genericTiers is broken for recursive types (missing addWeight somewhere?)

[jwaldmann]

This does not work (there is no output)

data P = Q | R P deriving (Show, Generic)
instance Listable P where tiers = genericTiers

take 3 ( tiers :: [[P]] )
^CInterrupted.

I guess that addWeight is missing somewhere. But where exactly?

This variant is OK (where we get the extra weight by going through Maybe on recursion)

data P = Q | R (Maybe P) deriving (Show, Generic)
instance Listable P where tiers = genericTiers

take 3 ( tiers :: [[P]] )
[[Q,R Nothing],[R (Just Q),R (Just (R Nothing))],[R (Just (R (Just Q))),R (Just (R (Just (R Nothing))))]]

[rudymatela]

@jwaldmann This is definitely a bug. Thank you for reporting. I'll have a new minor version of LeanCheck (v0.9.1) released on Hackage with the fix at the latest in a couple days.

You are right about the cause of the bug: an application of addWeight or delay is missing in at least one place in Test.LeanCheck.Generic:

instance Listable c => Listable' (K1 i c) where
  tiers' = mapT K1 tiers

should be

instance Listable c => Listable' (K1 i c) where
  tiers' = delay $ mapT K1 tiers

There may be a couple other places where I should add delay. I'll add some tests first :-) with a few different recursive types to make sure I'm not missing anything. My test files for the generic derivation are missing recursive types!

I'll probably want to try to make the genericTiers enumeration the same as the one produced by TH for consistency.

[jwaldmann]

Hi, thanks for the quick response.

I'll probably want to try to make the genericTiers enumeration the same as the one produced by TH for consistency.

Yes that would be ideal. And you could easily test it automatically.

[rudymatela]

@jwaldmann The bug is fixed:

$ cabal update
$ cabal install leancheck
Resolving dependencies...
Downloading  leancheck-0.9.1
Downloaded   leancheck-0.9.1
Starting     leancheck-0.9.1
Building     leancheck-0.9.1
Completed    leancheck-0.9.1
$ ghci -XDeriveGeneric
> import GHC.Generics
> import Test.LeanCheck
> import Test.LeanCheck.Generic
> data P = Q | R P deriving (Show, Generic)
> instance Listable P where tiers = genericTiers
> take 6 $ tiers :: [[P]]
[[Q],[R Q],[R (R Q)],[R (R (R Q))],[R (R (R (R Q)))],[R (R (R (R (R Q))))]]

I have released leancheck-0.9.1 with the fix, commits: a5ec90fa2dfddeae85bbf082cd13a212068a4e28 and a9e126a11db8c3d16a5fa9f740b398c195ed4c2a.

In addition to the fix, I have added a few automated tests using recursive types (5756b48f5b7e195fec19f6f272b2f225a985080d) and automated tests of consistency between deriveTiers and genericTiers (ce850e2e9d122c98dfbcfaaf658bad6f861aa321).

@jwaldmann Thanks again for the bug report.