feat(tvix/serde): add an example application to the project r/5785

This shows how people can use tvix_serde to deserialise configuration
structs for their programs from Nix code.

Change-Id: I71bf4e03dce19dddafe67dd729b4e4b10719a739
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7945
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

3 files changed, 57 insertions, 0 deletions

diff --git a/tvix/serde/.skip-subtree b/tvix/serde/.skip-subtree
new file mode 100644
index 0000000000..21b2d0d358
--- /dev/null
+++ b/tvix/serde/.skip-subtree
@@ -0,0 +1 @@
+The foods.nix can not be read by readTree.
diff --git a/tvix/serde/examples/cfg-demo.rs b/tvix/serde/examples/cfg-demo.rs
new file mode 100644
index 0000000000..8d31c9fb04
--- /dev/null
+++ b/tvix/serde/examples/cfg-demo.rs
@@ -0,0 +1,34 @@
+//! This program demonstrates how to use tvix_serde to deserialise
+//! program configuration (or other data) from Nix code.
+//!
+//! This makes it possible to use Nix as an embedded config language.
+//! For greater control over evaluation, and for features like adding
+//! additional builtins, depending directly on tvix_eval would be
+//! required.
+use serde::Deserialize;
+use std::collections::HashMap;
+
+#[derive(Debug, Deserialize)]
+enum Flavour {
+    Tasty,
+    Okay,
+    Eww,
+}
+
+#[derive(Debug, Deserialize)]
+struct Data {
+    name: String,
+    foods: HashMap<String, Flavour>,
+}
+
+fn main() {
+    // Get the content from wherever, read it from a file, receive it
+    // over the network - whatever floats your boat! We'll include it
+    // as a string.
+    let code = include_str!("foods.nix");
+
+    // Now you can use tvix_serde to deserialise the struct:
+    let foods: Data = tvix_serde::from_str(code).expect("deserialisation should succeed");
+
+    println!("These are the foods:\n{:#?}", foods);
+}
diff --git a/tvix/serde/examples/foods.nix b/tvix/serde/examples/foods.nix
new file mode 100644
index 0000000000..c8733cd3ef
--- /dev/null
+++ b/tvix/serde/examples/foods.nix
@@ -0,0 +1,22 @@
+# This is content for the `Data` struct, written in intentionally
+# convoluted Nix code.
+let
+  mkFlavour = flavour: name: {
+    inherit name;
+    value = flavour;
+  };
+
+  tasty = mkFlavour "Tasty";
+  okay = mkFlavour "Okay";
+  eww = mkFlavour "Eww";
+in
+{
+  name = "exhaustive list of foods";
+
+  foods = builtins.listToAttrs [
+    (tasty "beef")
+    (okay "tomatoes")
+    (eww "olives")
+    (tasty "coffee")
+  ];
+}