Newtypes in STG

[Christopher Done]

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:
>
>    1.
>
>    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.
>    2.
>
>    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
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/ghc-devs/attachments/20190330/cef4f274/attachment.html>