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MonadAccum instance for ParsecT #522
Description
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
Activity
olafklinke commentedon Mar 30, 2023
MonadAccumexists only inmtl >= 2.3and 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 commentedon Mar 31, 2023
It is not there because at the time no such thing as
MonadAccumexisted inmtl. You are very welcome to open a PR adding this instance, which I will gladly merge.Lev135 commentedon Jun 20, 2023
It's clear about
MonadAccum, but why there is noMonadWriterinstance (it seems to be old enough)? Are there any problems with it?olafklinke commentedon Jun 21, 2023
olafklinke commentedon Jun 21, 2023
The PR is able to merge.