Interpreter thread in ghc-8.10.6 in macOS

[Zubin Duggal]

A patch to run Template Haskell code in a forked thread was included in
ghc-8.10.6:  https://gitlab.haskell.org/ghc/ghc/-/merge_requests/5042

This was chosen as the only robust way to avoid reporting any
asynchronous exceptions thrown to the compilation process during TH
evaluation as a user error in TH code. If you cancel a compile by
throwing the compilation thread an exception, we don't want to catch
that error in GHC and report it as if the TH code had a regular
exception. By running the TH code in a forked thread and not sharing
the ThreadId, we ensure that any exception caught during the evaluation
of TH code either originated from the code itself or was thrown by the
RTS directly to the TH code (BlockedIndefinetelyOnMVar,... etc.).

There may be alternative solutions to this or ways to make this
behaviour configurable. I would urge you to open an ticket on the GHC
tracker detailing your use case.

On 21/11/16 11:26, Domínguez, Facundo wrote:
>Dear devs,
>
>I found recently that ghc-8.10.6 and ghc-8.10.7 might be behaving
>differently than previous compilers when running Template Haskell code.
>
>When upgrading HaskellR to use a newer ghc I found that it would work fine
>with ghc-8.10.4, but not ghc-8.10.6 or ghc-8.10.7. HaskellR, and the R
>runtime in particular, is sensitive to local state in the threads that are
>used to execute the runtime. HaskellR is arranging to run the R runtime
>when running TemplateHaskell splices, and this stopped working in MacOS
>with ghc-8.10.6.
>
>The error I got is not new. It comes from R and we would get it in ghci if
>we try to use HaskellR without passing the command line flag
>-fno-ghci-sandbox. But this is the first time we get the error when using
>TemplateHaskell.
>
>Does anyone know if there have been changes to the threads in which the TH
>splices are run? And were there any changes specific to TH in MacOS?
>
>Thanks in advance,
>Facundo
>
>[1] https://github.com/tweag/HaskellR/pull/368#issuecomment-968864170

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