sexpr.d

#!/usr/bin/env dub
/+ dub.sdl:
authors "Paul Backus"
license "MIT"
dependency "sumtype" version="~>0.8.0"
+/

import sumtype;

alias Cell = SumType!(string, This[]);
alias Sexpr = Cell[];

string eval(string s)
{
    return s;
}

string eval(Sexpr s)
{
    import std.exception: enforce;

    enforce(s.length > 0, "Empty sexpr");
    string fname = eval(s[0]);
    auto fp = fname in library;
    enforce(fp !is null, fname ~ " not in the library");
    return (*fp)(s[1..$]);
}

string eval(Cell c)
{
    return c.match!eval;
}

alias Builtin = string function(Cell[]);
Builtin[string] library;

static this()
{
    library["cat"] = (args) {
        string ret;
        foreach (arg; args) ret ~= eval(arg);
        return ret;
    };
}

void main()
{
    import std.stdio;
    Sexpr prog = [Cell("cat"), Cell("Hello "), Cell("world!")];
    writeln(eval(prog));
}