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? 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 _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post. |
Ben,
the solution might be simple: Don't express the property as an atomic function (a -> Bool). This reminds me a bit of the Cutty-Howard correspondence, only that proofs are replaced by counterexamples. A testing library should have an operator for each of the logical connectives, where - a counterexample to True is never found - a counterexample to False is () - a counterexample to (not x) is an example for x - a counterexample to (x && y) is either a counterexample to x or a counterexample to y, whichever is found first - a counterexample to (x || y) is a pair of counterexamples to both x and y - a counterexample to (x ==> y) is an example for x together with a counterexample for y Then, if you build your property out of smaller properties and the connectives above, the counterexample-finder would be able to inform you which part of the property failed. I'm pretty sure at least one of the testing libraries has a system like this. Cheers, Olaf _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post. |
In reply to this post by Ben Franksen
Perhaps the work in my "assert-explainer" project is relevant here -
https://github.com/ocharles/assert-explainer. The idea is your tests should just be writing "assert someArbitraryExpressionEvaluatingToBool", and then having some compiler magic recover the context for you when it goes wrong. To cite my own README: You write: assert (length xs == 4) No need for lots of special assertEqual etc functions. When the assertion fails, you will get much more context: ✘ Assertion failed! length xs == 4 /= True (at Test.hs:18:12-25) I found the following sub-expressions: - length xs = 3 - xs = [1,2,3] This is done via a GHC source plugin that rewrites "assert (length xs == 4)" into an expression that - if False - prints much more information. It's very early days for this plugin, but the goal is to reach parity with https://docs.pytest.org/en/latest/example/reportingdemo.html#tbreportdemo. Hope this helps, Ollie On Fri, Oct 19, 2018 at 8:20 AM Ben Franksen <[hidden email]> 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? > > 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 > > _______________________________________________ > Haskell-Cafe mailing list > To (un)subscribe, modify options or view archives go to: > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > Only members subscribed via the mailman list are allowed to post. Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post. |
Hi Ollie
This is fantastic! It's certainly much more than I hoped for. I haven't looked into the details of your plugin yet, but from your description it looks as if this is currently geared towards "classical" unit tests. Perhaps this is not a problem: I think quickcheck & co report exceptions as a failed property, and if they don't we'll just have to catch the exception, print it and return False. I will definitely try it out and report back. Cheers Ben Am 19.10.2018 um 23:25 schrieb Oliver Charles: > Perhaps the work in my "assert-explainer" project is relevant here - > https://github.com/ocharles/assert-explainer. > > The idea is your tests should just be writing "assert > someArbitraryExpressionEvaluatingToBool", and then having some > compiler magic recover the context for you when it goes wrong. To cite > my own README: > > > You write: > > assert (length xs == 4) > > No need for lots of special assertEqual etc functions. > > When the assertion fails, you will get much more context: > > ✘ Assertion failed! > length xs == 4 /= True (at Test.hs:18:12-25) > > I found the following sub-expressions: > - length xs = 3 > - xs = [1,2,3] > > > This is done via a GHC source plugin that rewrites "assert (length xs > == 4)" into an expression that - if False - prints much more > information. > > It's very early days for this plugin, but the goal is to reach parity > with https://docs.pytest.org/en/latest/example/reportingdemo.html#tbreportdemo. > > Hope this helps, > Ollie > On Fri, Oct 19, 2018 at 8:20 AM Ben Franksen <[hidden email]> 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? >> >> 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 >> >> _______________________________________________ >> Haskell-Cafe mailing list >> To (un)subscribe, modify options or view archives go to: >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> Only members subscribed via the mailman list are allowed to post. > _______________________________________________ > Haskell-Cafe mailing list > To (un)subscribe, modify options or view archives go to: > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > Only members subscribed via the mailman list are allowed to post. > _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post. |
In reply to this post by Ben Franksen
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-Property.html 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 > > _______________________________________________ > Haskell-Cafe mailing list > To (un)subscribe, modify options or view archives go to: > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > Only members subscribed via the mailman list are allowed to post. _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post. |
In reply to this post by Ben Franksen
On Mon, Oct 22, 2018 at 10:23 AM Ben Franksen <[hidden email]> wrote:
