guide: Basic nix

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posts/guides/basic-nix-haskell.md

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+---
+title: Nix and Haskell
+date: 2018-09-20
+---
+
+The simplest way you can use haskell and nix together is by calling
+`callCabal2nix` on the cabal file and using the resulting artifacts.
+
+I usually create two files `release.nix` and `default.nix`. At least when
+developing, I avoid using the `cabal2nix` tool and instead let nix handle
+calling it for me. If I ever published an actual package, I would probably
+provide the autogenerated `default.nix` and nothing else.
+
+**Build instructions** are managed by `default.nix`. At its simplest it's just
+a call to `callCabal2nix`, but might contain for example some extra cabal flags
+or adding patches.
+
+```nix
+# default.nix
+{ haskellPackages }:
+
+haskellPackages.callCabal2nix "foo" ./. {}
+```
+
+**Pinning** of haskell packages and system libraries are managed by this file.
+Nixpkgs requires a bootstrap `<nixpkgs>` in scope so that we can pin to a
+specific commit hash.
+
+```nix
+# release.nix
+{ pkgs ? import <nixpkgs> {} }:
+
+# No pinning yet
+pkgs.callPackage ./default.nix {}
+```