cabal run should send info logging to stderr not stdout

[gbaz]

cabal run prints stuff like "up to date" to stdout. So running an executable and cabal run an executable give different output to stdout, which is no good when the executable is producing output for e.g. machine consumption. We should do our info logging to stderr.

[fgaz]

Related/duplicates: #5595 #4652 #4789

[jneira]

Not sure about that, maybe there are programs expecting the actual behaviour, i.e. both cabal and the progam output in stdout.
What about separate both outputs in a standard way (using -?), including a comment

See comment in #4789: #4789

[andreasabel]

See comment in #4789: #4789

@jneira, something went wrong; this does not link to a comment, only to the PR.

[jneira]

tbh i cant determine what is the relevant comment right now 😅

[pacien]

Note that Cabal already prints some of its warnings on stderr.
This is also the case for GHC warnings.

As a result, we get logs split between both stderr and stdout,
before the output of the target executable that is being run.

The output is someting like this (prefixes and blank lines added):

> cabal run print-csv

STDERR: Warning: The package list for 'hackage.haskell.org' does not exist.
STDERR: Run 'cabal update' to download it.

STDOUT: Resolving dependencies...
STDOUT: Build profile: -w ghc-9.0.2 -O1
STDOUT: [...]

STDERR:
STDERR: lib/[...[:19:1: warning: [-Worphans]
STDERR:    Orphan instance: instance CSV.ToField URI
STDERR:   |
STDERR:23 | instance CSV.ToField URI where
STDERR:   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^...

STDOUT: Linking [...] ...

STDOUT: [CSV/JSON/stuff I would actually like to capture or redirect]

---

jneira:

Not sure about that, maybe there are programs expecting the actual behaviour,
i.e. both cabal and the progam output in stdout.

Given that there is no way of separating the output of Cabal, GHC and the
target executable, currently happening on both stderr and stdout, I doubt that
anyone is making use of the current behaviour.