> > Hi Ollie > > This is fantastic! It's certainly much more than I hoped for. > > I haven't looked into the details of your plugin yet, but from your > description it looks as if this is currently geared towards "classical" > unit tests. Perhaps this is not a problem: I think quickcheck & co > report exceptions as a failed property, and if they don't we'll just > have to catch the exception, print it and return False. Yea, everything is very proof-of-concept at the moment. Right now, we have assert :: Bool -> IO (), but that could be changed to have a richer return type (and likely will as this project progresses). > I will definitely try it out and report back. Super, please feel free to open issues/questions on GitHub! _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post. |
In reply to this post by Arjen
Am 22.10.2018 um 11:59 schrieb [hidden email]:
> 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-Property.html > 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. Thanks for the hint wrt (===) which I wasn't aware of. In the example I posted this would work, but not in more complex ones. There are properties that get more patches as input and require many combinations of e.g. commute to succeed. Any one of them can fail and the rest can only be performed if the previous ones have succeeded. Cheers Ben _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post. |
In reply to this post by Olaf Klinke
Am 19.10.2018 um 22:53 schrieb Olaf Klinke:
> the solution might be simple: Don't express the property as an atomic > function (a -> Bool). This reminds me a bit of the Cutty-Howard > correspondence, only that proofs are replaced by counterexamples. A > testing library should have an operator for each of the logical > connectives, where > > - a counterexample to True is never found > - a counterexample to False is () > - a counterexample to (not x) is an example for x > - a counterexample to (x && y) is either a counterexample to x or a counterexample to y, whichever is found first > - a counterexample to (x || y) is a pair of counterexamples to both x and y > - a counterexample to (x ==> y) is an example for x together with a counterexample for y > > Then, if you build your property out of smaller properties and the connectives above, the counterexample-finder would be able to inform you which part of the property failed. I'm pretty sure at least one of the testing libraries has a system like this. > Hi Olaf while this approach works in principle it has practical disadvantages. One charming feature of property testing in Haskell is that expressing properties does normally /not/ require to import the testing library. This allows me to write my properties in a generic style that works with e.g. quickcheck, leancheck, smallcheck etc. With your approach I would be tied to a single property testing library. I guess this would be less of a problem if in Haskell the boolean operators were overloaded i.e. part of a class. I think this was an oversight when the Prelude and the standard libraries were designed. IME boolean algebras (or perhaps more generally lattices) appear /everywhere/. I can't count the number of times I wanted to use these operators on structures containing Bools or functions returning Bool and couldn't. IMHO such a generalization would be a lot more useful and less controversial than the one from List to Foldable. (I can't remember a single instance where I missed this generalization of lists. In fact I think is completely useless. Foldable essentially allows me to say: "give me all your elements, one by one, in /your/ preferred order, so I can combine them to something new". Due to the laziness of lists this is really the same as having a toList function (modulo list fusion to take care of the constant factor overhead). Only that a container type may offer many different versions of toList but only a single Foldable instance. I understand that foldr and foldl have been added to the class to allow at least two different "preferred" traversal orders, but adding methods to a class in this way obviously doesn't scale and is poor design, again IMHO.) Cheers Ben _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post. |
In reply to this post by Arjen
Agreed, I think QuickCheck is already up to the task. The simpler prop_recommute could be rewritten with (===), (.&&.), and maybe "counterexample" to label which branch failed.
Tom > El 22 oct 2018, a las 05:59, [hidden email] escribió: > >> 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-Property.html > 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 >> >> _______________________________________________ >> Haskell-Cafe mailing list >> To (un)subscribe, modify options or view archives go to: >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> Only members subscribed via the mailman list are allowed to post. > > _______________________________________________ > Haskell-Cafe mailing list > To (un)subscribe, modify options or view archives go to: > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > Only members subscribed via the mailman list are allowed to post. 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I agree with Ben's point though that you have to "buy in" to the testing framework and write tests in a DSL, though. This seems unfortunate. On Wed, 24 Oct 2018, 2:44 pm , <[hidden email]> wrote: Agreed, I think QuickCheck is already up to the task. The simpler prop_recommute could be rewritten with (===), (.&&.), and maybe "counterexample" to label which branch failed. _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post. |
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