Is there any cabal port that actually sets DISTFILES ?

There are helpers that are for qmake, cmake, meson, they are all in bsd.options.mk.

I would prefer that all helpers be defined in one place, instead of having some magically happen when you add a feature.

This is not right, see Mk/bsd.port.mk lines 5186+ to see what to add to that variable.

Remove trailing /

Please remove this, we already have a standard way for ports to "do" things when they need, do not invent a new one
Ports should have their own post-install* targets.

Please do not piggy-back on an existing target, create your own.

I saw that, but don't get what's wrong with this line. Should I prepend some integer before the target name?

Wrong place in the Makefile. Should happen before OPTIONS_DEFINE. See Chapter 15. Order of Variables in Port Makefiles.

Shouldn't this just be removed?

Sure, I will remove it when landing this.

You should check if cabal_ARGS is empty and set IGNORE= when it is like other USES to prevent someone from adding bogus USES=cabal:foobar things.

It might be good to pass through MAKE_ENV too. This is important for stuff like BINARY_ALIAS etc. to work correctly.

Ports might want to overwrite do-build, so maybe guard it with .if !target(do-build). Same for do-install.

The new options helpers need to be documented in the header and added to _OPTIONS_FLAGS/_ALL_OPTIONS_HELPERS too.

Is this really needed when we already have the opt_USE helper?

These are in the wrong place now. See 15. Order of Variables in Port Makefiles.

What does MAN1PAGES do? With bsd.cabal.mk gone it does not seem to do anything.

Not sure about last part. ${opt}_CABAL_FLAGS works differently from other opt helpers - it appends some value in any case, so it actually handles _OFF case too.

As for ${opt}_USE_CABAL and ${opt}_EXECUTABLES - they don't need any handling for _OFF case.

It is not, but I'm hesistant to change that now.

Moreover, I find opt_USE to look untidy when there are many items and you have to list them separated by , instead of .

Adding them to just _ALL_OPTIONS_HELPERS directly should be fine then.

grep -r _ALL_OPTIONS_HELPERS Mk yields nothing. What's _ALL_OPTIONS_HELPERS and where is it located?

In bsd.options.mk. Update your ports tree to after rP499267.

Put the .if !defined(SKIP_CABAL_PLIST) around the target, no need to create an empty target.

Note that this is not needed, you already have <opt>_USE= cabal=foo.

If all you are doing is:

.if defined(<opt>_FOO)
FOO+= <opt>_FOO
.endif

all you need to do is add FOO to _OPTIONS_FLAGS.

Please, let's leave these 3 lines as they are. While opt_USE= xorg:xft,xwhatever looks nice, because there aren't many arguments for USE_XORG exists, the opt_USE= cabal:foo-1.2.3_4,bar-5.6.7_8,baz-9.10.11_12,... is awful, IMO. And these lines can be pretty large, in case some option pulls a bunch of Haskell packages.

Remove EXECUTABLES here.

Remove USE_CABAL here.

Note that this file can be replaced by having:

PORTEXAMPLES=   xmobar.config

in the port's Makefile.