[Haskell-cafe] Ensuring all values of an ADT are explicitly handled OR finding all occurrences of type X in my app

[Saurabh Nanda]

>
> Actually in team, one who writes `_` match, is very useful as that
> prevents breaking code when adding new value...I can't really see any
> problem here. There is real world use case when member of team don't need
> to cover all cases therefore `_`.
>
I **want** code to break when I add a new value to an ADT. I want the
programmer to go through all the sites where a pattern match was done on a
particular ADT and explicitly consider the impact of the new value on these
sites.

Actual use-case -- we start with the following ADT definition and
call-sites:

data BookingStatus = Confirmed | Cancelled | Abandoned
> computeRemainingSeats :: BookingStatus -> (...)
> computeBilling :: BookingStatus -> (...)


Now, assume that within `computeAvailability` only `Confirmed` is
explicitly matched to result in a reduction of available seats, everything
else is matched by `_` and results in a no-op. What happens when
`BookingStatus` evolves to have a new value of  `ManualReview`? We want to
reduce the number of seats till the manual-review of the booking is
complete. However, the compiler will not force us to look at this
particular call-site to evaluate this impact.

Another one: assume that within `computeBilling` only `Confirmed is
explicitly matched to trigger an invoicing action, others are matched via
`_` to a no-op. What happens when `BookingStatus` evolves to have a new
value of `Refunded`, which should trigger a credit-note action? Again, the
compiler is not going to help us here.

One may reason that this is conflation of concerns; that this should be
split into **three** ADTs -- one for BookingStatus, one for
AvailabilityStatus and one for BillingStatus. And pedantically this might
be right. But all of these learnings evolve over time and one may not make
the right decisions when writing v1 of the system. Under such
circumstances, can Haskell still help in helping the programmer ensure
correctness?

-- Saurabh.
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