Re: help needed configuring GHC API client

24 Sep 2021

      Alfredo,

I've confirmed that I can use the stage1 compiler and it doesn't
panic.

It occurs to me that I might be able to *compile* my code with the
stage1 compiler while using the bootstrap compiler and Haskell
Language Server.   I'll give that a shot.

Norman
...
Hi Norman,
in addition to what Ben already said: is there any particular reason you
wanted to use the stage0 compiler? I have written a small program against
HEAD which used the GHC API fairly recently, and I have simply used the
`stage1` compiler. An excerpt from my little code snippet:
```
-- This is just the output of '_build/stage1/bin/ghc --print-libdir'
myGhcLibDir :: FilePath
myGhcLibDir = "./_build/stage1/lib"
playground :: FilePath -> IO ()
playground fn = do
  res <- runGhc (Just myGhcLibDir) $ do
  ...
```
As you can see that's exactly what you are doing, modulo the fact I was
using the `stage1` (where _build is the default directory for Hadrian
builds).
Hope this helps in some way!
A.
On Fri, 24 Sept 2021 at 03:37, Ben Gamari  wrote:
...
Norman Ramsey  writes:
...
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 suspect that the stages are getting mixed up here. Would it be
possible to post a full reproducer? I'd be happy to investigate further,
but without being able to reproduce locally it's a bit hard to say
anything useful. My recollection is that we look for DerivedConstants.h
in the usual include paths, so there are many ways in which things could
go wrong.
Cheers,
- Ben
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