Specialisation doesn't kick in (RE: Instantiation of overloaded definition *in Core*)

[Erdi, Gergo]

PUBLIC


PUBLIC

Hi,

Thanks! Originally I was going to reply to this saying that my transformation isn't running in CoreM so where do I get that environment from, but then I realized I can just build it from the md_insts field of ModDetails. However, after thinking more about it, I also realized that I shouldn't ever really need to conjure up dictionaries from thin air: the whole reason I am making a specific specialization of an overloaded function is because I found somewhere a call at that type. But then, that call also gives me the dictionary!

Of course at this point, this sounds exactly like what GHC already does in `specProgram`. So maybe I should be able to just use that?

Unfortunately, my initial testing seems to show that even if I run `specBind` manually on my whole-program collected CoreProgram, it doesn't do the work I would expect from it!

In the following example, I have only kept the definitions that are relevant. Before specialisation, I have the following whole-program Core:

(>>=)
  :: forall (m :: * -> *) a b. Monad m => m a -> (a -> m b) -> m b
[GblId[ClassOp], Arity=1, Caf=NoCafRefs, Str=<S(LSL),U(A,U,A)>]
(>>=)
  = \ (@(m :: * -> *)) (v_sGm [Occ=Once1!] :: Monad m) ->
      case v_sGm of
      { C:Monad _ [Occ=Dead] v_sGp [Occ=Once1] _ [Occ=Dead] ->
      v_sGp
      }
$dm>> :: forall (m :: * -> *) a b. Monad m => m a -> m b -> m b
[GblId, Arity=3, Unf=OtherCon []]
$dm>>
  = \ (@(m :: * -> *))
      ($dMonad [Occ=Once1] :: Monad m)
     (@a)
      (@b)
      (ma [Occ=Once1] :: m a)
      (mb [Occ=OnceL1] :: m b) ->
      let {
        sat_sGQ [Occ=Once1] :: a -> m b
        [LclId]
        sat_sGQ = \ _ [Occ=Dead] -> mb } in
      >>= @m $dMonad @a @b ma sat_sGQ
C:Monad [InlPrag=NOUSERINLINE CONLIKE]
  :: forall (m :: * -> *).
     Applicative m
     -> (forall a b. m a -> (a -> m b) -> m b)
     -> (forall a b. m a -> m b -> m b)
     -> Monad m
[GblId[DataCon], Arity=3, Caf=NoCafRefs, Cpr=m1, Unf=OtherCon []]
C:Monad
  = \ (@(m :: * -> *))
      (eta_B0 [Occ=Once1] :: Applicative m)
      (eta_B1 [Occ=Once1] :: forall a b. m a -> (a -> m b) -> m b)
      (eta_B2 [Occ=Once1] :: forall a b. m a -> m b -> m b) ->
      C:Monad @m eta_B0 eta_B1 eta_B2
$fMonadIO [InlPrag=NOUSERINLINE CONLIKE] :: Monad IO
[GblId[DFunId]]
$fMonadIO = C:Monad @IO $fApplicativeIO bindIO $fMonadIO_$c>>;
$fMonadIO_$c>> [Occ=LoopBreaker]
  :: forall a b. IO a -> IO b -> IO b
[GblId]
$fMonadIO_$c>> = \ (@a) (@b) -> $dm>> @IO $fMonadIO @a @b;
sat_sHr :: IO ()
[LclId]
sat_sHr = returnIO @() ()
sat_sHq :: IO ()
[LclId]
sat_sHq = returnIO @() ()
main :: IO ()
[GblId]
main = $fMonadIO_$c>> @() @() sat_sHq sat_sHr


Now I pass this to GHC's `specBind`, but the output is exactly the same as the input! (or it's close enough that I can't spot the difference).

