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

[Richard Eisenberg]

I don't know the "official" way to do this, but `Module` is a very simple type. You could always build it yourself, using the unit ID from another `singletons` module and just change the module name to suit your needs.

Richard

> On Feb 1, 2020, at 11:59 AM, 石井大海 <konn.jinro at gmail.com> wrote:
> 
> 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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