scopedSort and kind variable left-biasing

Thu Feb 14 23:08:58 UTC 2019

Yes -- sweep it away!

> On Feb 14, 2019, at 5:30 PM, Simon Peyton Jones via ghc-devs <ghc-devs at haskell.org> wrote:
> 
> What do you (or anyone else) think about sweeping all that stuff away?  See my comments on
> https://gitlab.haskell.org/ghc/ghc/merge_requests/361 <https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitlab.haskell.org%2Fghc%2Fghc%2Fmerge_requests%2F361&data=02%7C01%7Csimonpj%40microsoft.com%7Cfc2d7f5498a34d68153008d692aaa9ae%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636857659024990967&sdata=H7euZSJmjiP%2BNE8KD9WD7TeeuHGFvuHzBAGLebPIMPM%3D&reserved=0>
>  
> Simon
>  
> From: Ryan Scott <ryan.gl.scott at gmail.com> 
> Sent: 14 February 2019 18:31
> To: Simon Peyton Jones <simonpj at microsoft.com>
> Cc: ghc-devs at haskell.org
> Subject: Re: scopedSort and kind variable left-biasing
>  
> Ah, I somehow forgot all about FreeKiTyVars. It turns out that the `freeKiTyVarsAllVars` function [1] is exactly what drives this behavior:
> 
>     freeKiTyVarsAllVars :: FreeKiTyVars -> [Located RdrName]
>     freeKiTyVarsAllVars (FKTV { fktv_kis = kvs, fktv_tys = tvs }) = kvs ++ tvs
>  
> That's about as straightforward as it gets. Thanks!
>  
> Ryan S.
> -----
> [1] https://gitlab.haskell.org/ghc/ghc/blob/5c1f268e2744fab2d36e64c163858995451d7095/compiler/rename/RnTypes.hs#L1604-1605 <https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitlab.haskell.org%2Fghc%2Fghc%2Fblob%2F5c1f268e2744fab2d36e64c163858995451d7095%2Fcompiler%2Frename%2FRnTypes.hs%23L1604-1605&data=02%7C01%7Csimonpj%40microsoft.com%7Cfc2d7f5498a34d68153008d692aaa9ae%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636857659024990967&sdata=QaWLPwctKWcuIFJFdPAqBhJzA99%2FFtsftSuuAstHVEQ%3D&reserved=0>
>  
> On Thu, Feb 14, 2019 at 12:46 PM Simon Peyton Jones <simonpj at microsoft.com <mailto:simonpj at microsoft.com>> wrote:
> See Note [Kind and type-variable binders] in RnTypes, and Note [Ordering of implicit variables].
> And the data type FreeKiTyVars.
>  
> But NB: that in https://gitlab.haskell.org/ghc/ghc/merge_requests/361 <https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitlab.haskell.org%2Fghc%2Fghc%2Fmerge_requests%2F361&data=02%7C01%7Csimonpj%40microsoft.com%7Cfc2d7f5498a34d68153008d692aaa9ae%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636857659024990967&sdata=H7euZSJmjiP%2BNE8KD9WD7TeeuHGFvuHzBAGLebPIMPM%3D&reserved=0>, I argue that with this patch we can sweep all this away.
>  
> If we did, we’d probably end up with [j,a,k,b].  
>  
> Perhaps that’s an ergonomic reason for retaining the current rather cumbersome code.  (Maybe it could be simplified.)
>  
> Simon
>  
> From: ghc-devs <ghc-devs-bounces at haskell.org <mailto:ghc-devs-bounces at haskell.org>> On Behalf Of Ryan Scott
> Sent: 14 February 2019 15:35
> To: ghc-devs at haskell.org <mailto:ghc-devs at haskell.org>
> Subject: scopedSort and kind variable left-biasing
>  
> Consider this function:
> 
>     f :: Proxy (a :: j) -> Proxy (b :: k)
> 
> If you just collect the free type variables of `f`'s type in left-to-right order, you'd be left with [a,j,b,k]. But the type of `f` is not `forall (a :: j) j (b :: k) k. Proxy a -> Proxy b`, as that would be ill scoped. `j` must come before `a`, since `j` appears in `a`'s kind, and similarly, `k` must come before `b`.
> 
> Fortunately, GHC is quite smart about sorting free variables such that they respect dependency order. If you ask GHCi what the type of `f` is (with -fprint-explicit-foralls enabled), it will tell you this:
> 
>     λ> :type +v f
>     f :: forall j k (a :: j) (b :: k). Proxy a -> Proxy b
> 
> As expected, `j` appears before `a`, and `k` appears before `b`.
> 
> In a different context, I've been trying to implement a type variable sorting algorithm similar to the one that GHC is using. My previous understanding was that the entirely of this sorting algorithm was implemented in `Type.scopedSort`. To test my understanding, I decided to write a program using the GHC API which directly uses `scopedSort` on the example above:
> 
>     main :: IO ()
>     main = do
>       let tv :: String -> Int -> Type -> TyVar
>           tv n uniq ty = mkTyVar (mkSystemName (mkUniqueGrimily uniq) (mkTyVarOcc n)) ty
>           j = tv "j" 0 liftedTypeKind
>           a = tv "a" 1 (TyVarTy j)
>           k = tv "k" 2 liftedTypeKind
>           b = tv "b" 3 (TyVarTy k)
>           sorted = scopedSort [a, j, b, k]
>       putStrLn $ showSDocUnsafe $ ppr sorted
> 
> To my surprise, however, running this program does /not/ give the answer [j,k,a,b], like what :type reported:
> 
>     λ> main
>     [j_0, a_1, k_2, b_3]
> 
> Instead, it gives the answer [j,a,k,b]! Strictly speaking, this answer meets the specification of ScopedSort, since it respects dependency order and preserves the left-to-right ordering of variables that don't depend on each other (i.e., `j` appears to the left of `k`, and `a` appears to the left of `b`). But it's noticeably different that what :type reports. The order that :type reports, [j,k,a,b], appears to bias kind variables to the left such that all kind variables (`j` and `k`) appear before any type variables (`a` and `b`).
> 
> From what I can tell, scopedSort isn't the full story here. That is, something else appears to be left-biasing the kind variables. My question is: which part of GHC is doing this left-biasing?
> 
>  
> 
> Ryan S.
> 
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