(>>=)
  :: forall (m :: * -> *) a b. Monad m => m a -> (a -> m b) -> m b
[GblId[ClassOp], Arity=1, Caf=NoCafRefs, Str=<S(LSL),U(A,U,A)>]
(>>=)
  = \ (@(m :: * -> *)) (v_sGm [Occ=Once1!] :: Monad m) ->
      case v_sGm of
      { C:Monad _ [Occ=Dead] v_sGp [Occ=Once1] _ [Occ=Dead] ->
      v_sGp
      }
$dm>> :: forall (m :: * -> *) a b. Monad m => m a -> m b -> m b
[GblId, Arity=3, Unf=OtherCon []]
$dm>>
  = \ (@(m :: * -> *))
      ($dMonad [Occ=Once1] :: Monad m)
      (@a)
      (@b)
      (ma [Occ=Once1] :: m a)
      (mb [Occ=OnceL1] :: m b) ->
      let {
        sat_MHt [Occ=Once1] :: a -> m b
        [LclId]
        sat_MHt = \ _ [Occ=Dead] -> mb } in
      >>= @m $dMonad @a @b ma sat_MHt
C:Monad [InlPrag=NOUSERINLINE CONLIKE]
  :: forall (m :: * -> *).
     Applicative m
     -> (forall a b. m a -> (a -> m b) -> m b)
     -> (forall a b. m a -> m b -> m b)
     -> Monad m
[GblId[DataCon], Arity=3, Caf=NoCafRefs, Cpr=m1, Unf=OtherCon []]
C:Monad
  = \ (@(m :: * -> *))
      (eta_B0 [Occ=Once1] :: Applicative m)
      (eta_B1 [Occ=Once1] :: forall a b. m a -> (a -> m b) -> m b)
      (eta_B2 [Occ=Once1] :: forall a b. m a -> m b -> m b) ->
      C:Monad @m eta_B0 eta_B1 eta_B2
$fMonadIO [InlPrag=NOUSERINLINE CONLIKE] :: Monad IO
[GblId[DFunId]]
$fMonadIO = C:Monad @IO $fApplicativeIO bindIO $fMonadIO_$c>>;
$fMonadIO_$c>> [Occ=LoopBreaker]
  :: forall a b. IO a -> IO b -> IO b
[GblId]
$fMonadIO_$c>> = \ (@a) (@b) -> $dm>> @IO $fMonadIO @a @b;
sat_sHr :: IO ()
[LclId]
sat_sHr = returnIO @() ()
sat_sHq :: IO ()
[LclId]
sat_sHq = returnIO @() ()
main :: IO ()
[GblId]
main = $fMonadIO_$c>> @() @() sat_sHq sat_sHr


Why is that? I would have expected that the call chain main >-> $fMonadIO_$c>>  >-> $dm>> would have resulted in a specialization along the lines of:

$dm>>_IO :: forall a b. IO a -> IO b -> IO b
>>=_IO :: forall a b. IO a -> (a -> IO b) -> IO b

With appropriate definitions that can then be simplified away.

But none of this seems to happen -- $dm>> doesn't get an IO-specific version, and so $fMonadIO_$c>> still ends up with a dictionary-passing call to $dm>>. Isn't this exactly the situation that the specialiser is supposed to eliminate?

Thanks,
            Gergo

From: Simon Peyton Jones <simonpj at microsoft.com>
Sent: Monday, October 4, 2021 7:29 PM
To: Erdi, Gergo <Gergo.Erdi at sc.com>
Cc: Montelatici, Raphael Laurent <Raphael.Montelatici at sc.com>; GHC <ghc-devs at haskell.org>
Subject: [External] RE: Instantiation of overloaded definition *in Core*


PUBLIC
You can look it up in the class instance environment, which the Simplifier does have access to it.  That's relatively easy when you have a simple dictionary like (Monad IO).  But if you want (Eq [Int]) you first of all have to look up the (Eq [a]) dictionary, then the Eq Int dictionary, and apply the former to the latter.  We don't (yet) have a simple API to do that, although it would not be hard to create one.

Simon

PS: I am leaving Microsoft at the end of November 2021, at which point simonpj at microsoft.com<mailto:simonpj at microsoft.com> will cease to work.  Use simon.peytonjones at gmail.com<mailto:simon.peytonjones at gmail.com> instead.  (For now, it just forwards to simonpj at microsoft.com<mailto:simonpj at microsoft.com>.)

From: ghc-devs <ghc-devs-bounces at haskell.org<mailto:ghc-devs-bounces at haskell.org>> On Behalf Of Erdi, Gergo via ghc-devs
Sent: 04 October 2021 10:30
To: 'GHC' <ghc-devs at haskell.org<mailto:ghc-devs at haskell.org>>
Cc: Montelatici, Raphael Laurent <Raphael.Montelatici at sc.com<mailto:Raphael.Montelatici at sc.com>>
Subject: Instantiation of overloaded definition *in Core*


PUBLIC

Hi,

I'd like to instantiate Core definitions. For example, suppose I have the following Core definition:

foo :: forall m a b. Monad m => m a -> m b -> m b
foo = \ @m ($d :: Monad m) @a @b (ma :: m a) (mb :: m b) -> ...

Now let's say I'd like to instantiate it for m ~ IO. It is quite straightforward to go from the above to:

foo_IO_0 :: forall a b. Monad IO => IO a -> IO b -> IO b
foo_IO_0 = \ ($d :: Monad IO) @a @b (ma :: IO a) (mb :: IO b) -> ...

However, I would like to go all the way to:

foo_IO :: forall a b. IO a -> IO b -> IO b
foo_IO = \ @a @b (ma :: IO a) (mb :: IO b) -> ...

Because instances are coherent, it should be sound to replace all occurrences of $d with "the" dictionary for Monad IO. However, the places I've found for this kind of query seem to live in the typechecker. How do I access this information while working with Core?

Thanks,
            Gergo

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