Tsonnet #17 - Indexing a String: From copy-paste to unification

How Tsonnet learned to index strings

Welcome to the Tsonnet series!

If you're just joining, you can check out how it all started in the first post of the series.

In the previous post, we added support for late binding, and a little bit of Jsonnet's inconsistency rant:

Tsonnet #16 - Late binding and Jsonnet inconsistency

We made array access by index possible, but we are still missing another indexable expr variant: String.

Let's add index access capability to it.

Photo by Maksym Kaharlytskyi on Unsplash

Getting string characters

The simplest test sample:

local name = "Tsonnet";
name[0]

$ jsonnet samples/strings/get_char_zero.jsonnet
"T"

The index out of bounds case:

local name = "Tsonnet";
name[1234]

The non-indexable index value:

local str = "Tsonnet";
local index = false;
str[index]

The Jsonnet error messages hurt my eyes:

$ jsonnet samples/errors/string_index_out_of_bounds.jsonnet
RUNTIME ERROR: Index 1234 out of bounds, not within [0, 7)
	samples/errors/string_index_out_of_bounds.jsonnet:2:1-11	$
	During evaluation

$ jsonnet samples/errors/string_index_not_int.jsonnet
RUNTIME ERROR: Unexpected type boolean, expected number
	samples/errors/string_index_not_int.jsonnet:3:1-11	$
	During evaluation

Let's get this working:

$ dune exec -- tsonnet samples/strings/get_char_zero.jsonnet
samples/strings/get_char_zero.jsonnet:2:0 Expected "Array", found "String"

2: name[0]
   ^^^^^^^

With a bit of copy and paste from the array implementation, it's easily adaptable. It's just a matter of using the String equivalent functions accordingly to get the values given an index:

And now it works like a charm. Simple as that:

$ dune exec -- tsonnet samples/strings/get_char_zero.jsonnet
"T"

$ dune exec -- tsonnet samples/errors/string_index_out_of_bounds.jsonnet
samples/errors/string_index_out_of_bounds.jsonnet:2:0 Index out of bounds. Trying to access index 1234 but "name" length is 7

2: name[1234]
   ^^^^^^^^^^

$ dune exec -- tsonnet samples/errors/string_index_not_int.jsonnet
samples/errors/string_index_not_int.jsonnet:3:0 Expected Integer index, got Bool

3: str[index]
   ^^^^^^^^^^

The cram tests:

Looking good, but we can make it even better.

The indexable abstraction

There is a lot of duplication that we can encapsulate in a well crafted Indexable module:

This module provides polymorphic functions (length, nth, and get) that operate on indexable expression types, specifically the Array and String variants of the expr type.

And with that we can remove all the duplicated code in the interpreter:

The code is now cleaner and easier to understand, isn't it?

Conclusion

With string indexing now implemented, Tsonnet can handle both arrays and strings as indexable types. The refactoring into an Indexable module not only eliminated code duplication but also created a clean abstraction that will make it easier to add support for other indexable types in the future. The error handling remains consistent and informative, helping developers understand what went wrong when things don't work as expected.

The diff with all the changes can be seen here.

There's still something bothering me about testing, but I will leave it as a surprise for the next post.

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