On Fri, 2018-10-19 at 09:19 +0200, Ben Franksen wrote:
Hi everyone
it seems to be the season for new variations on the "property testing" theme, so I would like to chime in... not to announce a new library, sadly, but with a rough idea how the existing ones could perhaps be improved, based on practical experience in Darcs.
The problem I have is that there is a tension between
(a) stating a property in a clear and simple way, so its code doubles as a formal specification
and
(b) writing the property in such a way that when it fails, the reported value(s) give enough information about the context to be useful for finding the cause of the problem.
Let me give an example to demonstrate what I mean.
There is a simple law that says if a sequential pair of patches A;B commutes to B';A' then B';A' commutes back to A;B. In code this looks (more or less) like this:
prop_recommute :: Commute p => (p :> p) wA wB -> Bool prop_recommute (x:>y) | Just (y':>x') <- commute (x:>y) = case commute (y':>x')of Just (x'':>y'') -> x==x'' && y==y'' Nothing -> False | otherwise = True
This is a bit more verbose than the informal spec but still quite readable.
Now suppose this property fails. So quickcheck reports the counter example pair (X:>Y) for some X and Y. But that isn't too useful in itself. We'd like to know a bit more:
* did the second commute fail? * or did it succeed but x/=x'' or y/=y''? * and in the latter case, which of the two?
I think that this is possible by simply using QuickCheck's === and ==> (if you have Show and Eq instances for :>): prop_recommute :: Commute p => (p :> p) wA wB -> Bool prop_recommute (x:>y) = isJust commuted ==> commute commuted === Just (x:>y) where commuted = commute (x:>y) See https://hackage.haskell.org/package/QuickCheck-2.11.3/docs/Test-QuickCheck-P... for information on ==> and ===. This is more readable and quite similar to your example above. It would print both left and right side of === when a counter-example is found. Depending on your implementation of Show for :>, it could look like: Nothing /= Just (...A... :> ...B...) or Just (...A... :> ...B...) /= Just (...C... :> ...D...). I did not try this myself, so I could have made a mistake or I may have missed why this is not good enough for your case.
So in practice our property code looks more like this:
prop_recommute :: (ShowPatch p, Commute p) => (p :> p) wA wB -> Bool prop_recommute (x :> y) | Just (y' :> x') <- commute (x :> y) = case commute (y' :> x') of Nothing -> failed (redText "failed, where x" $$ displayPatch x $$ redText ":> y" $$ displayPatch y $$ redText "y'" $$ displayPatch y' $$ redText ":> x'" $$ displayPatch x') Just (x'' :> y'') -> if y'' /= y then failed (redText "y'' =/\\= y failed, where x" $$ displayPatch x $$ redText ":> y" $$ displayPatch y $$ redText "y'" $$ displayPatch y' $$ redText ":> x'" $$ displayPatch x' $$ redText "x''" $$ displayPatch x'' $$ redText ":> y''" $$ displayPatch y'') else if x'' /= x then failed (redText "x'' /= x, where x" $$ displayPatch x $$ redText ":> y" $$ displayPatch y $$ redText "y'" $$ displayPatch y' $$ redText ":> x'" $$ displayPatch x' $$ redText "x''" $$ displayPatch x'' $$ redText ":> y''" $$ displayPatch y'') else True | otherwise = True
Now this code tells us exactly what went wrong when the property fails but it is ugly and repetitive and the additional code obscures the simple logical content. So this is no longer quite as suitable for a (human readable) formal spec.
I wonder if displaying
(1) all relevant contextual variables and (2) "where in the code it fails"
could be automated away, somehow. I guess this is not trivial and may require syntactic analysis, so perhaps expecting a /library/ to solve the problem is unrealistic, except perhaps by using Template Haskell. I'd also go with a separate tool that extracts properties from a module and enriches them with code that displays the additional information.
Tackling this problem might be an interesting theme for a master thesis... ;-)
Cheers Ben
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