Just the other day, while programming in Scala, I was thinking how nice it is that Haskell hides non-strictness (a.k.a. laziness) underneath, so no obnoxious lazy stream creation/forcing needs to be done. And then it struck me: how cool would it be to treat Promises in a language the same way that Haskell treats lazy values? Would such a language even make sense? How would you program in such a language?

Let's explore! You can find all the code included in this post in a more useful form here.

Promise

(define *promises* '())

(define (make-promise value state then handle thunk)
  (list '&promise value state then handle thunk))

;; Field accessors omitted.

(define (promise fun)
  (let* ((val '())
         (state 'pending)
         (on-resolve id)
         (on-reject id)
         (resolve (lambda (v)
                    (set! val v)
                    (set! state 'resolved)
                    (on-resolve val)))
         (reject (lambda (e)
                   (set! val e)
                   (set! state 'rejected)
                   (on-reject val)))
         (then (lambda (t)
                 (if (equal? state 'resolved)
                     (t val)
                     (set! on-resolve t))))
         (handle (lambda (h)
                   (if (equal? state 'rejected)
                       (h val)
                       (set! on-reject h))))
         (p (make-promise (lambda () val)
                          (lambda () state)
                          then
                          handle
                          (lambda () (fun resolve reject)))))
    (set! *promises* (cons p *promises*))
    p))

Recently, I came across this post about implementing custom type systems using Prolog and it inspired me to revisit an old logic-programming-related project of mine.

About a year and a half ago I took a shot at using Prolog for type reconstruction (a.k.a. type inference), but that resulted in a load of not much for one reason or another. I did write this Rule-based system shortly thereafter, however I did not give much thought to using it for type reconstruction. Let's change it...

(assert! (: map (-> ((-> (?a) ?b) (list ?a)) (list ?b))))

In case you are wondering, here are some questions & answers regarding λ-blog:

"Kajtek, you incredibly handsome stallion, what have you been doing these past few months?" - you might ask, concerned about the lack of ASM dev logs recently...

Well, I've mostly been studying various PL design quirks and prototyping neat features such as vau calculus flavoured fexprs or the following piece of what I consider art.

As mentioned before, I was working on an extensible reader that would support user-defined reader macros for easy DSL programming. It took me quite some time, effort and researching but eventually I came up with an awesome solution. It's worthy to mention here, that it's in no way a new solution. I prefer to figure stuff out myself and often times it turns out that similar concept already existed a few years before mine. Bummer u_u.

One of the most distinctive features of Common Lisp and Lisp in general, are its code-generation and code-manipulation capabilities.

Probably the best example is the LOOP macro - a Swiss Army knife of iteration that can do pretty much anything. The following snippet iterates a list of random numbers collecting some statistics of its contents and does that while being very concise and readable:

(let ((random (loop with max = 500
                    for i from 0 to max
                    collect (random max))))
  (loop for i in random
          counting (evenp i) into evens
          counting (oddp i) into odds
          summing i into total
          maximizing i into max
          minimizing i into min
        finally (format t "Stats: ~A"
                          (list min max total evens odds))))

Stats: (0 499 120808 261 240)

Here's a cool hack I use to optimize my docs searching.

Let's start off with DuckDuckGo search engine. By itself it's a pretty powerful tool thanks to its numerous features like the !bang syntax. For example searching for:

!cpp std::string::clear

...takes me exactly where I want.

Let's use it to our advantage, shall we?

StumpWM is a tailing window manager that allows you to define system-wide key bindings that work and feel pretty much like Emacs ones. Combining that with DuckDuckGo'es !bang syntax makes you just a few clicks away from anything out there:

(defcommand duckduckgo (phrase) ((:string "Search: "))
  "Searches for something on DuckDuckGo."
  (run-shell-command
    (concatenate 'string
                 *your-fav-webbrowser*
                 " http://duckduckgo.com/?q="
                 (substitute #\+ #\Space phrase))))
(define-key *root-map* (kbd "d") "duckduckgo")

Now, if you want to find out if I used substitute correctly all you have to do is:

C-t d !lisp substitute

...what will take you directly there. Turns out I did.

But wait, there's more!

I had very little time for this blog lately, but this is about to end soon enough. Here's a quick post showing ASM' keyboard distribution.

It's high time for more Another Syntactic Monstrosity examples, but first let's take a look at its development.

devLog

The new parsing strategy reminds of CommonLisp reader and reader-macros:

• The raw input is passed to the lexer (written in D) that uses dynamic syntax table - a set of regular expressions describing the syntax (defined in ASM) to tokenize the input into a token stream.
• Next, the token stream is passed to the parser (written in ASM with default implementation in D), that uses ASM functions, correlated to the syntax table, to dispatch the token stream and translate it to basic, lispy S-expressions (see LR parser for details on how it's done).This will allow ASM to act like any other language, given there's the syntax table available, making it perfect for DSL programming.

For example:

(lambda (x y) (pow (+ x y) 2))

...might be written as:

[x y => (x + y)^2]

...and ASM will happily accept it. Awesome!

Finally. Good job, Lazy-me, keep it up! Or shall I say, 'keep it down', dohoho!

So we've seen latmab - the calculator and Scheme - RNoRS Scheme interpreter, both leading to the following, somewhat fun story... After having basics of Scheme implemented even though I did not have any previous Lisp experience, I figured I could do even better. Hell, let's actually learn some Lisp, said my inner smoking-hot stallion-programmer-guy!

It's been a while since my last post, more than I'd wish for actually, and that is because I was hunting a man by the name Iain Buclaw on the IRC. I had to make sure he was ok with me using some of his work in this next project/post (and he obviously is, oh the kind gentleman he is).

Last time I shared some thoughts on latmab, a simple algebraic calculator, I have... commited and todays topic is somewhat related. It's a continuation I'd say, but let's go on with the story...