Gradual Soundness: Lessons from Static Python
Posted on 28 June 2022.
It is now virtually a truism that every dynamic language adopts a gradual static type system. However, the space of gradual typing is vast, with numerous tradeoffs between expressiveness, efficiency, interoperability, migration effort, and more.
This work focuses on the Static Python language built by the Instagram team at Meta. Static Python is an interesting language in this space for several reasons:
It is designed to be sound.
It is meant to run fast.
It is reasonably expressive.
The static type system is a combination of what the literature calls concrete and transient types. Concrete types provide full soundness and low performance overhead, but impose nonlocal constraints. Transient types are sound in a shallow sense and easier to use; they help to bridge the gap between untyped code and typed concrete code. The net result is a language that is in active use, with high performance, inside Meta.
Our work here is to both formalize and assess the language. We investigate the language’s soundness claims and report on its performance on both micro-benchmarks and in production systems. In particular, we find that the design holds up the intent of soundness well, but the act of modeling it uncovered several bugs (including one that produced a segmentation fault), all of which have now been fixed.
We believe Static Python is a particularly interesting language for the gradual typing community to study. It is not based on virtuoso theoretical advances; rather, it takes solid ideas (e.g., from Muehlboeck and Tate’s Nom language), combines them well, and pays attention to the various affordances needed by practicing programmers. It therefore offers useful advice to the designers of other gradually-typed languages, in particular those who confront large code-bases and would like incremental approaches to transition from dynamic safety to static soundness. You can read much more about this in the paper.
As an aside, this paper is a collaboration that was born entirely thanks to Twitter and most probably would never have occurred withtout it. An aggrieved-sounding post by Guido van Rossum led to an exchange between Carl Meyer at Meta and Shriram, which continued some time later here. Eventually they moved to Direct Messaging. Shriram pointed out that Ben Greenman could investigate this further, and from that, this collaboration was born.