When trying to build haskell packages from local source using nix (
source code you have used `git clone` to retrieve and have modified)
it is often annoying that small changes can trigger a lot of components
to be built unnecessarily. For instance if you change the code for
a test in a package it will often trigger builds of the library and
all its dependencies.
To avoid this problem we could manually set up `cleanSourceWith` calls
in nix to filter out files that are not need, but to do this every
component would be difficult and error prone.
This change automates this process using the information in the
packages `.cabal` file to filter the source of each component
automatically when using `callCabalPlanToNix` (we should be able to
add it to `callStackToNix` too).
## How does it work?
* A new version on `cleanSourceWith` is used that adds a `subDir`
argument. It allows descending into a subdirectory to compose with
cleaning the source.
* A new `--full` argument is passed to `plan-to-nix` by
`callCabalPlanToNix` telling it to output extra information about
the cabal packages in the plan (directories and filenames).
* `callCabalProjectToNix` uses the new `subDir` argument of
`cleanSourceWith` when setting the packageSrc.
* `comp-builder.nix` and `setup-builder.nix` use a new
`cleanCabalComponent` function to further clean the packageSrc
using the new information in the `package` and `component`.
## hpack wildcards
`package.yaml` files can include wild cards and these are not handled
yet (they do not map to anything in the `.cabal` file and so are lost).
They will need to be added manually to the appropriate field
(eg. `packages.xyz.package.dataFiles`)
If your hpack file does not contain wildcards or if you are willing to
add the wildcards entries manually in nix it is still useful to
enable cleaning on hpack.
For example:
```
packages.ghc-toolkit.package.cleanHpack = true;
packages.ghc-toolkit.components.library.extraSrcFiles = [
"genapply/**/**"
"boot-libs/**/**"
"ghc-libdir/**/**"
];
```
- Dev note about nixpkgs pin
- More information about selecting a GHC version for cabal projects
- A little bit more information about cross compiling, but this is still incomplete.
Hamish Mackenzie
Rodney Lorrimar