Is it necessary to specify every superclass in a class context of a class declaration?

Question

The ArrowList class from the hxt package has the following declaration:

class (Arrow a, ArrowPlus a, ArrowZero a, ArrowApply a) => ArrowList a where ...

The ArrowPlus class is declared as: class ArrowZero a => ArrowPlus a where ...

The ArrowZero class is declared as: class Arrow a => ArrowZero a where ...

And the ArrowApply class is declared as: class Arrow a => ArrowApply a where ...

Why can't it just be written as: class (ArrowPlus a, ArrowApply a) => ArrowList a where ...?

Answer 1

No, it's not necessary to include all the superclasses. If you write

class (ArrowPlus a, ArrowApply a) => ArrowList a where

it will work. However, here are two possible reasons for mentioning all the superclasses explicitly.

1. It might be more readable as you can tell at a glance what all the superclasses are.

2. It might be slightly more efficient, as listing the superclasses explicitly will result in a direct dictionary lookup at runtime, while for a transitive superclass it will first lookup the dictionary for the superclass and then lookup the class member in that.

   For example, take this inheritance chain:

   module Example where
   
   class Foo a where
       foo :: a -> String
   
   class Foo a => Bar a
   class Bar a => Baz a
   class Baz a => Xyzzy a
   
   quux :: Xyzzy a => a -> String
   quux = foo
   

   Looking at the generated core for this (with ghc -c -ddump-simpl), we see that this generates a chain of lookup calls. It first looks up the dictionary for Baz in Xyzzy, then Bar in that, then Foo, and finally it can look up foo.

   Example.quux
     :: forall a_abI. Example.Xyzzy a_abI => a_abI -> GHC.Base.String
   [GblId, Arity=1, Caf=NoCafRefs]
   Example.quux =
     \ (@ a_acE) ($dXyzzy_acF :: Example.Xyzzy a_acE) ->
       Example.foo
         @ a_acE
         (Example.$p1Bar
            @ a_acE
            (Example.$p1Baz @ a_acE (Example.$p1Xyzzy @ a_acE $dXyzzy_acF)))
   

   Modifying the definition of Xyzzy to explicitly mention Foo:

   class (Foo a, Baz a) => Xyzzy a
   

   We see that it can now get the Foo dictionary straight from the Xyzzy one and look up foo in that.

   Example.quux
     :: forall a_abD. Example.Xyzzy a_abD => a_abD -> GHC.Base.String
   [GblId, Arity=1, Caf=NoCafRefs]
   Example.quux =
     \ (@ a_acz) ($dXyzzy_acA :: Example.Xyzzy a_acz) ->
       Example.foo @ a_acz (Example.$p1Xyzzy @ a_acz $dXyzzy_acA)
   

   Note that this may be GHC-specific. Tested with version 7.0.2.