Superclass Cycle via Associated Type
Simon Peyton-Jones
simonpj at microsoft.com
Mon Jul 25 10:46:03 CEST 2011
On further reflection I have a question.
Under the limited design below, which Edward says will do all he wants:
· The mutually recursive classes (call them A, B, C) must be defined all together. Like
class B a => A a; class C a => B a; class A a => C a
· If a type T is an instance of any of those classes, it must be a member of all of them
· If a function f has type f :: A a => blah, then the signature f :: B a => blah and f :: C a => blah would work equally well
In short, I see no advantage to making A,B,C separate classes compared to simply unioning them into a single class.
Bottom line: adding recursive superclasses with the restrictions I describe below would add no useful expressive power. But it would cost effort to implement. So why do it?
Maybe I’m missing something.
Simon
From: Edward Kmett [mailto:ekmett at gmail.com]
Sent: 22 July 2011 20:07
To: Simon Peyton-Jones
Cc: Gábor Lehel; glasgow-haskell-users at haskell.org
Subject: Re: Superclass Cycle via Associated Type
2011/7/22 Simon Peyton-Jones <simonpj at microsoft.com<mailto:simonpj at microsoft.com>>
I talked to Dimitrios. Fundamentally we think we should be able to handle recursive superclasses, albeit we have a bit more work to do on the type inference engine first.
The situation we think we can handle ok is stuff like Edward wants (I've removed all the methods):
class LeftModule Whole m => Additive m
class Additive m => Abelian m
class (Semiring r, Additive m) => LeftModule r m
class Multiplicative m where (*) :: m -> m -> m
class LeftModule Natural m => Monoidal m
class (Abelian m, Multiplicative m, LeftModule m m) => Semiring m
class (LeftModule Integer m, Monoidal m) => Group m
class Multiplicative m => Unital m
class (Monoidal r, Unital r, Semiring r) => Rig r
class (Rig r, Group r) => Ring r
The superclasses are recursive but
a) They constrain only type variables
b) The variables in the superclass context are all
mentioned in the head. In class Q => C a b c
fv(Q) is subset of {a,b,c}
Question to all: is that enough?
This would perfectly address all of the needs that I have had!
-Edward
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