I don't know boo about Scala, but I was watching this intro talk by Dean
Wampler, who wrote the O'Reilly book on Scala:
http://at.pscdn.net/008/00102/videoplatform/kv/100917scalaKV.html
I learned that every Scala object that defines an apply method can be
used as a function. Cute. It's nice to see a language emphasize the
symmetry between objects and functions.
So then I took a few minutes to code up the idea in Racket:
#lang racket
(require test-engine/racket-tests)
(define applicable<%>
(interface* ()
([prop:procedure
(λ (obj x) (send obj apply x))])
apply))
(define fun%
(class* object% (applicable<%>)
(init-field f)
(define/public (apply x)
(f x))
(super-new)))
(define deriv
(new (class* object% (applicable<%>)
(define d #e0.001)
(define/public (apply g)
(new fun% [f (λ (x)
(/ (- (g (+ x d))
(g (- x d)))
(* 2 d)))]))
(super-new))))
(define sqr (new fun% [f (λ (x) (* x x))]))
(check-expect (send sqr apply 4) 16)
(check-expect (sqr 4) 16)
(check-expect ((send deriv apply sqr) 4) 8)
(check-expect ((deriv sqr) 4) 8)
(test)
It's nicer to see a language that enables creating languages that
emphasize the symmetry between objects and functions.
Thanks PLT.
David
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