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

5 Oct 2021

      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=]
(>>=)
  = \ (@(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=]
(>>=)
  = \ (@(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 
Sent: Monday, October 4, 2021 7:29 PM
To: Erdi, Gergo 
Cc: Montelatici, Raphael Laurent ; GHC 
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@microsoft.commailto:simonpj@microsoft.com will cease to work.  Use simon.peytonjones@gmail.commailto:simon.peytonjones@gmail.com instead.  (For now, it just forwards to simonpj@microsoft.commailto:simonpj@microsoft.com.)

From: ghc-devs mailto:ghc-devs-bounces@haskell.org> On Behalf Of Erdi, Gergo via ghc-devs
Sent: 04 October 2021 10:30
To: 'GHC' mailto:ghc-devs@haskell.org>
Cc: Montelatici, Raphael Laurent mailto:Raphael.Montelatici@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

This email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please delete all copies and notify the sender immediately. You may wish to refer to the incorporation details of Standard Chartered PLC, Standard Chartered Bank and their subsidiaries at https: //www.sc.com/en/our-locations

Where you have a Financial Markets relationship with Standard Chartered PLC, Standard Chartered Bank and their subsidiaries (the "Group"), information on the regulatory standards we adhere to and how it may affect you can be found in our Regulatory Compliance Statement at https: //www.sc.com/rcs/ and Regulatory Compliance Disclosures at http: //www.sc.com/rcs/fm

Insofar as this communication is not sent by the Global Research team and contains any market commentary, the market commentary has been prepared by the sales and/or trading desk of Standard Chartered Bank or its affiliate. It is not and does not constitute research material, independent research, recommendation or financial advice. Any market commentary is for information purpose only and shall not be relied on for any other purpose and is subject to the relevant disclaimers available at https: //www.sc.com/en/regulatory-disclosures/#market-disclaimer.

Insofar as this communication is sent by the Global Research team and contains any research materials prepared by members of the team, the research material is for information purpose only and shall not be relied on for any other purpose, and is subject to the relevant disclaimers available at https: //research.sc.com/research/api/application/static/terms-and-conditions. 

Insofar as this e-mail contains the term sheet for a proposed transaction, by responding affirmatively to this e-mail, you agree that you have understood the terms and conditions in the attached term sheet and evaluated the merits and risks of the transaction. We may at times also request you to sign the term sheet to acknowledge the same.

Please visit https: //www.sc.com/en/regulatory-disclosures/dodd-frank/ for important information with respect to derivative products.

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

Erdi, Gergo

Matthew Pickering

Simon Peyton Jones

Erdi, Gergo

Erdi, Gergo

Simon Peyton Jones

Erdi, Gergo

tags

participants (3)