Tagged Scheme

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.


Here's a very simple Promise implementation written in Scheme, with support for asynchronous resolving & error rejection, which we'll need in order to explore this weird proposition:

(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)
                          (lambda () (fun resolve reject)))))
    (set! *promises* (cons p *promises*))

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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))))

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So, I finally found some time for a Template Metaranting follow-up post. This time let's get down to business as this one contains a fair amount of code.

Sadly, I won't rant as much but instead I'll try to show how awesome D's templates really are. We'll write a piece of code, based on this Scheme implementation, that is, a simple monad) that we'll use to build a binary tree, with uniquely numbered nodes containing their height, without any global state (therefore purely) entirely at compile time.

Quick Reader, grab my code!


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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...

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