Avoid Data.List.{head,tail}

[Bodigrim]

CLC has approved the proposal to add {-# WARNING #-} to Data.List.{head,tail} (haskell/core-libraries-committee#87). It means that usage of head and tail will emit compile-time warnings.

This patch eliminates the only usage of head in parsec.

[phadej]

TBH, I'd rather disable the warning. Newer version is not clearer at all.

[Bodigrim]

No problem, warning disabled.

[phadej]

I'll hold merging this until ghc-proposals/ghc-proposals#541 is reviewed and hopefully implemented. Disabling all warnings is too blunt.

I feel Data.List.head/tail warning/deprecations was rushed.

[Bodigrim]

I'm not a fan of disabling warnings myself. I've made another attempt, which not only avoids head but also hopefully has a benefit of being more clear than the original. How does it look now?

[Bodigrim]

@phadej any chance to look at this please?

FWIW I do not plan to backport the {-# WARNING #-} into ghc-9.6 branch, which should give proponents of more granular warnings plenty of time to implement them.

[phadej]

has a benefit of being more clear than the original. How does it look now?

I disagree it being clearer. I'll try myself.

Is there a hurry?

[Bodigrim]

Is there a hurry?

I'd be very grateful if we make some progress here, parsec is the last stumbling block remaining, other boot packages are already compliant.

[phadej]

other boot packages are already compliant.

Is there a requirement for them [boot packages] to be warning free? What set, -Wall? Why?

[Bodigrim]

Yes.

[phadej]

Why? That's stupid. There's nothing wrong in parsec. Why I need to change it. Yeah, sure head is bad, but it was fine here for 10 years, and you are not even going to remove it from Prelude in any foreseeable future. Are you?

[Bodigrim]

It's not me setting rules here, just hadrian/build --flavour=validate barks at any boot library warning.

At the moment I have no plans to remove Prelude.head indeed.

[phadej]

I complained to GHC folks: https://gitlab.haskell.org/ghc/ghc/-/issues/22729 I'm very frustrated by this.

I made the change in #164 which I'm more happy with structurally. (I don't find your version clearer that what is in master)

[Bodigrim]

Thanks @phadej, I'm very happy with this resolution.