In a lot of programming discussions, especially those about Haskell or those about Lisp, types invariably come up as a feature (or a bug). Some even suggest static or dynamic types are most useful at different phases of a programming project. I've heard opinions go every which way with types. By and large, the concept of static types (to Haskell levels) and dynamic types (to Smalltalk levels) are typically spoken about as a sort of dichotomy.

Many technologies have tested this dichotomy; even very mainstream languages, like Python with type annotations and mypy. However, what I've found consistently lacking is the "completeness" of either side of the experience: either half-baked dynamicism buried in a static language, or type annotations that actually don't have predictable correctness guarantees—typically, type checkers in this arena are "best effort".

Coalton doesn't try to paper over Lisp as a dynamic language, or try to change the philosophy of Common Lisp code by restricting it, but instead invites static features as a separate, well-defined language within Lisp. Critically, though, inter-operation between Lisp and this separate language must be essentially free.

Coalton makes use of what many cite as Lisp's greatest power: the ability to do seamless metaprogramming and syntactic abstraction. Often, non-Lisp programmers, especially in forums like these, see these features get used for programming party tricks. (I myself have even engaged in such shenanigans, like saying macros are good for making DSLs like

    (rpn 1 2 + 3 *)        ; RPN notation embedded in Lisp

even though the next natural question would be, "Why would I ever actually use that?") In that regard, Coalton is implemented actually as quite sophisticated Lisp macros, without any separate (pre-)processors.

Though it has some sharp edges as an alpha release, we think Coalton satisfies the objective of bridging static and dynamic, and opens a very interesting and relatively under-explored paradigm of functional programming.

Coalton isn't the first to explore this space in earnest. In fact, Racket, Typed Racket, and Hackett have done so quite successfully. There are several others, as well.

Lastly—though I think it goes without saying—there's value in having an entire language buy in to some core ideas, because it means library authors and application developers can have solid expectations of one another. As a Lisp developer, though, I've witnessed that a clean, modular coding style allows many programming styles to participate with one another without the headache one might imagine.