[Haskell-cafe] Looking-up a TyCon of a type exposed from public module but defined in hidden module

[石井大海]

Hi cafe,

I'm currently writing GHC Type-Checker Plugin [ghc-typelits-presburger], which enhances type-inference involving type-level naturals using Presburger arithmetic.

To process constraints properly, I need an access to TyCons of `TrueSym0`,
`FalseSym0`, `GTSym0`, etc. from `singletons` package.
These types are exposed by public modules, e.g. Data.Singletons.Prelude, but
it seems that they are originally defined in private module `Data.Singletons.Prelude.Instances`.

I first tried, to retrieve the `TyCon` for `TrueSym0`, the following:

```
  insts <- lookupModule (mkModuleName "Data.Singletons.Prelude.Instances") (fsLit "singletons")
  singTrueSym0 <- tcLookupTyCon =<< lookupOrig insts (mkTcOcc "TrueSym0")
```

But this results in the following compile-time error:

```
ghc: panic! (the 'impossible' happened)
  (GHC version 8.6.5 for x86_64-apple-darwin):
       Unable to resolve module looked up by plugin:
  Data.Singletons.Prelude.Instances

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
```

Then, I tried to lookup `TrueSym0` from exposed module, say Data.Singletons.Prelude:

```
  prel <- lookupModule (mkModuleName "Data.Singletons.Prelude") (fsLit "singletons")
  singTrueSym0 <- tcLookupTyCon =<< lookupOrig prel (mkTcOcc "TrueSym0")
```

But this resulted in the following error:

```
• Can't find interface-file declaration for type constructor or class singletons-2.5.1:Data.Singletons.Prelude.TrueSym0
    Probable cause: bug in .hi-boot file, or inconsistent .hi file
    Use -ddump-if-trace to get an idea of which file caused the error
• In the type signature: ...
```

Is there any way, in GHC API, to lookup symbols exported from public module but originally defined in hidden module?

Thanks,

[ghc-typelits-presburger]: https://github.com/konn/ghc-typelits-presburger

-- Hiromi ISHII
konn.jinro at gmail.com



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