

There exists an instance

(Stream s, MonadState st m) => MonadState st (ParsecT e s m)

and State is more expressive than Writer, in the sense that any State monad can implement the MonadWriter interface for monoidal state. Hence I wonder why there are no instances

(Stream s, MonadAccum w m) => MonadAccum w (ParsecT e s m)
(Stream s, MonadWriter w m) => MonadWriter w (ParsecT e s m)

I claim that accumulating some value while parsing is a frequent use case and therefore should be supported by any parsing library. After all, ParsecT does accumulate its own errors and hints. Certainly tell a.k.a. add is easy enough to implement:

addP :: MonadAccum w m => w -> ParsecT e s m ()
addP w = ParsecT (\s _ _ whenOK _ -> add w >> (whenOK () s mempty))

and look can be implemented via lift:

lookP :: MonadAccum w m => ParsecT e s m w
lookP = lift look
olafklinke commented

MonadAccum exists only in mtl >= 2.3 and the current lower bound is 2.2.2 so the bound in the dependency must be increased to be able to implement the instance.

mrkkrp commented

It is not there because at the time no such thing as MonadAccum existed in mtl. You are very welcome to open a PR adding this instance, which I will gladly merge.

Lev135 commented

It's clear about MonadAccum, but why there is no MonadWriter instance (it seems to be old enough)? Are there any problems with it?

olafklinke commented
olafklinke commented

The PR is able to merge.