How do I keep both sides of a match?

[atrauzzi]

How do I keep both sides of a match?

[RobRamSystems]

I think query syntax is the clearest (but watch out for performance!):

from c in Char('U')
from number in Digit.Repeat(9)
select $"{c}{number}"

In general, use Map if you need to combine the results of multiple sequential parsers in some way. The syntax above is basically a prettier way of using Bind which also allows you to do things like have a parser depend on the result of a previous parser, e.g.

from c in Char('U').Or(Char('S'))
from sign in c switch {
    'U' => Return(1),
    'S' => Char('-').Optional().Select(x => x.HasValue ? -1 : 1)
from number in Digit.Repeat(9)
select sign * int.Parse(number)

where it would only allow an optional minus sign if it started with 'S' instead of 'U'.

[benjamin-hodgson]

Thanks for fielding this @RobRamSystems!