Abstract:

This video introduces Coalton, a statically typed functional programming language embedded within Common Lisp, addressing the desire for a more complete type system beyond Common Lisp's dynamic nature and SDCL's type inference. Leveraging Common Lisp's powerful macro system, Coalton compiles its code directly into Common Lisp, enabling rigorous compile-time type checking. The video demonstrates defining functions with explicit type signatures, illustrating how Coalton catches type errors at compile time. It also explores the interoperability between Coalton and Common Lisp, detailing how to call Common Lisp functions from Coalton (requiring explicit type declarations for Common Lisp objects) and how to call Coalton functions from Common Lisp (with considerations for polymorphic types). The powerful concept of Algebraic Data Types (ADTs) is also showcased through the example of a binary-tree. Ultimately, Coalton offers Common Lisp programmers the flexibility to selectively apply static typing to critical software components, balancing Common Lisp's expressive freedom with enhanced type safety.

Exploring Coalton: Statically Typed Functional Programming in Common Lisp

• 0:00 A Complete Type System for Common Lisp: Common Lisp, while dynamic, benefits from SDCL's type inference. Coalton is introduced as a way to bring a complete, Haskell-like type system to Common Lisp.
• 0:24 Coalton: Embedded and Statically Typed: Coalton is a statically typed functional programming language embedded within Common Lisp, utilizing its macro system for deep language modification.
• 0:34 Setting Up Coalton: To use Coalton, one must load coalton and coalton-prelude libraries within a new package, notably without loading standard Common Lisp symbols directly into the Coalton environment.
• 0:50 Defining Functions with Types: Coalton functions are defined within a coalton-toplevel block. An example is-even function demonstrates declaring its type signature (integer to boolean) before its implementation.
• 1:10 Compile-Time Error Checking: Coalton code compiles to Common Lisp. Critically, type errors (e.g., trying to divide by a string) are caught at compile time, preventing runtime failures.
• 2:33 Calling Common Lisp from Coalton: Coalton can call Common Lisp functions (e.g., cl:concatenate for string concatenation). However, types for Common Lisp objects must be explicitly specified using the lisp operator.
• 3:21 Calling Coalton from Common Lisp: Simple Coalton functions (without polymorphic types) can be called directly from Common Lisp as they compile to standard defun forms.
• 3:50 Polymorphic Types and Export: When a Coalton function uses polymorphic types (e.g., Integral class), its Common Lisp compilation includes additional parameters, making direct calls from Common Lisp more complex. It's recommended to avoid polymorphic types or provide specific non-polymorphic definitions for functions intended for Common Lisp use.
• 4:26 Algebraic Data Types (ADTs): Coalton supports ADTs. An example binary-tree is defined with Leaf and Node constructors, and a bt-height function uses pattern matching to calculate tree height.
• 5:47 Modularity and Programmer Freedom: Coalton's primary advantage is its seamless integration with Common Lisp. Programmers can choose to implement critical or complex parts of their software in Coalton for enhanced static type safety, while utilizing standard Common Lisp for other modules, leveraging its modularity and existing type inference capabilities.