Allow more expressive functions to be given as input in operator tables for buildExpressionParser

[ahmadsalim]

Currently, the Operator constructors used for building expression parsers are required to be pure functions wrapped in the ParsecT monad, i.e.:

Infix (ParsecT s u m (a -> a -> a)) Assoc    
Prefix (ParsecT s u m (a -> a))  
Postfix (ParsecT s u m (a -> a))

This however limits what can be done with the operators, for example it is not possible to perform additional checks of the parsed argument under the ParsecT monad and then fail using fail or unexpected, or build up new information in the complete term using the parsed sub-terms.

I would therefore like to ask for a feature request that allows taking more expressive functions where only the resulting term needs to be in the ParsecT monad. In order to not break backwards compatibility, one could imagine having an M postfix to each new constructor, so the following constructors are added:

InfixM (a -> a -> ParsecT s u m a) Assoc     
PrefixM (a -> ParsecT s u m a)   
PostfixM (a -> ParsecT s u m a)

[ahmadsalim]

I would happily submit a PR if the feature request if there are not any major objections.

[mrkkrp]

@ahmadsalim, this is interesting. We're considering replacing of old constructors with something like this in Megaparsec. You can discuss it here: mrkkrp/megaparsec#22.