iface HasEnv {
    env: Map<str, str>,
}

iface HasVal {
    val: str,
}

alias Ident = /([a-zA-Z][a-zA-Z0-9_]*)|(_[a-zA-Z0-9_]+)/

node Expr : HasEnv + HasVal {
    "let" <name:Ident> "=" <val:Expr> "in" <body:Expr> => {
        body.env = self.env.with(name, val);
        self.val = body.val;
    }
    <name:Ident> => {
        // TODO: does env need to be referenced here?
        // TODO: how to check for unbound names ahead of time
        //       (for self-implementation)
        self.val = self.env.lookup(name);
    }
    <string:StringLit> => {
        self.val = string;
    }
}