Weird test failure due to type error with GHC 9.2.1

[sjakobi]

bsb-http-chunked:test:doctests fails with a weird type error that I can't reproduce with older GHC versions or cabal repl.

Please see sjakobi/bsb-http-chunked#38.

[sol]

@sjakobi invoking doctest via cabal repl works for me:

$ cabal install doctest --overwrite-policy=always && cabal build && cabal repl --build-depends=QuickCheck --build-depends=template-haskell --with-ghc=doctest
Examples: 5  Tried: 5  Errors: 0  Failures: 0

Starting with doctest-0.20.0 that's how I recommend to use doctest with cabal-v2.

[sol]

As I describe here sjakobi/bsb-http-chunked#38 (comment), if I invoke ghci the same way as doctest is invoked, I get the same error. So as far as I can tell, this is not a bug (by design, doctest tries to mimic ghci).

I'm closing this, but please don't be shy to open a new issue if you think there is something that we can do in doctest to improve the situation.

[sjakobi]

So as Ryan explains in https://gitlab.haskell.org/ghc/ghc/-/issues/20833#note_398289, the problem seems to be that the doctests invoked via cabal test end up using the default GHC2021 language variant, which leads to the type error in bsb-http-chunked.

To me that seems like a bug somewhere in cabal test or doctest. It's good that the workaround with cabal repl exists, but I hope that doctests will eventually be properly supported in cabal test again.

EDIT: For reference: haskell/cabal#4500 contains some discussion on better doctest support in cabal.

[sol]

To me that seems like a bug somewhere in cabal test or doctest.

@sjakobi I'm not convinced that this is the case. doctest behaves the same as ghci, which is by design.

cabal test on the other hand merely runs your tests/Doctests.hs. I don't think it is in a position to influence how doctest is invoked.

What I assume is that you just need to pass -XHaskell2010 to doctest. Specifically, change tests/Doctests.hs to:

import Test.DocTest

main :: IO ()
main = doctest ["-isrc", "-XHaskell2010", "Data/ByteString/Builder/HTTP/Chunked.hs"]

(untested)

I still think that with doctest-0.20.0 the most reliabl way to use doctest is via cabal repl. Because this delegates the handling of all the details to cabal. So specifically this should work:

cabal install doctest --overwrite-policy=always && cabal build && cabal repl --build-depends=QuickCheck --build-depends=template-haskell --with-ghc=doctest

[sjakobi]

What I assume is that you just need to pass -XHaskell2010 to doctest.

Thank you, @sol! I had totally forgotten to check how these doctests are invoked.

I still think that with doctest-0.20.0 the most reliabl way to use doctest is via cabal repl.

This is good to know, but I'd really to prefer to be able to run the tests with just cabal test. The main problems I see with the cabal repl command is that I'd need to communicate this with contributors, and I'd also need to customize the CI configs which I usually just generate with haskell-ci.

I'd love to have good support for doctests built into cabal though. Maybe this cabal repl command or something like it can be built into cabal as a part of a new type: doctest test type?!