posts/working-with-nix-shells.md

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posts/working-with-nix-shells.md

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+# Working with Nix Shell
+
+Nix shells are quite useful for a number of different reasons, but the main reason is that it can provide you programs you want at specific versions.
+
+## Shells with `-p`
+
+Using `nix-shell` with `-p` can get you quickly set up with a shell with packages from NixPkgs installed.
+
+```bash
+# from your top level
+$ nix-shell -p hello
+
+# nix shell with hello installed
+$ which hello
+/nix/store/some-hash-hello-2.10/bin/hello
+$ hello
+Hello, world!
+$ exit
+exit
+
+# back to your top level shell
+```
+
+### Expressions as arguments
+
+But instead of only top level attributes, you can also put entire expressions in the `-p` argument. For example, with the [easy-purescript-nix](https://github.com/justinwoo/easy-purescript-nix/) project:
+
+```bash
+$ nix-shell \
+     -p 'let ep = import ./default.nix {}; in [ ep.purs ep.spago ]' \
+    --run 'which purs; which spago; purs --version; spago version'
+/nix/store/some-hash-purescript/bin/purs
+/nix/store/some-hash-spago/bin/spago
+0.13.3
+0.9.0.0
+```
+
+As mentioned before, we can use Nix shells to get specific versions of packages.
+
+```bash
+$ nix-shell -p '(import ./default.nix {}).purs' --run 'which purs; purs --version'
+/nix/store/some-hash-purescript/bin/purs
+0.13.3
+```
+
+```bash
+$ nix-shell -p '(import ./default.nix {}).purs-0_13_0' --run 'which purs; purs --version'
+/nix/store/some-hash-purescript/bin/purs
+0.13.0
+```
+
+## NixPkgs mkShell
+
+To work with Nix shells in some organized manner, you probably will want to use a source-controlled file defining a derivation. To make this concise and explicit, you can use `mkShell` from NixPkgs.
+
+```nix
+# shell.nix
+{ pkgs ? import <nixpkgs> {} }:
+
+pkgs.mkShell {
+  buildInputs = [ pkgs.hello ];
+}
+```
+
+And we can see this used like so:
+
+```nix
+$ nix-shell --run 'which hello; hello'
+/nix/store/some-hash-hello-2.10/bin/hello
+Hello, world!
+```
+
+## Links
+
+* Nix manual nix-shell: <https://nixos.org/nix/manual/#sec-nix-shell>
+* NixPkgs manual mkShell: <https://nixos.org/nixpkgs/manual/#sec-pkgs-mkShell>