Merge #1956

1956: Revert "Remove unused type variables." r=dcoutts a=mrBliss

Revert "Remove unused type variables."

This reverts commit 156e38b.

This change was not compatible with GHC 8.6.5:

    src/Cardano/Ledger/Era.hs:92:25: error:
        Unexpected type ‘_’
        In the default declaration for ‘TranslationError’
        A default declaration should have form
          default TranslationError a b = ...
       |
    92 |   type TranslationError _ _ = Void

I don't think we (read: all downstream repos) are ready yet to burn that bridge,
at least not for something so trivial as this.

Co-authored-by: Thomas Winant <thomas@well-typed.com>

shelley-ma/impl/src/Cardano/Ledger/Allegra/Translation.hs

@@ -42,7 +42,7 @@ type instance PreviousEra (AllegraEra c) = ShelleyEra c
 --
 -- Note: if context is needed, please coordinate with consensus, who will have
 -- to provide the context in the right place.
-type instance TranslationContext (AllegraEra _) = ()
+type instance TranslationContext (AllegraEra c) = ()
 
 instance Crypto c => TranslateEra (AllegraEra c) NewEpochState where
  translateEra ctxt nes =

shelley-ma/impl/src/Cardano/Ledger/Mary/Translation.hs

@@ -35,7 +35,7 @@ type instance PreviousEra (MaryEra c) = AllegraEra c
 --
 -- Note: if context is needed, please coordinate with consensus, who will have
 -- to provide the context in the right place.
-type instance TranslationContext (MaryEra _) = ()
+type instance TranslationContext (MaryEra c) = ()
 
 instance Crypto c => TranslateEra (MaryEra c) NewEpochState where
  translateEra _ = error "TODO Allegra to Mary translation"

shelley/chain-and-ledger/executable-spec/src/Cardano/Ledger/Era.hs

@@ -89,7 +89,7 @@ class (Era era, Era (PreviousEra era)) => TranslateEra era f where
  -- concrete error type.
  type TranslationError era f :: Type
 
- type TranslationError _ _ = Void
+ type TranslationError era f = Void
 
  -- | Translate a type @f@ parameterised by the era from an era to the era
  -- after it.

shelley/chain-and-ledger/executable-spec/src/Cardano/Ledger/Shelley.hs

@@ -24,7 +24,7 @@ data ShelleyEra c
 instance CryptoClass.Crypto c => Era (ShelleyEra c) where
  type Crypto (ShelleyEra c) = c
 
-type instance Value (ShelleyEra _) = Coin
+type instance Value (ShelleyEra c) = Coin
 
 type TxBodyConstraints era =
  ( ChainData (TxBody era),