Newcomer help wanted in the Haskell Debugger

I’ll have a stab at the Stack installation angle.

EDIT1: I tried (on Windows):

> stack unpack haskell-debugger
> cd haskell-debugger-0.13.1.0
> # Create a stack.yaml file with a suitable snapshot (see below)
> stack build

with stack.yaml:

snapshot: nightly-2026-05-27 # GHC 9.12.4
compiler: ghc-9.14.1 # GHC 9.14.1 or later is needed

extra-deps:
- dap-0.5.0.0@sha256:09f487806436461b0af84c15d82e4b74d361a4238480d8e80e8ade358a50f08e,2324
- ghc-stack-annotations-0.1.0.0@sha256:56bdded7dc6a72ea338b96fdb9da1806c583bfdf3286aa9d092bf072d36bac21,1643
- haskell-debugger-view-0.2.1.0@sha256:a6cad0c90bcc03be18b5435fc9222a9c97b3dc53b2b1c66c068ee5bfaa9af57e,1973

GHC 9.14.1 could not compile module GHC.Debugger.Monad, failing with:

haskell-debugger\GHC\Debugger\Monad.hs:629:13: error: [GHC-83865]
    • Couldn't match expected type: IO ProcessHandle
                  with actual type: Win32-2.14.2.1:System.Win32.DebugApi.Internal.PHANDLE
                                    -> IO ProcessHandle
    • Probable cause: ‘mkProcessHandle’ is applied to too few arguments
      In a stmt of a 'do' block:
        ph <- mkProcessHandle (fromIntegral pid) False
      In the expression:
        do bi_h <- socketToHandle sock ReadWriteMode
           pidLine <- annotateCallStackIO $ hGetLine bi_h
           pid <- case readMaybe pidLine :: Maybe Int of
                    Just pid -> pure pid
                    Nothing
                      -> fail
                           $ "invalid external interpreter PID on socket: " ++ show pidLine
           ph <- mkProcessHandle (fromIntegral pid) False
           ....
      In the third argument of ‘Control.Exception.bracketOnError’, namely
        ‘(\ (sock, _)
            -> do bi_h <- socketToHandle sock ReadWriteMode
                  pidLine <- annotateCallStackIO $ hGetLine bi_h
                  pid <- case readMaybe pidLine :: Maybe Int of
                           Just pid -> pure pid
                           Nothing
                             -> fail
                                  $ "invalid external interpreter PID on socket: " ++ show pidLine
                  ....)’
    |
629 |       ph <- mkProcessHandle (fromIntegral pid) False
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

That does not appear to be a Stack-related problem.

EDIT2: I then looked at the GitHub repository, and tried:

> git clone https://github.com/well-typed/haskell-debugger.git
> cd haskell-debugger
> # Edit the custom snapshot.yaml (see below)
> stack -w stack-windows.yaml build

using snapshot.yaml:

# A bespoke snapshot for GHC 9.14.1. There is no Stackage snapshot for
# GHC 9.14.1.
#
# The relevant package versions in this snapshot are known, by testing, to work
# together, not withstanding the upper bounds on dependencies specified in their
# Cabal files.
snapshot: nightly-2026-05-27 # GHC 9.12.4.
compiler: ghc-9.14.1

packages:
- dap-0.7.0.0@sha256:4eb00ced16dfec6ed1ef4705db8c57a32b657978bcd773c06a027fcf15e4a8f6,2324
- ghc-stack-annotations-0.1.0.0@sha256:56bdded7dc6a72ea338b96fdb9da1806c583bfdf3286aa9d092bf072d36bac21,1643

That failed during the build with:

haskell-debugger     > configure (lib + sub-lib + exe)
haskell-debugger     > Configuring haskell-debugger-0.13.1.0...
haskell-debugger     > Error: [Cabal-8010]
haskell-debugger     > Encountered missing or private dependencies:
haskell-debugger     >     dap >=0.7 && <0.8 (missing)
haskell-debugger     >

which I do not understand because:

> stack -w stack-windows.yaml exec -- ghc-pkg list dap
D:\sr\programs\x86_64-windows\ghc-9.14.1\lib\package.conf.d
    (no packages)
D:\sr\snapshots\0f13af98\pkgdb
    dap-0.7.0.0

D:\Users\mike\Code\Haskell\haskell-debugger\.stack-work\install\0f13af98\pkgdb
    (no packages)

shows that dap-0.7.0.0 is an installed package and (extract)

stack -w stack-windows.yaml ls dependencies
Cabal 3.16.0.0
...
cryptohash-sha1 0.11.101.0
data-fix 0.3.4
...
zlib-clib 1.3.1

appears to imply that dap is, in fact, not a direct or indirect dependency. So, I am puzzled …

EDIT3: … but now, looking at haskell-debugger.cabal, dap is also a named private sub-library (an internal library). However, it is not referenced as haskell-debugger:dap in the Cabal file, as I would expect. Perhaps that is the source of the problem …

EDIT4: If I rename the internal library in the Cabal file to be distinct from the name of a dependency package (dap → hdb-dap), Stack no longer falls over. (I don’t know if that reflects a Stack problem or a problem with the Cabal library that ships with GHC 9.14.1.) However, GHC 9.14.1 still cannot compile module GHC.Debugger.Monad - for what appears to be the same reason as above.