是否有必要在 class 声明的 class 上下文中指定每个超类？

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.