Forsp: A Forth+Lisp Hybrid Lambda Calculus Language

Original Article ↗ Hacker News Discussion ↗

Overall reception & goals

  • Many commenters find the Forsp idea “very cool” and conceptually elegant.
  • It’s seen as a minimal core that unifies Forth-like stacks with Lisp-like semantics via a CBPV-style lambda calculus.
  • Some view it as a candidate “more fundamental” layer beneath both Lisp and Forth, and as a neat discovery rather than just another syntax.

Semantics, CBPV, thunks, and closures

  • Discussion clarifies that Forsp is based on Call-By-Push-Value (CBPV), not linear types, though linear types might help with memory management.
  • Thunks are described as delayed computations (functions of zero arguments), aligning with Forsp’s lazy “block/thunk” flavor reminiscent of Forth and Rebol.
  • A key subtlety: ^a (push without evaluating) differs from (a) (builds a closure to compute a); without ^ you’d accumulate nested closures and need extra forcing.

Syntax, variables, and sigils

  • Some like $ and ^ as variable/stack sigils; others prefer Forth-like !/@ or </> or even +/-.
  • There’s debate whether 'x-style quoting could be replaced by (x); in Forsp they are intentionally different to keep semantics simple and expressive.

Minimal core & primitives

  • The language intentionally minimizes primitives (e.g., only subtraction and multiplication).
  • Users show how addition can be derived from subtraction; division is acknowledged as “harder but doable.”
  • The interpreter is under 1,000 lines of C with a single tagged-union object type and lots of cons cells, essentially a very small Lisp.

Implementation, compilation, and GC

  • Questions arise about how a compiler would differ from the C interpreter; suggestions reference Forth compilers and register renaming.
  • Ideas floated: simple reference counting plus linear types, or more general real-time GC for complex scenarios.

Relations to Forth, Lisp, and other languages

  • Forsp is compared to historic and modern Forths, HP’s RPL, Kitten, Cognition, Dreams, Rebol, “fe”, “aria”, FreeForth, ableForth, and other Forth/Lisp hybrids.
  • Some note that many stack-based VMs for Lisp already combine Lisp semantics with a stack machine.

Practicality, usability, and learning curve

  • Several commenters express excitement to tinker, see it as a strong teaching/experimental tool, and praise its small, comprehensible implementation.
  • Others question whether it will see real use beyond niche language exploration, given the abundance of small experimental languages.
  • There’s an extensive side debate about Forth vs Lisp readability; claims that Lisp’s parentheses encode more structure are countered with arguments about style, habit, and tooling, with no consensus reached.