mzero >>= f  =  mzero
v >> mzero   =  mzero

Since ParsecT also has a MonadPlus instance, can we have different implementations of `empty` and `mzero` to have these 2 separate semantics expressible:

*) parse to ignored result from some input

*) un-parsable (input consumed or not is irrelevant as to err out anyway)

	parse to no significant result	unparsable
consumed input	empty -> throw ??	mzero -> throw
no input consumed	empty -> nothrow	mzero -> throw

I'd much like the behavior above `empty -> throw ??` changed to `empty -> nothrow`

On 2020-10-27, at 17:47, Jaro Reinders <jaro.reinders@gmail.com> wrote:

The 'empty' value should always be the unit of <|>, that is specified in the documentation of the Alternative class. The problem starts when you build composite parsers. E.g. (char 'a' *> empty) does not need to be a unit of <|>. I thought of 'fixing' this by adding another law 'u <*> empty = empty', but that disregards all effects that u can have.

On 10/27/20 10:26 AM, YueCompl via Haskell-Cafe wrote:
In [1], Alternative is said being most commonly considered to form a monoid, so that:
```hs
-- empty is a neutral element
empty <|> u = u
u <|> empty = u
```
In my particular case wrt Megaparsec, when the artifact parser evaluates to `empty` at eof, I suppose the outer `many` should evaluate to whatsoever previously parsed, but current implementation of Megaparsec makes it conditional:
*) in case the parser hasn't consumed any input, it works the way as expected
*) incase the parser has consumed some input (whitespaces), the outer `many` throws error
So can I say this is a violation regarding [1]?
Best regards,
Compl
On 2020-10-27, at 04:18, Olaf Klinke <olf@aatal-apotheke.de> wrote:

I used to think that an Alternative is just an Applicative which is
also a Monoid but apparently there is no consensus about this [1,2].
Actually it kind of makes sense to make the 'empty' parser fail:
Consider the parser combinator

choice = Data.Foldable.asum = foldr (<|>) empty

which folds over a list of Alternatives. Its semantics can be regarded
analogous to 'any' for a list of Booleans, and in the latter the empty
list evaluates to False.
Put differently: The parser (p <|> q) matches at least as many inputs
than either p or q. Hence the neutral element for <|> ought to be the
parser that matches the least amount of inputs, but a parser that
succeeds on the empty string _does_ match some input. It would be the
neutral element for the monoid operation that concatenates parsers.

Olaf

[1] https://en.wikibooks.org/wiki/Haskell/Alternative_and_MonadPlus
[2] https://wiki.haskell.org/MonadPlus

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