A Typed Lisp in Elm

The code you find here outlines how one might go about writing a kind of typed Lisp in Elm — an educational experiment for myself. The files are

• Parse.elm, exporting one type, AST, and one function parse.
• ParserHelpers.elm, exporting one function, many, a clip from the package Punie/parser-extras. The many combinator is used in module Parse
• Lisp.elm, exporting one function, eval
• Lisp2.elm, a superset of Lisp.elm, also exporting evalToString

Medium article on this code

One can evaluate Lisp expressions using elm-repl or by using the little node repl program
described in the last section.

Using elm-repl

Examples:

$ elm repl

> import Parse exposing(parse)
> import Lisp2 exposing(eval)

> parse "(+ 1 2 3)"
Just (LIST [Str "+",Num 1,Num 2,Num 3])
    : Maybe Parse.AST

> eval "(+ 1 2 3)"
Just (VNum 6) : Maybe Lisp2.Value

> eval "(+ 1 2 haha!)"
Just Undefined : Maybe Lisp2.Value

Note that eval handles nonsense input in a somewhat graceful way.

The repl

To use the repl:

$ elm make --optimize src/Repl/Main.elm --output=src/Repl/main.js
$ node src/Repl/repl.js

Type ':help' for help

> (+ 1 2)
3

The repl program uses the code in ./src/Repl following the outline described in the Medium article Elm as BlackBox. The idea is to use the Platform.Worker program in Repl.Main to communicate with the terminal via node.js.