Add a command line option to print the Pancake news file

[IlmariReissumies]

The flag is --pancake_news. Close #1469

[mktnk3]

I suppose this assumes that NEWS.md is already updated for the feature being added. I wonder if there is a way to ensure this is done. For example, requiring a non-empty (markdown?) string describing the changes that gets concatenated to the content of NEWS.md for both command line printing and updating the file... won't work probably :D Not sure how to go about it, but maybe it's worth stating that "NEWS.md should be updated" somewhere...

[IlmariReissumies]

Yes that's a good point, this works so long as we're disciplined about updating NEWS.md when merging Pancake PRs with user-facing changes. But it doesn't do anything to enforce this discipline.

I think that's difficult to enforce entirely automatically, because there's no good check for figuring out which changes are user-facing. But there are things that would probably help. Maybe a github hook that checks if PRs make any changes in the pancake/ directory and encourages NEWS.md updates or makes you tick a box that it's not needed?

Even so I guess this at least gives people a lower bound on the features they have :)

[KurtWu10]

Is there a simple way to make it more machine-readable?

The motivation for the issue is to improve the compatibility of Pancake programs, i.e. to enable programmers to write a single Pancake program that selects language features based on availability, similar to feature test macros in C. In the existing form, I guess I would write another wrapper to retrieve the latest date in the news, and then use the date to query a pre-defined list of dates to know the supported features.

[mktnk3]

Even so I guess this at least gives people a lower bound on the features they have :)

I can't quite decide whether that is a plus or a potential cause for confusion, but either way, that only won't be good enough to close the issue. I guess we can keep looking into more complete solutions.

@KurtWu10 would you like to add something on the motivation to the issue #1469 ?

[IlmariReissumies]

Is there a simple way to make it more machine-readable?

Don't see why not. For example, the NEWS.md could assign a tag to each new feature, say varshifts for variable-length shifts. You could then ask the binary whether certain tags are present with (say) cake --pancake_news varshifts, which would print true or false accordingly. Would that work for you?

[KurtWu10]

Yes, that would work.

[IlmariReissumies]

Okay, that's added now! Please check if it looks good to you :)

I added annotations for enabling and disabling features to NEWS.md, see here: https://github.com/CakeML/cakeml/blob/pan_news/pancake/NEWS.md

newsScript now parses these tags does some basic well-formedness checks on them. The compiler can query them with e.g. cake --pancake_feature=varshifts, which will print true if the most recent varshift entry in NEWS.md says "Feature enabled".

[KurtWu10]

Thanks for the work, it looks helpful. I noticed that some important features are not in the NEWS.md file, e.g. function inlining of PR 1323.

[IlmariReissumies]

Good catch! I added one retroactively.

[IlmariReissumies]

In the most recent commit I dropped the --pancake_news option, since @myreen pointed out that it's inadvisable to have huge one-time-use strings in memory. Now the PR only includes the option to query feature tags with --pancake_feature=<feature tag>. This seems closer to the spirit of #1469 and @KurtWu10's ideas anyway :)