lisp — a Scheme interpreter

Run it in the playground ↗ Source on GitHub ↗

A small but real Scheme, in two parts: a Raku grammar (plus an actions class) reads the source into Raku data structures, and a tree-walking evaluator runs them. The numeric tower rides directly on Raku's own, so exact Rats and unbounded Ints come for free.

Run it

build/rakupp showcase/lisp/lisp.raku showcase/lisp/examples/fact.scm
build/rakupp showcase/lisp/lisp.raku showcase/lisp/examples/closures.scm
build/rakupp showcase/lisp/lisp.raku                 # no file → a REPL
# or compile a standalone binary:
build/rakupp --exe -o lisp showcase/lisp/lisp.raku

What it can do

Exact bignums — (fact 100) prints all 158 digits, no overflow:

$ build/rakupp showcase/lisp/lisp.raku showcase/lisp/examples/fact.scm
5! = 120
10! = 3628800
20! = 2432902008176640000
50! = 30414093201713378043612608166064768844377641568960512000000000000
100! = 933262154439441526816992388562667004907159682643816214685929638952175999932299156089414639761565182862536979208272237582511852109168640000000…

Closures, mutable state, higher-order functions, recursion:

$ build/rakupp showcase/lisp/lisp.raku showcase/lisp/examples/closures.scm
add5(10)   = 15
add100(10) = 110
counter: 1 2 3
sum 1..10 = 55
squares   = (1 4 9 16 25)
even? 10  = #t
odd?  7   = #t

Language

Feature	Notes
`define` / `lambda` / `let`	including `(define (f x) …)` function shorthand
`if` / `cond` / `and` / `or`	`cond` supports `else`
`set!`	mutate a binding in its defining scope
`quote` / `quasiquote / `,unquote` / `,@splice`	list templating
lexical closures	functions capture their defining environment
numeric tower	exact `Int` / `Rat`, promotes to `Num` as Raku does
builtins	`+ - * / = < > cons car cdr list map filter fold-left append reverse length …`

How it works

Reader — a Raku grammar (token/rule) with an actions class turns text straight into Raku values: numbers become Int/Rat/Num, symbols an interned Sym, lists an Array. ;-comments are stripped first so the grammar's whitespace rule stays trivial.
Evaluator — a recursive evaluate($form, $env): self-evaluating literals return themselves, symbols look up the environment chain, and a list is either a special form (if, define, …) or a procedure application.
Environments — a chain of frames (Env with a parent), so closures just hold the environment they were defined in.