Closed stevana closed 6 years ago
For N-ary relations/relational algebra it could be helpful to do something like:
Or use ideas from Datafun: https://github.com/rntz/datafun ?
Another idea is to have pre- and post-conditions use a new datatype instead of Bool
, which gives better indication of what went wrong. For example:
precondition :: Model -> Action -> Logic
precondition _ (Apa x) = Boolean "x < 5" (x < 5) .&. Boolean "x > 10" (x > 10)
data Logic = Boolean String Bool | Logic .&. Logic
test :: Logic -> Either String ()
test (Boolean str b) | b = Right ()
| otherwise = Left str
test (p .&. q) = do
Right () <- test p
test q
We can then instead of just printing "precondition failed for Apa" show why it failed.
We can also be more fancy and have test
accumulate error strings. Or make Boolean
take a data Predicate = forall a. (Ord a, Show a) => LessThan a a | ...
instead of a String
and a Bool
, the advantage of this approach apart from not having to provide the string is that we can also negate the predicate, which is helpful when adding disjunction to Logic
.
Ideas from the above comment were added in PR #192.
The above ideas are as of #209 part of the counterexample output.
PR #171 introduces a small language, inspired by Z/Event-B and TLA+, for specifying concise and showable models.
The, perhaps naive, idea that I have is that it will allows us to more easily port examples from Event-B and TLA+ to our setting. I say naive because the settings are obviously very different. They use classical meta-theory where as we have to be more constructive (unless we want to implement a theorem prover), and it shows in that many operations in the PR are partial.
I'd be curious to hear what other people think about this topic!