Re: Implicit actions & program correctness

[deadhacker <gstover@gmail.com>]

On Friday, May 15, 2020 at 3:10:02 PM UTC-7, Paul Rubin wrote:
> You might like this article (not Ada specific):
> 
> http://blog.tmorris.net/posts/understanding-practical-api-design-static-typing-and-functional-programming/

Paul, thank you so much for the link.  It's good reading.

To me, seems that his third requirement is the tough one.  (Quoting it here: "If I call move on a tic-tac-toe board, but the game has finished, I should get a compile-time type-error. In other words, calling move on inappropriate game states (i.e. move doesn’t make sense) is disallowed by the types.")

It seems that if I have a Move subprogram that operates on a Game, it & produces another Game, I don't see how it could sometimes return a FinishedGame that can be detected at compile-time.  (It could return a Game that could be checked for Finished at run-time.)

Sadly, his Java API is no longer available, so I can't see how he solved it.

Do you (or anyone else here) know how it could be solved with Ada?