Let me see if I understand what is going on here:

1. haskell-gi is in stackage LTS-16, so it is kept back at version 0.23.
2. gi-javascriptcore is not in stackage LTS-16, so it is just pulled from the latest version on hackage, which is currently 4.0.22.
3. gi-javascriptcore-4.0.22 requires haskell-gi-0.24.
4. However, gi-javascriptcore doesn't actually need haskell-gi-0.24, it works fine with haskell-gi-0.23.

This means that in order to build gi-javascriptcore in nixpkgs currently, we either need a whole bunch of overrides, or we just need to jailbreak gi-javascriptcore to build with an older version of haskell-gi?

The code in this PR is as follows:

gi-javascriptcore =
  # Remove these jailbreaks, when assert fails.
  assert (pkgs.lib.versionOlder super.haskell-gi-base.version "0.24");
  doJailbreak super.gi-javascriptcore;

This means that gi-javascriptcore will fail to build when the haskell-gi version is 0.24 or newer.

I don't have a strong opinion for or against this, but I do have a couple concerns:

1. If there was a revision to gi-javascriptcore-4.0.22 on Hackage, then this assert/doJailbreak could potentially not be needed, even if haskell-gi-base is still 0.23. This assert doesn't capture this possibility.

2. I wonder how an expression like this would deal with haskellPackages getting some overrides applied. For instance, if haskell-gi-base was overridden to a more recent version, then I guess this would still work?

   However, what if gi-javascriptcore was overridden to be an older version? Would that be okay as well?

   I guess what I'm mainly worried about is that doJailbreak is able to be disabled with the dontJailbreak function, but I imagine it is not as easy to remove the assert without directly editing the source file.

3. It might be nice to have a version of doJailbreak that specifically includes this assert. Something you could call like this:

   doJailbreakFor "haskell-gi-base" "older" "0.24" super.gi-javascriptcore

Let me see if I understand what is going on here:

1. `haskell-gi` is in stackage LTS-16, so it is kept back at version `0.23`.

2. `gi-javascriptcore` is not in stackage LTS-16, so it is just pulled from the latest version on hackage, which is currently `4.0.22`.

3. `gi-javascriptcore-4.0.22` requires `haskell-gi-0.24`.

4. However, `gi-javascriptcore` doesn't _actually_ need `haskell-gi-0.24`, it works fine with `haskell-gi-0.23`.

This means that in order to build gi-javascriptcore in nixpkgs currently, we either need a whole bunch of overrides, or we just need to jailbreak gi-javascriptcore to build with an older version of haskell-gi?

Yes, that is exactly right.

The code in this PR is as follows:

gi-javascriptcore =
  # Remove these jailbreaks, when assert fails.
  assert (pkgs.lib.versionOlder super.haskell-gi-base.version "0.24");
  doJailbreak super.gi-javascriptcore;

This means that gi-javascriptcore will fail to build when the haskell-gi version is 0.24 or newer.

I don't have a strong opinion for or against this, but I do have a couple concerns:

1. If there was a revision to `gi-javascriptcore-4.0.22` on Hackage, then this assert/doJailbreak could potentially not be needed, even if `haskell-gi-base` is still `0.23`.  This `assert` doesn't capture this possibility.

That is right. But my primary concern is to not have this jailbreak linger here forever.

2. I wonder how an expression like this would deal with `haskellPackages` getting some overrides applied.  For instance, if `haskell-gi-base` was overridden to a more recent version, then I guess this would still work?
   However, what if `gi-javascriptcore` was overridden to be an older version?  Would that be okay as well?
   I guess what I'm mainly worried about is that `doJailbreak` is able to be disabled with the `dontJailbreak` function, but I imagine it is not as easy to remove the `assert` without directly editing the source file.

That is a valid concern I hadn‘t thought about.

1. I think updating haskell-gi-base to a newer version would work because it uses super. So the assert happens against the old package. If someone where to fork nixpkgs and bump the version in hackage-packages.nix they would run into this assert, but that’s intentional.
2. I think downgrading gi-javascriptcore to an older version with an override might fail late in the build because of the jailbreak. But I don‘t think the assert increases the severity of that problem.

So my summary is. I agree that it might be a problem that you cannot override the assert, but I can‘t think of a situation where the assert would fail without that being intended.

3. It might be nice to have a version of `doJailbreak` that specifically includes this `assert`.  Something you could call like this:
   ```nix
   doJailbreakFor "haskell-gi-base" "older" "0.24" super.gi-javascriptcore
   ```

I am not a big fan of the stringly typed "older" in there, but yeah, that might be a good idea. In general I think it would be great to annotate overrides in configuration-common.nix with informations/asserts so that they can‘t get stale. But I haven‘t thought about the whole design space yet.

@GrahamcOfBorg build haskellPackages.reflex-dom

I agree that it might be a problem that you cannot override the assert, but I can‘t think of a situation where the assert would fail without that being intended.

Yeah, that's sort of the conclusion I came to as well. In the example in this PR here, whether you overrode haskell-gi-base to an older or newer version, or you overrode gi-javascriptcore to an older or newer version, I couldn't come up with a time when you would accidentally trigger the alert.

(Although I do think that we should provide a separate doJailbreakFor-like function, since most people would probably pass self.haskell-gi-base to the assert, instead of the correct super.haskell-gi-base.)

I am not a big fan of the stringly typed "older" in there, but yeah, that might be a good idea.

Ah, yeah, I completely agree.

Ping @peti to see how he feels about this.