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

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 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@gmail.com

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