BLisp: A Statically Typed Lisp Like Language

Make that three bugs. The macro is called "double," but the numeric result is square.

In any case, it looks neat, I think the intersection of static types and Lisps is a space that needs more experimentation with, so I'm happy to see that.

I hope eventually it gains more effects as well, with only Pure and IO, you can't do much of the cool things that effect systems bring.

  (defmacro double (x)
    (let ((temp (gensym))
    `(let ((,temp ,x))
       (+ temp temp))))

  (let ((#:uniqueName123 (parse-integer (read-line)))
    (+ #:uniqueName123 #:unique_symbol_123))

To explain a little bit what is going on: normally if you want to have an s-expression as a piece of data, you can use the quote special form - (quote (a b c)), usually shortened to '(a b c), returns a list containing three symbols (think of these as special strings), "a", "b", "c". If you want instead to evaluate a variable named "a", you can use the ` syntax, together with , and ,@. That is, `(,a b ,@c) will produce a list containing the value of a variable named "a", the symbol "b", and the value of a variable named "c", spliced in. If a is '(1 2 3) and c is '(4 5 6), `(,a b ,@c) will return the 5-element list ((1 2 3) b 4 5 6). Depending on how this is used further, b itself may be evaluated or just printed as is.

So, when expanding the macro, x will be initialized to the form provided as argument to double (not the value of that form). Then, temp will first be assigned a value that is produced by gensym, which generates a unique symbol; then, we'll return a list that represents some Lisp code binding the form represented by x to a variable whose name is the value returned by gensym, and then using this same variable name in a call to +. Finally, if this macro was "called" from regular lisp code, the expression it returned will be compiled or interpreted.

The macro could also be called from a special form like macroexpand-1, which would just return the list returned by the macro, without evaluating it; or macroexpand, which would do the same but recursively until there are no more macros in the expansion.

Note: a symbol is basically a string that can be used as a Lisp identifier, and is registered as such in the Lisp runtime. It is a separate type from string, but you can create a string from a symbol, or try to create a symbol from a string (which fails if the string is not a valid Lisp identifier).