Re: [Haskell-cafe] Proposal: Shorter Import Syntax

On 2015-06-07 at 02:18:47 +0200, Anthony Cowley wrote:

On Sat, Jun 6, 2015 at 6:35 PM, Herbert Valerio Riedel wrote:

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On 2015-06-04 at 23:36:01 +0200, Anthony Cowley wrote:

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On Jun 4, 2015, at 5:22 PM, Herbert Valerio Riedel wrote:

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On 2015-06-04 at 23:05:42 +0200, Alexey Shmalko wrote: If I understand correctly, the initial proposal was to enable the new syntax by default and it mustn't break any code (full backward-compatible).

That would be a departure from how language extensions were handled in GHC in the past afaik, and if there's no language pragma to toggle this new syntax, Cabal has no way to know that a new language syntax is required and that thereby needs exclude (not implemented yet) the affected package versions from the install-plan configuration space.

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I can't parse your last sentence.

I'll rephrase: Extending the syntax GHC accepts beyond H2010 w/o a language flag (but if I understood you correctly, you stated that there's gonna be an associated language pragma) is not desirable, since you wouldn't be able communicate that requirement to Cabal via the other-extensions/default-extensions mechanism (albeit that information is currently not used early enough to affect the install-plan solver, see also [1])

While I'm mildly against having this new syntax enabled in GHC's default-mode (i.e. when neither -XHaskell2010 nor -XHaskell98 is set), I'm strongly against enabling it implicitly via -XHaskell2010, as when you request -XHaskell2010 you want GHC ideally to tell you when you're using syntax beyond H2010.

Just to be clear, that ideal is not reality, is it? I can pass ghc -XHaskell2010 and use syntactic extensions in my programs today (e.g. LambdaCase).

*Only* if you also explicitly enable them, e.g. if you have a module which only contains {-# LANGUAGE Haskell2010 #-} and let's assume for simplicity there are no additional -X* flags passed via the CLI to GHC, then you can't use syntactic extensions like LambdaCase; i.e. the following module {-# LANGUAGE Haskell2010 #-} module M1 where f :: () -> () f = \case () -> () when compiled with `ghc -Wall -c M1.hs` is rightfully rejected with: M1.hs:6:5: parse error: naked lambda expression '' However, if an additional `{-# LANGUAGE LambdaCase #-} is added, GHC compiles it w/o any complaint.

I can additionally pass ghc -Wall and not hear about it. So I think what you're writing here is a proposal for a new GHC feature that restricts available syntactic extensions in the presence of a language standard flag?

Maybe I was unclear, but I didn't mean that -XHaskell2010 should enable additional warnings for additionally *requested* language extensions. If I write {-# LANGUAGE Haskell2010 #-} {-# LANGUAGE LambdaCase #-} {-# LANGUAGE DefaultSignatures #-} {-# LANGUAGE RecordWildcards #-} into a module, I explicitly state that the code requires Haskell2010 syntax *with* additionally those 3 syntax extensions enabled (which btw are syntax extensions that afaik don't break any legal Haskell2010, and hence could be enabled by default -- but GHC doesn't enable them by default either!). There's obviously no point in warning if I really make use of those additional language extension, as that's what the pragma-mechanism is for in the first place.

The interplay between things like Haskell2010 and other language extensions isn't great. It would be nice to have a better story, but I think that really is a separate issue wherein you want Haskell2010 to act like a meta-pragma to ensure that the file really is Haskell2010 compatible. I am not volunteering to make that happen :)

To be clear, I only meant to say that the presence of -XHaskell2010 (or equivalently, {-# LANGUAGE Haskell2010 #-}) *without* any additional -X flags or LANGUAGE-pragmas is supposed to enable only the minimum required syntax extensions to provide the Haskell2010 syntax, as any syntax beyond Haskell2010 ought to be provided via -X/LANGUAGE pragmas. See also https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/bugs-and-inf... which states that | By default, GHC mainly aims to behave (mostly) like a Haskell 2010 | compiler, although you can tell it to try to behave like a particular | version of the language with the -XHaskell98 and -XHaskell2010 flags. and fwiw GHC's default-mode is not the same as -XHaskell2010