How to override default-language to GHC2021 (for use with GHC9.2.2)

[maerten]

How to set default-language to GHC2021 instead of Haskell2010?
When using stack in a project, it sets the default-language in the generated cabal file to Haskell2010. Afaik, this disables some modules that would otherwise be enabled by default in GHC9.2.x (via the GHC2021 language extension).

Any suggestions on how to work around this behavior of stack?

[mpilgrem]

stack can use hpack (but does not need to) and hpack before version 0.35.0 assumes Haskell2010 by default. Very recently, the stack code on this repository was updated to use hpack-0.35.0. So you could upgrade to the unreleased version of stack, by stack upgrade --source-only.

EDIT: Alternatively, you could install locally version 0.35.0 of the hpack executable and then use stack --with-hpack <hpack executable> to use that version of hpack rather than the version 'built in' to your existing version of the stack executable (revealed by the command stack --hpack-numeric-version).

Alternatively, you could not use hpack at all. Delete the package.yaml file (which hpack uses to create a .cabal file) and specify the .cabal file directly, including your preferred default-language.

[maerten]

Thanks @mpilgrem, I did not know about hpack and package.yaml.

FYI: stack upgrade --source-only did not update stack, it said the newest version was already installed. This might be the case because I installed it with ghcup.

The trick with hpack worked perfectly though:

1. stack install hpack-0.35.0 --resolver=nightly-2022-05-05
2. stack --with-hpack=/Users/<username>/.local/bin/hpack ghci --test
3. the language: GHC2021 entry in package.yaml is now included in the generated cabal file

[maerten]

One additional question, can this setting be controlled via stack.yaml?

UPDATE: it seems that adding with-hpack: <path>/hpack to stack.yaml works

[mpilgrem]

Yes, that is correct - see the documentation on the .yaml file: https://docs.haskellstack.org/en/stable/yaml_configuration/#with-hpack

[carlwr]

I still get GHC2010 in the .cabal file, although stack now seems to ship with hpack >= 0.35.0.

Any simple workaround? (I'd prefer to keep having the .cabal file auto-generated.)

I was also a bit confused about GHC and GHCi not defaulting to the same base languages:

stack ghci
λ> :show language
base language is: GHC2021
with the following modifiers:
λ>

---

Project was created with

stack new --resolver=lts-20.24 proj

Verifying hpack >= 0.35.0

stack --hpack-numeric-version
0.35.1

Still:

grep default-language proj.cabal
  default-language: Haskell2010
  default-language: Haskell2010
  default-language: Haskell2010

(All stack commands above were run in the project dir.)

Versions:

`stack exec -- which ghc` --version
The Glorious Glasgow Haskell Compilation System, version 9.2.7

stack --version | head -n 1
Version 2.9.3, Git revision d0e984f00034552aac7c67675d49ef0807c10c7b (dirty) (112 commits) aarch64

# macOS version:
sw_vers -productVersion
13.4

[mpilgrem]

@LeuschkeTressa, if you specify language: GHC2021 in a package's package.yaml file, it should work - it does on my machine. EDIT: Stack's default template with stack new (new-template.hsfiles) does not specify a 'language' and, so, Hpack applies its default of Haskell2010.

[carlwr]

@LeuschkeTressa, if you specify language: GHC2021 in a package's package.yaml file, it should work

It does!

Thanks.

[carlwr]

For future readers:

Regarding (from my initial comment):

I was also a bit confused about GHC and GHCi not defaulting to the same base languages:

stack ghci
λ> :show language
base language is: GHC2021
with the following modifiers:
λ>

This inconsistency was previously reported as #5973.

b6d11f7 is intended to fix this. It was committed to release 2.11.1, which is a later release than what I am using.