Newtypes in STG

Mon Apr 1 08:49:21 UTC 2019

Hi,

I'm a bit confused about your findings because I'm unable to reproduce them. I
wasn't aware that we're generating terms for newtype constructors, and when I
try I can see that this is really the case. For example, when I compile a module
with just this line:

    newtype MyInt = MyInt Int

in the Core or STG dumps I don't see any terms for the `MyInt` constructor.

I also checked the Core for System.Timeout.timeout module, and I don't see any
`Timeout` constructor applications anywhere. I compiled with -O0 -ddump-simpl.
Here's what I get for the `fmap Timeout` application in that module:

    (fmap
        @ IO
        GHC.Base.$fFunctorIO
        @ Unique
        @ Timeout
        ((\ (v_B1 :: Unique) -> v_B1)
         `cast` (<Unique>_R ->_R Sym (System.Timeout.N:Timeout[0])
                 :: (Unique -> Unique) ~R# (Unique -> Timeout)))
        newUnique)

No `Timeout` constructors here.

For reference, I'm attaching the full Core and STG dumps of System.Timeout
module.

It'd be helpful if you could share the build.mk you used when buildling GHC.

Ömer

Christopher Done <chrisdone at gmail.com>, 30 Mar 2019 Cmt, 17:07
tarihinde şunu yazdı:
>
> Oh, also my complete pipeline is here: https://github.com/chrisdone/prana/blob/0cbb7b4b96bbfdb4f0d6a60e08f4b1f53abdfb15/prana-ghc/src/Prana/Ghc.hs#L106-L154
>
> Parse/typecheck/desugar, tidy, prep, core-to-stg (ripped from HscMain) and then I try to resolve all names in the AST and that leads me to this.
>
> On Sat, 30 Mar 2019 at 14:05, Christopher Done <chrisdone at gmail.com> wrote:
>>
>> Hi all,
>>
>> Could you offer some insight into newtypes at the STG level? Here’s the
>> context:
>>
>> I’m working on this interpreter for STG
>> (https://github.com/chrisdone/prana) and I’m trying to generate a
>> pristine AST where all names in it are resolved to something known to
>> me.
>>
>> I’ve compiled ghc-prim and integer-gmp without issue, and I’m
>> compiling base and there remains one last frontier which is newtypes.
>>
>> These are the culprits pointed out if I compile base:
>>
>> chris at precision:~/Work/chrisdone/prana/ghc-8.4/libraries/base-4.11.1.0$ PRANA_INDEX=../prana.idx stack build --exec './Setup build --ghc-options=-O0' --file-watch
>> Preprocessing library for base-4.11.1.0..
>> Building library for base-4.11.1.0..
>> [1 of 244] Compiling GHC.Base
>> [2 of 244] Compiling GHC.IO
>> [3 of 244] Compiling GHC.Real
>> [4 of 244] Compiling Data.Semigroup.Internal
>> ... [snip] ...
>> [242 of 244] Compiling Data.Functor.Compose
>> [243 of 244] Compiling Data.Fixed
>> [244 of 244] Compiling Data.Complex
>>
>> Errors in Data.Foldable:
>>   Variable name not found: base:Data.Semigroup.Internal.All
>>   Variable name not found: base:Data.Semigroup.Internal.Any
>>
>> Errors in Foreign.Marshal.Pool:
>>   Variable name not found: base:Foreign.Marshal.Pool.Pool
>>
>> Errors in GHC.ExecutionStack.Internal:
>>   Variable name not found: base:GHC.ExecutionStack.Internal.StackTrace
>>
>> Errors in Data.Bifoldable:
>>   Variable name not found: base:Data.Semigroup.Internal.All
>>   Variable name not found: base:Data.Semigroup.Internal.Any
>>
>> Errors in System.Timeout:
>>   Variable name not found: base:System.Timeout.Timeout
>>
>> Errors in Data.Data:
>>   Variable name not found: base:Foreign.Ptr.WordPtr
>>   Variable name not found: base:Foreign.Ptr.IntPtr
>>   Variable name not found: base:Data.Semigroup.Internal.Any
>>   Variable name not found: base:Data.Semigroup.Internal.All
>>
>> I looked these up, and they all appear to be uses of a
>> newtype.
>>
>> For example, in the Timeout function:
>>
>> http://hackage.haskell.org/package/base-4.12.0.0/docs/src/System.Timeout.html#timeout
>>
>> I printed out the full [StgTopBinding] list and got:
>>
>> let {
>>   sat_s27NC [Occ=Once] :: IO Timeout
>>   [LclId] =
>>       [] \u [] fmap $fFunctorIO Timeout newUnique;
>> } in  >>= $fMonadIO sat_s27NC sat_s27NT;
>>
>> Oddly (or not?), they’re used as values, not constructors. This error
>> comes from this part of my code:
>>
>> StgSyn.StgApp occ arguments ->
>>   AppExpr <$> lookupSomeVarId occ <*> traverse fromStgGenArg arguments
>>
>> If it was used as a constructor, it’d appear in this position, where
>> looking up the ID would produce a “Data constructor name not found”
>> error:
>>
>> StgSyn.StgConApp dataCon arguments types ->
>> ConAppExpr <$> lookupDataConId dataCon <> traverse fromStgGenArg arguments <>
>> pure (map (const Type) types)
>>
>> My understanding of newtypes at this stage is hazy. It seems like:
>>
>> They ought to be erased and replaced with coercions by now. If
>> they’re not replaced, it’s because they’re in a not-quite-id position
>> like fmap Timeout .... (Arguably these could be fixed in base with a
>> Data.Coerce.coerce?)
>>
>> However, the CoreTidy/PrepPgm processing modules
>> don’t seem to have removed or replaced these, or introduced a binding
>> that would do something.
>>
>> At this stage what would you recommend? At this point type-checking is
>> done, and I only care about interpreting the code. So I suppose they
>> could be id for all it matters to the interpreter?
>>
>> I imagine they aren’t actually supposed to allocate something here. And
>> I’m certain that any pattern matching on a newtype is also erased away
>> by this point.
>>
>> Cheers!
>>
>> Chris
>
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