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Hamish Mackenzie
5c25ce919f
* Delays cabal configure errors until after evaluation when possible.
* Avoids rerunning long `cabal configure` if we already know it will fail.
* Provides a way to get a component from a package or project using a `cabal` like reference.
Code using the `tool` functions will automatically use `getComponent`.
For `(hackage-package {...}).components.library` is also ok.
```
# Consider changing hackage-package use:
(pkgs.haskell-nix.hackage-package {...}).components.exes.something
(pkgs.haskell-nix.hackage-package {...}).getComponent "exe:something"
# For any cabal project:
project.hsPkgs.somepackage.components.exes.something
project.getComponent "somepackage:exe:something"
# or do it in two steps
(project.getPackage "somepackage").getComponent "exe:something"
```
The reason for the new function is that we cannot provide the attribute interface without knowing that packages are in the project first.
Here is how the `cabal configure` error output is handled:
* The `plan-nix` derivation builds even if `cabal configure` fails.
* When it fails, it copies `failed-cabal-configure.nix` to the `$out/default.nix` along with a copy of the `cabal configure` output.
* When `failed-cabal-configure.nix` is imported and used in any way it writes the `cabal configure` output with `__trace` so it will always be visible.
* Instead of a `plan` the imported nix contains a `configurationError` pointing the `cabal configure` output.
* The intermediate functions `configurationError` and bubble it up to where it is needed.
* `getPackage` returns a mostly empty proxy for a real package when there is a `configurationError`
* The `getComponent` function always returns a derivation, but the version in the proxy writes the `cabal configure` output to stdout and calls `exit 1` (so that it will never build).