Tsonnet #29 - Making inner references work

Source: Dev.to

Welcome to the Tsonnet series!

If you’re not following along, check out how it all started in the first post of the series.

In the previous post, I added ppx_deriving.show to help debug the AST and centralized configuration.

Tsonnet #29 – Making inner references work

What we’re building

Here’s the sample file we need to handle:

// samples/tutorials/inner-reference.jsonnet
{
  Martini: {
    local drink = self,
    ingredients: [
      { kind: "Farmer's Gin", qty: 1 },
      {
        kind: 'Dry White Vermouth',
        qty: drink.ingredients[0].qty,
      },
    ],
    garnish: 'Olive',
    served: 'Straight Up',
  },
}

The tricky bit is the local drink = self inside the Martini object. The variable drink holds a reference to the object itself, and we then use it to access drink.ingredients[0].qty. This is a self‑reference through an intermediate variable.

The expected output (as a cram test) is:

diff --git a/test/cram/tutorials.t b/test/cram/tutorials.t
index b9c1f6f..9fe544c 100644
--- a/test/cram/tutorials.t
+++ b/test/cram/tutorials.t
@@ -79,3 +79,15 @@
       "served": "Tall"
     }
   }
+
+  $ tsonnet ../../samples/tutorials/inner-reference.jsonnet
+  {
+    "Martini": {
+      "garnish": "Olive",
+      "ingredients": [
+        { "kind": "Farmer's Gin", "qty": 1 },
+        { "kind": "Dry White Vermouth", "qty": 1 }
+      ],
+      "served": "Straight Up"
+    }
+  }

Extending the AST with variable references

We already have Self and TopLevel for object scopes. Now we need a way to reference objects through variables:

@@ -76,6 +76,7 @@ and object_entry =
 and object_scope =
   | Self
   | TopLevel
   | ObjVarRef of string
 [@@deriving show]

let dummy_expr = Unit

Objects also need their own environment. Before, we were storing just an… (the article continues with the implementation details).