wiki:GhcKinds/KindInference

Version 8 (modified by simonpj, 5 years ago) (diff)

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Kind inference for types and classes

This page summarises several alternative designs, which are debated on #9200. (See also #9201.)

Baseline strategy (BASELINE)

This plan, originally due to Mark Jones, is the strategy that GHC 7.8 follows for ordinary, recursive term-level functions, and for recursive data types. I'll describe it for data types, with this example:

   data SS f a b = MkSS (TT a f) (SS f a b)
   data TT (a::k) (f::k -> *) :: * where
      MkTT :: f a -> SS f a Maybe -> SS f a Int -> TT a f
  1. Identify which type constructors have Complete User Type Signatures (CUSK). In this example, TT does. Extend the environment with these, fixed, kinds:
           TT :: forall k. k -> (k->*) -> *
    
  2. Perform strongly-connected component (SCC) analysis on the non-CUSK decls, ignoring dependencies on a type constructor with a CUSK. In our example, we get a single recursive SCC, containing SS.
  1. For each SCC in turn:
    • Bind the type constructor to a fresh meta-kind variable:
              SS :: kappa0
      
    • Kind-check all the declarations of the SCC in this environment. This will generate some unifications, so in the end we get
              kappa0 ~ (kappa1 -> *) -> kappa1 -> kappa2 -> *
      
      The kappa1 arises from instantiating TT at its call site in SS
    • Generalise. So we get
              SS :: forall k1 k2. (k1->*) -> k1 -> k2 -> *
      
  2. Extend the environment with these generalised kind bindings, and kind-check the CUSK declarations.

The Key Point is that we can kind-check SS without looking at TT's definition at all, because we completely know TT's kind. That in turn means that we can exploit inferred polymorphism for SS when kind-checking TT. As we do here: TT uses SS in two different ways (SS f a Maybe) and (SS f a Int).

Note that for a non-recursive type or class declaration, (BASELINE) always works fine.

Partial kind signature strategy (PARTIAL)

The key idea is that all polymorphism is declared, so nothing gets to be kind-polymorphic unless you say so. But the payoff is that you can give partial kind signatures. Here's the strategy.

  1. Sort the declarations into SCCs. No special treatment for CUSKs.
  1. For each declaration, extend the environment with a kind binding that has a forall for each explicit user-written kind variable, but meta-kind variables otherwise. These kind annotations amount to partial kind signatures. For example
          data Foo (a :: k1 -> k1) b c = ...
    
    would get a kind binding
          Foo :: forall k1. (k1->k1) -> kappa1 -> kappa2 -> *
    
    Our earlier example would give
          T :: forall k. k -> (k->*) -> *
          S :: kappa3 -> kappa4 -> kappa5 -> *
    
  1. Kind-check the declartions in this environment. At a call of Foo, say, we'd instantiate the forall k1 with a fresh meta-kind variable, but would share kappa1, kappa2 among all calls to Foo.
  1. Default any unconstrained meta kind variables to *

That's it! No generalisation step. The only polymorphism is that declared by the user.

So our earlier SS/TT example would be rejected because it relies on S being polymorphic in its third parameter. If you want the SS/TT example to work you could write

   data SS (f::k1->*) (a::k1) (b::k2) = MkSS (TT a f) (SS f a b)
   data TT (a::k) (f::k->*) where
      MkTT :: f a -> SS f a Maybe -> SS f a Int -> TT a f

Declarative typing rules for (PARTIAL)

Richard: I'm similarly unsure of this section. I like thinking in terms of typing rules, but I want rules about datatype declarations, not function declarations. I agree with the conclusions here, but I can't figure out how (PARGEN) would look different from (BASELINE) in this presentation. End Richard

I think that (PARTIAL) has a nice declarative typing rule.

Here is what the conventional declarative typing rule, in the absence of polymorphism for a single self-recursive function looks like. (I'm using the term language but the same thing happens at the type level.)

        G, f:t |- e:t
        G, f:t |- b:t'
      ---------------------------
        G |- letrec f = e in b : t'

Here the "t" is a monotype (no foralls) that the declarative typing rules clairvoyantly conjures up out of thin air.

