cabal repl doesn't respect flag constraints for dependencies

[turion]

Describe the bug

The bug was originally discovered in a special case here: turion/essence-of-live-coding-tutorial#3
In that case, the application has a dependency to gloss, which has the flags GLUT and GLFW. We tried setting these flags with cabal repl --constraint="gloss +GLFW -GLUT" and by writing them down in cabal.project.local. Nevertheless, always the GLUT backend is selected.

A clear and concise description of what the bug is.

To Reproduce

git clone https://github.com/turion/essence-of-live-coding-tutorial/
cd essence-of-live-coding-tutorial
nix-shell # or instead install GLFW, GLUT, OpenGL
cabal v2-repl --constraint="gloss +GLFW -GLUT"
:livelaunch
# Exit the repl by pressing Ctrl+D

You will see an error message saying something with GLUT.

[hvr]

I've tried to reproduce this with cabal 3.2 but from my POV, cabal v2-repl --constraint="gloss +GLFW -GLUT" appears to be honored (you notice this if you change the flag settings and cabal hasn't cached yet the different flag configuration it'll rebuild a different instance of `gloss` and its rev-deps); in fact, if I invoke cabal v2-repl --constraint="gloss -GLFW -GLUT" `gloss` would fail to build because it include a CPP conditional that will trigger a compile error if neither the GLFW nor the GLUT backend have been compile-time enabled: ,---- | $ cabal v2-repl --constraint="gloss -GLFW -GLUT" | Resolving dependencies... | Build profile: -w ghc-8.10.2 -O1 | In order, the following will be built (use -v for more details): | - gloss-1.13.1.2 (lib) (requires build) | - essence-of-live-coding-tutorial-0.2.2 (exe:essence-of-live-coding-tutorial) (first run) | Starting gloss-1.13.1.2 (lib) | Building gloss-1.13.1.2 (lib) | | Failed to build gloss-1.13.1.2. | Build log ( | /home/hvr/.cabal/logs/ghc-8.10.2/gloss-1.13.1.2-2d63cd816ebde03d84ef0dbc18eee2e68e2f3e34a8344cb19fd6ff5dc5cdfbcb.log | ): | Configuring library for gloss-1.13.1.2.. | Preprocessing library for gloss-1.13.1.2.. | Building library for gloss-1.13.1.2.. | | Graphics/Gloss/Internals/Interface/Backend.hs:31:2: error: | error: #error No default backend defined | #error No default backend defined | ^~~~~ | | | 31 | #error No default backend defined | | ^ | | Graphics/Gloss/Internals/Interface/Backend.hs:1:1: error: | `cc' failed in phase `C pre-processor'. (Exit code: 1) | | | 1 | {-# LANGUAGE CPP #-} | | ^ | cabal: Failed to build gloss-1.13.1.2 (which is required by | exe:essence-of-live-coding-tutorial from | essence-of-live-coding-tutorial-0.2.2). See the build log above for details. `---- PS: I don't have nix-shell (as I don't use Nix myself), so I couldn't properly reproduce your instructions, however from the looks of it I don't think it would have made any significant difference

[turion]

Thank you for trying to reproduce! This is the behaviour I would expect. Then it must be an interaction with Nix. The nix-shell supplies a particular version of gloss to GHC, namely one built with default flags (-GLFW +GLUT), and it seems like cabal repl (or in fact all of cabal? not sure) doesn't try to do anything about that.

[hvr]

Then it must be an interaction with Nix. The nix-shell supplies a particular version of `gloss` to GHC, namely one built with default flags (`-GLFW +GLUT`), and it seems like `cabal repl` (or in fact all of cabal? not sure) doesn't try to do anything about that.

Aha! That's actually a good point! If your dependency is already contained in the global or user package-db (which is what you'd end up e.g. when you install Debian-packaged Haskell packages via `apt-get` -- or like nix-shell seems to do), then those packages will be preferred. And the problem with flags in "(pre)installed" (this is the technical term for those and a keyword for constraints) packages is that the flag settings aren't recorded in in ghc-pkg's metadata, so cabal has no way to know that a preinstalled package doesn't have the requested flag. so what you *can* do, is to explicitly tell cabal via the constraint specification to force using a non-preinstalled from-"source" package: long story short, try adding a

…

--constraint "gloss source" flag, in addition to your

--constraint "gloss +GLWF -GLUT" flag, and see if that makes any difference. For the record, these special constraint-types are mentioned e.g. in https://cabal.readthedocs.io/en/3.4/setup-commands.html?highlight=source#cmdoption-runhaskell-Setup.hs-configure-constraint

[turion]

Yes, that resolves the cabal issue! Thanks