help needed configuring GHC API client

[Matthew Pickering]

Hi Norman,

I think you are overcomplicating things quite a bit.

1. Build a HEAD (9.3) compiler
2. Setup a normal cabal project to depend on the `ghc` library
3. Use the cabal -w option to point to your HEAD compiler to build the
project (`cabal build -w /path/to/head`)

That will work reliably rather than messing around with package databases.

Matt

On Thu, Sep 23, 2021 at 7:31 PM Norman Ramsey <nr at cs.tufts.edu> wrote:
>
> I'm writing client code against the GHC API in HEAD (version 9.3),
> using 9.0.1 as my bootstrap compiler.  To make it possible to build
> this code, I've set up cabal using
>
>    cabal v1-configure \
>       --package-db clear \
>       --package-db $STAGE0/lib/package.conf.d/      # stage0 libraries
>
> In my Haskell code I'm invoking `runGhc (Just thelibdir)` where
>
>    thelibir = "/home/nr/asterius/ghc/_build/stage0/lib"
>
> which is my `$STAGE0/lib`.
>
> Unfortunuately, when I launch my app, `setSessionDynFlags` panics.
> The output, along with some diagnostic information about some dflags
> that seemed relevant, looks like this:
>
>   libdir = /home/nr/asterius/ghc/_build/stage0/lib
>   includePaths = IncludeSpecs {includePathsQuote = [], includePathsGlobal = [], includePathsQuoteImplicit = []}
>   libraryPaths = []
>   packageDBFlags = []
>   packageEnv = Nothing
>   panic! (the 'impossible' happened)
>     GHC version 9.3.20210918:
>           GHC couldn't find the RTS constants (#define HS_CONSTANTS ") in /home/nr/.ghcup/ghc/9.0.1/lib/ghc-9.0.1/include/DerivedConstants.h: the RTS package you are trying to use is perhaps for another GHC version(e.g. you are using the wrong package database) or the package database is broken.
>
>   CallStack (from HasCallStack):
>     error, called at _build/stage0/compiler/build/GHC/Platform/Constants.hs:143:20 in ghc:GHC.Platform.Constants
>
>   Please report this as a GHC bug:  https://www.haskell.org/ghc/reportabug
>
>
> I'm a little suprprised that my app is hunting for 9.3 information in
> the tree that belongs to the bootstrap compiler.
>
> I could start tinkering with `packageEnv` or other dflags, but I'm on
> thin ice here, and I thought it made more sense to ask for advice.
>
> What is going on in my configuration, and how can I set up my app so
> it is using the library in my `libdir` rather than the library that
> belongs to the bootstrap compiler?
>
>
> Norman
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