Fix shrinker for ProtocolParameters.

[jonathanknowles]

Currently, the shrinker for ProtocolParameters always returns the empty list:

> :set -XTypeApplications
> import Cardano.Wallet.Primitive.Types (ProtocolParameters)
> import Cardano.Wallet.DB.Arbitrary
> import Test.QuickCheck
> generate (arbitrary @ProtocolParameters)
ProtocolParameters {decentralizationLevel = ...}
> a = it
> shrink a
[]

By using genericRoundRobinShrink from Test.QuickCheck.Extra, we can get actual shrinking:

> generate (arbitrary @ProtocolParameters)
ProtocolParameters {decentralizationLevel = ...}
> a = it
> length $ shrink a
68

[sevanspowell]

Ah interesting, the default shrink implementation for Arbitrary is simply to return an empty list.

When an empty list is used applicatively like this, it acts as an annihilator:

> [(+1)] <*> []
[]

Some elements of the ProtocolParameters use the default instance for shrinking, hence the empty list when shrinking ProtocolParameters.

We should probably audit our shrinkers for this.

This is a case where Hedgehog provides much more sensible defaults, in that it provides default shrinkers that actually shrink.

[Unisay]

Yeah this applicative annihilation is a real gotcha!
(and the best shrinker is that one that you don't have to write)

[jonathanknowles]

bors r+

[iohk-bors]

Build failed:

• ci/hydra-build:required

Final output of log:

  Sign transaction
    signTransaction adds reward account witness when necessary (AnyCardanoEra ByronEra)
      +++ OK, passed 100 tests (100% feature not supported in ByronEra.).
    signTransaction adds reward account witness when necessary (AnyCardanoEra ShelleyEra) (30749ms)
      +++ OK, passed 100 tests.
    signTransaction adds reward account witness when necessary (AnyCardanoEra AllegraEra) (31215ms)
      +++ OK, passed 100 tests.
    signTransaction adds reward account witness when necessary (AnyCardanoEra MaryEra) (30973ms)
      +++ OK, passed 100 tests.
    signTransaction adds reward account witness when necessary (AnyCardanoEra AlonzoEra) (30539ms)
      +++ OK, passed 100 tests.
    signTransaction adds extra key witness when necessary (AnyCardanoEra ByronEra) (1ms)
      +++ OK, passed 100 tests (100% feature not supported in ByronEra.).
    signTransaction adds extra key witness when necessary (AnyCardanoEra ShelleyEra)
      +++ OK, passed 100 tests (100% feature not supported in ShelleyEra.).
    signTransaction adds extra key witness when necessary (AnyCardanoEra AllegraEra)
      +++ OK, passed 100 tests (100% feature not supported in AllegraEra.).
    signTransaction adds extra key witness when necessary (AnyCardanoEra MaryEra)
      +++ OK, passed 100 tests (100% feature not supported in MaryEra.).
    signTransaction adds extra key witness when necessary (AnyCardanoEra AlonzoEra) (72958ms)
      +++ OK, passed 100 tests.
    signTransaction adds tx in witnesses when necessary (AnyCardanoEra ByronEra) (1ms)
      +++ OK, passed 100 tests.

Looks like another instance of:

https://input-output.atlassian.net/browse/ADP-1485

[jonathanknowles]

Going to merge manually, since:

• Every test passed on both buildkite and hydra, with the sole exception of the above known flaky test failure (See Fix shrinker for ProtocolParameters. #3205 (comment)).
• Rerunning the flaky test passed. (See https://hydra.iohk.io/build/13547582/nixlog/3/tail)
• We've observed the same flaky test on multiple other PRs, so it's extremely unlikely to have been caused by this PR. (See https://input-output.atlassian.net/browse/ADP-1485)