Fiddle 🎻

It’s just a two-man con...Like the guy with the fiddle, and the guy who wants to buy the fiddle, and the poor sap in between them who pays for the fiddle. Two men, who appear to be on opposite sides, playing the same game

• Neil Gaiman, American Gods

This repo contains an experimental, Scheme-like dynamically typed variant of Paul Blain Levy's Call-by-push-value (CBPV), called Fiddle. Fiddle is based on the "Scheme-like" model of gradual typing in a CBPV setting we designed in a paper on gradual typing in a CBPV-like metalanguage. Being based on CBPV, Fiddle allows for call-by-value and call-by-name programming styles, resulting in a language that feels a bit like a combination of Scheme, Haskell and continuation-passing style.

The basic idea is that instead of being uni-typed, Fiddle is "dityped" in that there are 2 kinds of expressions: inert values and effectful computations. Values are S-expressions as in a typical Scheme: they include symbols, boolean values, strings, ..., as well as immutable cons pairs and "thunked" computations. Computations are effectful programs that interact with the stack. The current stack is always either an opaque continuation waiting for the computation to ret a value to it, or is a value pushed onto a larger stack. The basic primitive for interacting with the stack is case-lambda which co-pattern matches on the stack to see if there are any arguments left. This stack-manipulation provides a slightly lower level interface for implementing variable-arity functions than typical Schemes do.

The language is implemented as a "#lang" in Racket, using the turnstile library.

Repository Structure

1. The actual library is in the fiddle directory, which can be installed using raco. It also contains a bare-bones stdlib.
2. More example code can be found in the examples directory.
3. The cbpv directory contains the beginnings of a typed version of the language.
4. The hs directory contains the beginnings of an interpreter written in Haskell.
5. The notes directory contains some notes on language design and comments on programming in this unfamiliar style.

Performance

Performance is pretty bad. Macro-expansion can take a while, but worse is the runtime performance, though I haven't looked into exactly why the Racket's optimizer doesn't do better on it.