Fail error messages do not show choices of attempted elements as an option

[bookofproofs]

I have a grammar that has choices of elements, some of which have to be proceeded by the attempt parser (otherwise, I would get different grammar).

My problem is that a failing parser does not reflect the actual possibilities of the grammar.

To illustrate this behavior, check out this example:

let a = pchar 'a'
let bra = pchar '<'
let ket = pchar '>'
let comma = skipChar ','
let aBraKet = a .>> bra .>> ket 
let aChoice = attempt aBraKet <|> a
// note that `let aChoice = aBraKet <|> a` would define a different grammar, in which we would never be able to parse a single `a`
// neither would `let aChoice = a <|> aBraKet` do the trick, because we would never be able to parse a single `a<>`.
let list = sepBy1 aChoice comma

let input = "a<>,a<>,a,axxx"
let result = run (list .>> eof) input
printf "%O" result

This will output

Failure:
Error in Ln: 1 Col: 12
a<>,a<>,a,axxx
           ^
Expecting: end of input or ','

The actual grammar, however, expects '<', ',' or end of input.

This behavior may be a feature and not a bug of FParsec, but would it be possible to enhance the error message to display every choice that is available in the grammar at this parsing state?