Tweaks to cabal nix idea

content/ideas/cabal-nix-workflow.md

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+---
+title: Haskell specific tooling for working with nix projects
+---
+
+[Nix](https://nixos.org/nix/) is becoming more and more widely used as a way to
+manage package dependencies.  This is despite the approach being quite low level
+and difficult to use. There are very few layers of abstraction which isolate
+less experienced uses from the internal workings of the nix machine.
+
+There are currently three main ways in which people use nix and Haskell together.
+All of these have different benefits and tradeoffs.
+
+1. Using `stack` with nix (somewhat common)
+2. Using `cabal` with the nix option (very uncommon)
+3. Using nix directly (the most common)
+
+To take each option in turn.
+
+1.  only uses nix to manage non-haskell dependencies. This is clearly not ideal
+    as we can't make use of the binary caching or anything else which is great
+    about nix.
+
+2.  is quite simple minded currently and relies on the presence of an already
+    generated `shell.nix` file. When the option is set several commands are run
+    in this shell instead of using cabal's normal dependency management.
+
+3.  The most flexible option is to invoke `cabal2nix` yourself and then
+    manipulate the environment using `nix-shell` but there are several
+    redundancies in this approach such as having to regenerate the `shell.nix`
+    file every time your cabal file changes.  It is also quite low level and
+    requires in-depth knowledge about how nix works. We want to abstract away
+    from this.
+
+However, the ideal tool doesn't yet exist. We want a tool that has the following philosophy:
+Nix, you are responsible for provisioning the correct environment but I will take
+care of the all important aspects of the build.
+
+The user provides a declarative specification of their necessary environment (by a cabal file
+or some other means), then when a user runs a command, nix provisions this
+environment and then the tool runs the haskell specific commands necessary to
+build the package locally.
+
+As an exemplification of this, using workflow (3), by default invoking `cabal2nix --shell`
+will generate a nix expression which loads both the build and test dependencies into the
+environment. It is not usual for the test dependency tree to quite a bit larger than the
+build dependency tree. Ideally, when a user runs "cabal build", cabal should enter
+a nix shell with the appropriate build dependencies for building whichever component
+it wants to build and no more. Similarly, "cabal test" should enter an
+environment with test dependencies. It is currently possible to achieve this
+for benchmarking dependencies by the somewhat archaic `nix-shell --argstr doBenchmark true`.
+
+Some more possible angles to explore are:
+
+* In a `cabal.project` file we can specify additional local dependencies.
+In `--nix` mode, these should turn into overrides of the local package set and `nix` should
+build them.
+
+* There should be an easy way to "pin" a nixpkgs version so that builds are reproducible.
+This could take the form of specifying a hash directly of a nixpkgs commit or more indirectly
+such as specifying a `lts` version (with an appropriately generated package set) and so on.
+
+* `cabal build --nix -w ghc-8.0.2` should modify the environment to provision the
+  8.0.2 package set rather than rely on the user to have already installed the
+  compiler locally.
+
+* [Extending `cabal2nix` to work with `cabal.project` files.](https://github.com/haskell-org/summer-of-haskell/pull/45#issuecomment-361255425)
+
+This is more of a framework for a project proposal rather than a concrete idea,
+and there are many more angles to explore. A successful proposal will need to
+flesh out in detail what would be necessary to implement one or perhaps two of
+these ideas:
+
+- <https://github.com/haskell-org/summer-of-haskell/pull/45>
+- <https://github.com/Gabriel439/haskell-nix/>
+- <https://docs.haskellstack.org/en/stable/nix_integration/>
+- <https://www.haskell.org/cabal/users-guide/nix-integration.html>
+- <https://github.com/jyp/styx>
+- <https://nixos.org/nix-dev/2016-September/021765.html>
+
+**Difficulty**: Advanced
+
+**Mentors**: Matthew Pickering, Will Fancher