Document that default-language defaults to Haskell98

[alt-romes]

I believe in the last Cabal dev meeting we agreed that it would be better for Cabal to default to Haskell98 when no default-language is specified in the package. This is already the current behaviour, however, I believe it is undocumented.

This ticket tracks documenting that not specifying a default-language makes Cabal default to Haskell98

[ffaf1]

I was going to update the docs (and manage the relevant tickets), when I noticed occourrences of defaultLanguge = which are not Haskell 98. First one in repl

cabal/cabal-install/src/Distribution/Client/CmdRepl.hs

Lines 304 to 309 in 46a7bfc

	lBuildInfo =
	emptyBuildInfo
	{ targetBuildDepends = [baseDep]
	, defaultLanguage = Just Haskell2010
	}
	baseDep = Dependency "base" anyVersion mainLibSet

Second one in withContextAndSelectors, which is used in a number of places (build, run, repl itself).

cabal/cabal-install/src/Distribution/Client/ScriptUtils.hs

Lines 374 to 377 in 46a7bfc

	executable' =
	executable
	& L.buildInfo . L.defaultLanguage %~ maybe (Just Haskell2010) Just
	& L.buildInfo . L.options %~ fmap (setExePath exePathRel)

[alt-romes]

@ffaf1 Oh dear...

What do you think is the main default, the one by which more things are determined?
The actual option passed to GHC defaults to Haskell98, but if the option is set to Haskell2010 before then that would be the "main" default.

Could you investigate to determine if actually we use Haskell2010 by default in most places?

An easy way could be to write a simple package without default-language, and print the ghc invocations with -v2 and grep for the Haskell version passed to them.

[ffaf1]

-v2 only mentions Haskell98, as you correctly guessed.

I have created a draft (#9675) to see if nothing breaks changing 2010→98. If everything is green, steps will be:

• bless Haskell98 as a default
• add a non-"Hackage will refuse this package" cabal check warning
• update documentation to reflect the default

[ulysses4ever]

Why default to Haskell98, which, I believe, is superceded by Haskell2010? 2010 is the current standard. Is there any context one can think of where 98 is preferable to 2010?

[ffaf1]

Nope there isn't. My idea was just to: a) get $BLESSED in one place and b) document current behaviour.

[ulysses4ever]

Yeah, having ^a^ consistent global default would be a big step forward regardless of the actual value.

[alt-romes]

Why default to Haskell98, which, I believe, is superceded by Haskell2010? 2010 is the current standard. Is there any context one can think of where 98 is preferable to 2010?

I'm not sure to what extent users are writing cabal packages without default-language fields, however, if they are, changing the default from Haskell98 to Haskell2010 may break things.

In having no reason to change the default (which seems to be rarely used), I don't think we should. Haskell98 is already the implicit default, so we simply document it as such.

[ffaf1]

CI passed meanwhile, which most likely means that most likely those Haskell2010 were overwritten along the way.