Once you add Hindley-Milner style polymorphism, the rule gets a bit more complicated

        G, f:t |- e:t
        G, f:gen(G,t) |- b:t'
      ---------------------------
        G |- letrec f = e in b : t'

where 'gen' is generalising.

The (PARTIAL) rule might look like this:

        t = forall vs. sig[t1..tn/_]
        vs \not\in ti
        G, f : t |- e : forall vs.t
        G, f : t |- b:t'
      --------------------------- (T-PARTIAL)
        G |- letrec f :: forall vs. sig; f = e in b : t'

Here I'm expressing the user-specified knowledge as a signature forall vs.sig, with '_' for bits you don't want to specify.

       f :: forall a. _ -> a -> _

Then the rule intantiates each '_' independently with a clairvoyantly guessed monotype (provided it does not mention the 'vs', or 'a' in this example), and off you go.

A tricky point about (PARTIAL)

Notice that in this typing rule I say vs \not\in ti. If you don't have that side condition I think complete inference becomes very hard. Suppose MT :: (*->*) -> *, and consider

   data Foo f (a::k) = MkFoo (Foo Maybe Int) (Foo MT Maybe)

Because of the partial kind signature we'll kind-check Foo's RHS with this kind signature for Foo:

   Foo :: forall k. kappa1 -> k -> *

using the unification variable kapp1 for f. Now, if we clairvoyantly decide kappa1 := k->*, as would be allowed by (T-PARTIAL), then indeed the definition if well-kinded. So we'd better infer that, if we are to be complete wrt (T-PARTIAL). But the algorithm will share kappa1 among both calls to Foo, and will therefore unify Maybe with MT and fail.

To gain completeness we need to be less ambitious; hence the side condition in (T-PARTIAL) vs \not\in ti.

But that side condition, in turn, means that this will fail:

  data Foo f (a::k) = MkFoo (f a) (Foo f a)

because here kappa1 must be unified with k->*, which isn't allowed by (T-PARTIAL). Maybe that is acceptable; you can always decorate both of Foo's arguments.

Richard: I don't understand this last point. Are you saying that the described algorithm does not do this unification and thus would reject Foo? Or, are you saying that the proposed strategy should not do this unification, as a point of design that might be friendlier to users? End Richard Simon: clearer now?

Generalised partial kind signature strategy (PARGEN)

The (PARGEN) strategy is exactly like (PARTIAL) except that step 4 is different:

  1. Generalise over any unconstrained meta kind variable, rather than defaulting to *. Since we are operating at top level, there are no kind variables mentioned in the environment, so no need for the ususal "not free in the environment" check.

So we use the partial kind signatures to express any polymorphism necessary for recursion inside the SCC, but perhaps infer yet more polymorphism that can be used after the SCC. Thus:

data T f a = MkT (f a) (T f a)
  -- Success:  T :: forall k. (k->*) -> k -> *

data S f a = MkS (f a) (S Maybe Int) (S Monad Maybe)
  -- Failure: needs polymorphic recursion

data S2 f (a::k) = MkS (f a) (S Maybe Int) (S Monad Maybe)
  -- Success: needs polymorphic recursion

Declarative typing rules for (PARGEN)

The declarative rule for (PARGEN) is a combination of the one for (PARTIAL) with the standard generalisation:

        t = forall vs. sig[t1..tn/_]
        vs \not\in ti
        G, f : t |- e : forall vs.t
        G, f : gen(G,t) |- b:t'
      --------------------------- (T-PARGEN)
        G |- letrec f :: forall vs. sig; f = e in b : t'

The difference from (PARTIAL) is that before type-checking b we generalise t.

All of the above (ALL)

Combine (BASELINE), for the CUSK stuff, with (PARGEN) for type with partial kind signatures.

Type signatures

Richard: I'm not sure what the upshot of this section is. In type signatures, it feels like we're using an algorithm other than (BASELINE), because polymorphic recursion on kinds works just fine without any mention of kind variables. I suppose this is because the body of a function is considered outside of its type signature's SCC and is not considered when doing kind inference. Given that recursion in a type signature is not possible (we can't mention terms in types), I can't quite figure out what differentiates the strategies in type signatures. End Richard Simon: true; but if we used (PARTIAL), which does no kind-generalisation, it would be consistent not to kind-generalise in type signatures either. I've tried to clarify End Simon

Another place that we currently (i.e. using (BASELINE)) do kind generalisation is in type signatures. If you write

f :: m a -> m a 
f = ...

then the type signature is kind-generalised thus:

This user-written signature 
  f :: m a -> m a 
means this (BASELINE)
  f :: forall k (a:k) (m:k->*). m a -> m a

And f's RHS had better be that polymorphic.

