Build failure with hashable-1.4.0.0

[RyanGlScott]

Trying to build parameterized-utils with hashable-1.4.0.0 fails thusly:

$ cabal build --allow-newer=hashable
Build profile: -w ghc-8.10.7 -O1
In order, the following will be built (use -v for more details):
 - parameterized-utils-2.1.4.0 (lib) (first run)
Preprocessing library for parameterized-utils-2.1.4.0..
Building library for parameterized-utils-2.1.4.0..
[ 3 of 35] Compiling Data.Parameterized.Classes

src/Data/Parameterized/Classes.hs:354:10: error:
    • Could not deduce (TestEquality f)
        arising from the superclasses of an instance declaration
      from the context: HashableF f
        bound by the instance declaration
        at src/Data/Parameterized/Classes.hs:354:10-46
      Possible fix:
        add (TestEquality f) to the context of the instance declaration
    • In the instance declaration for ‘Hashable (TypeAp f tp)’
    |
354 | instance HashableF f => Hashable (TypeAp f tp) where
    |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

This is because hashable-1.4.0.0 makes Eq a superclass of Hashable. Because TypeAp's Eq instance has a TestEquality constraint, the Hashable instance for TypeAp now also requires TestEquality. This isn't a problem currently, as parameterized-utils constraints hashable < 1.4 in its .cabal file, but it will be an issue eventually.

Two possible fixes:

1. Add TestEquality constraints to all instances that need them. Something like this:

   diff --git a/parameterized-utils.cabal b/parameterized-utils.cabal
   index 6df2300..aa86ded 100644
   --- a/parameterized-utils.cabal
   +++ b/parameterized-utils.cabal
   @@ -56,8 +56,8 @@ library
                   , containers
                   , deepseq
                   , ghc-prim
   -               , hashable       >=1.2  && <1.4
   -               , hashtables     ==1.2.*
   +               , hashable       >=1.2  && <1.5
   +               , hashtables     >=1.2  && <1.4
                   , indexed-traversable
                   , lens           >=4.16 && <5.1
                   , mtl
   @@ -142,4 +142,4 @@ test-suite parameterizedTests
    
      if impl(ghc >= 8.6)
        build-depends:
   -      hedgehog-classes
   \ No newline at end of file
   +      hedgehog-classes
   diff --git a/src/Data/Parameterized/Classes.hs b/src/Data/Parameterized/Classes.hs
   index b90754d..df1072a 100644
   --- a/src/Data/Parameterized/Classes.hs
   +++ b/src/Data/Parameterized/Classes.hs
   @@ -351,7 +351,7 @@ instance OrdF f => Ord (TypeAp f tp) where
    instance ShowF f => Show (TypeAp f tp) where
      showsPrec p (TypeAp x) = showsPrecF p x
    
   -instance HashableF f => Hashable (TypeAp f tp) where
   +instance (HashableF f, TestEquality f) => Hashable (TypeAp f tp) where
      hashWithSalt s (TypeAp x) = hashWithSaltF s x
    
    ------------------------------------------------------------------------
   diff --git a/src/Data/Parameterized/Context/Safe.hs b/src/Data/Parameterized/Context/Safe.hs
   index 96da3c0..5bdd196 100644
   --- a/src/Data/Parameterized/Context/Safe.hs
   +++ b/src/Data/Parameterized/Context/Safe.hs
   @@ -595,10 +595,10 @@ instance Hashable (Index ctx tp) where
    instance HashableF (Index ctx) where
      hashWithSaltF s i = hashWithSalt s (indexVal i)
    
   -instance HashableF f => HashableF (Assignment f) where
   +instance (HashableF f, TestEquality f) => HashableF (Assignment f) where
      hashWithSaltF = hashWithSalt
    
   -instance HashableF f => Hashable (Assignment f ctx) where
   +instance (HashableF f, TestEquality f) => Hashable (Assignment f ctx) where
      hashWithSalt s AssignmentEmpty = s
      hashWithSalt s (AssignmentExtend asgn x) = (s `hashWithSalt` asgn) `hashWithSaltF` x
    
   diff --git a/src/Data/Parameterized/Context/Unsafe.hs b/src/Data/Parameterized/Context/Unsafe.hs
   index 43e9def..e3f5845 100644
   --- a/src/Data/Parameterized/Context/Unsafe.hs
   +++ b/src/Data/Parameterized/Context/Unsafe.hs
   @@ -850,10 +850,10 @@ instance HashableF (Index ctx) where
    instance Hashable (Index ctx tp) where
      hashWithSalt = hashWithSaltF
    
   -instance HashableF f => Hashable (Assignment f ctx) where
   +instance (HashableF f, TestEquality f) => Hashable (Assignment f ctx) where
      hashWithSalt s (Assignment a) = hashWithSaltF s a
    
   -instance HashableF f => HashableF (Assignment f) where
   +instance (HashableF f, TestEquality f) => HashableF (Assignment f) where
      hashWithSaltF = hashWithSalt
    
    instance ShowF f => Show (Assignment f ctx) where
   diff --git a/src/Data/Parameterized/Some.hs b/src/Data/Parameterized/Some.hs
   index 3df9359..75e9c55 100644
   --- a/src/Data/Parameterized/Some.hs
   +++ b/src/Data/Parameterized/Some.hs
   @@ -33,7 +33,7 @@ instance TestEquality f => Eq (Some f) where
    instance OrdF f => Ord (Some f) where
      compare (Some x) (Some y) = toOrdering (compareF x y)
    
   -instance HashableF f => Hashable (Some f) where
   +instance (HashableF f, TestEquality f) => Hashable (Some f) where
      hashWithSalt s (Some x) = hashWithSaltF s x
      hash (Some x) = hashF x
   

2. In the spirit of hashable-1.4.0.0, we could make TestEquality a superclass of HashableF. This comes with its own set of challenges, however, as there are data types that have HashableF instances but not TestEquality instances. For instance, naïvely adding a TestEquality instance to HashableF will result in a different build error in parameterized-utils:

   [ 3 of 35] Compiling Data.Parameterized.Classes
   
   src/Data/Parameterized/Classes.hs:331:10: error:
       • Could not deduce (TestEquality (Const a))
           arising from the superclasses of an instance declaration
         from the context: Hashable a
           bound by the instance declaration
           at src/Data/Parameterized/Classes.hs:331:10-42
       • In the instance declaration for ‘HashableF (Const a)’
       |
   331 | instance Hashable a => HashableF (Const a) where
       |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   

   It's not entirely clear to me how you'd even define TestEquality for Const.

[RyanGlScott]

This issue is blocking support for GHC 9.0.2, as only hashable-1.4.0.2 has version bounds that are compatible with 9.0.2.

[RyanGlScott]

Option (2) in #126 (comment) is further complicated by the fact that it's unclear if TestEquality is even the right notion of equality to use—see haskell/core-libraries-committee#21. In light of this, I'm hesitant to commit to adding a new superclass to HashableF that could change pending the results of haskell/core-libraries-committee#21.

In light of this, I think it would be best to go with option (1). A quick search reveals that the libraries that would be most impacted by this change are what and crucible. Luckily, these libraries can be patched before a new parameterized-utils release is made to make the migration easier.

[robdockins]

I agree, let's not bake in TestEquality as a superclass given the uncertainty about its future.

[RyanGlScott]

hashable-1.3.5.0 has been revised to allow building with GHC 9.0.2, so this issue is no longer blocking 9.0.2 support. Still, it would be nice to land #127 in time for the next major release of parameterized-utils.