However (PARTIAL) does no kind generalisation, and it would be consistent to cease doing so for type signatures too. so:

This user-written signature 
  f :: m a -> m a 
means this (PARTIAL)
  f :: forall (a:*) (m:k->*). m a -> m a

If you want the kind-polymorphic one, you'd have to write thus

This user-written signature 
  f :: forall k (a:k) (m:k->*). m a -> m a
means this (PARTIAL)
  f :: forall k (a:k) (m:k->*). m a -> m a

Reflection

I think we could reasonably switch to (PARTIAL) throughout.

As Richard's comments in TcHsType point out, we don't want maximal polymorphism. His example is:

    type family F a where
      F Int = Bool
      F Bool = Char

We could generate

   F :: forall k1 k2. k1 -> k2

so that (F Maybe) is well-kinded, but stuck. But that's probably not what we want. It would be better to get F :: * -> *

But what about

    type family G a f b where
      G Int  f b = f b
      G Bool f b = Char -> f b

You could just about argue that the programmer intends

   F :: forall k. * -> (k->*) -> k -> *

It's quite similar to this:

  data PT f a = MkPT (f a)

which today, using (BASELINE), we infer to have kind

  PT :: forall k. (k->*) -> k -> *

But I'd be perfectly happy if PT got a monomorphic inferred kind, which is what (PARTIAL) would do:

  PT :: (*->*) -> * -> *

If you want the poly-kinded PT, use a signature:

  -- Any of these would do
  data PT f             (a :: k) = MkPT (f a)
  data PT (f :: k -> *) a        = MkPT (f a)
  data PT (f :: k -> *) (a :: k) = MkPT (f a)

One oddity is that we'd do (BASELINE) for terms and (PARTIAL) for types. But perhaps that's ok. They are different.

  • Terms ought to be as polymorphic as possible but arguably not types. Examples above. Also, since kind polymorphism is still in its infancy, maybe it's no bad thing if all kind polymorphism is explicitly signalled every time a kind-polymorphic binder is introduced.
  • Terms have well-established separate type signatures, but we don't have a syntax for separate kind signatures of types and classes.

If we moved from (BASELINE) to (PARTIAL), some programs that work now would fail:

  • the original S/T example above
  • a data type like PT where the user did actually want the kind-polymorphic version.

But that might be a price worth paying for the simplicity, uniformity, and predictability you'd get in exchange.

Richard: I think changing to (PARTIAL) throughout would be a mistake, as lots of code would fail to compile. Kind polymorphism by default in datatypes and classes has been around since 7.4, and I suspect there is quite a bit of code that such a change would disrupt.

On the other hand, I think changing to (PARGEN) throughout would work nicely. I believe that it would allow all current code to type-check (except for the weird example that probably should be rejected in #9201). If we were to choose (PARGEN) over (ALL), it's possible that some code would become more polymorphic, as (PARGEN) is more polymorphic than (BASELINE) in the presence of a CUSK. However, I don't believe that this could be a breaking change, and I would prefer going with (PARGEN) over (ALL) for the sake of simplicity -- no need to have two systems around.

I can't figure out a way that (BASELINE) and (PARGEN) are different in type signatures for terms. This version doesn't have quite as nice a declarative typing rule because the type is generalized over kind variables that go completely unmentioned in the type -- a straightforward forall ftv(t). t doesn't quite do it. We need to generalize over seen variables, infer kinds, and then generalize over meta-kind variables. But, this is what is done today.

(Because open type families do not have a body, they would still need their own kind inference story, where unconstrained meta-variables default to *.)

In comment:5:ticket:9200, I discuss "good" polymorphism and "bad" polymorphism. This discussion, in retrospect, seems tangential at this point. It really only makes sense when discussing closed type families, which aren't at the heart of the problems here. End Richard