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From: Slobodan Blazeski on 31 Mar 2010 14:52
On Mar 23, 7:55 am, Paul Donnelly <paul-donne...(a)sbcglobal.net> wrote:
> Hugh Aguilar <hughaguila...(a)yahoo.com> writes:
> > Another funky word is "curry." This seems to refer to nothing more
> > than providing default values for input parameters, which has always
> > been available in C++.
>
> Providing them after the function has been written, however, and
> reducing its arity. If you want to map a function over a list and add 4
> to each item, you can produce an adder bycurryingthe + function,
> without changing its definition or typing out the whole deal. You can
> curry the curried function too, if you've got nested iterations. It's
> particularly handy in functional programming, since mapping and
> higher-order functions are the main tools. In Factor as well, since
> quotations and combinators are used instead of loops structures that
> update variables and have bodies.
>
> (defun curry (function &rest args)
>   (lambda (&rest more-args)
>     (apply function (append args more-args))))
>
> (mapcar (curry #'+ 4) (list 1 2 3 4)) => (5 6 7 8)
>
> Strictly speaking, this use of the term is odd, and partial application
> is more appropriate.Currying, as I understand it, is a transformation
> like this:
>
> (lambda (x y z) (+ x y z))
>
> (lambda (z)
>   (lambda (y)
>     (lambda (x)
>       (+ x y z))))
>
> Or maybe with the nesting reversed; I don't know. But the term is
> generally used to mean what the curry function above does.
There's a cool joy(*) combinator called cleave, its useful when
several expressions need the same value but you don't want to
introduce new variable a.k.a tacit(**) style. And I just defined it
as below :
(defmacro cleave (outer value &rest args)
(let ((g (gensym)))
`(let ((,g ,value))
(,outer ,@(mapcar (lambda (x) (list x g)) args)))))

(defun avg (&rest args)
(cleave / args (lambda (x) (reduce #'+ x)) length))

CL> (avg 12 3 4 5)
6
Unfortunately it didn't worked with your curry so I rewrote it into:

(defmacro cleave (outer value &rest args)
(let ((g (gensym)))
`(let ((,g ,value))
(,outer ,@(mapcar (lambda (x) (if (symbolp x)
(list x g)
(list 'funcall x g))) args)))))


CL> (cleave / '(1 2 3 4) (curry #'reduce #'+) length)
5/2

Ugly but it seems to work. Anyway thanks for the bug report.

cheers
Slobodan

(*) http://en.wikipedia.org/wiki/Joy_(programming_language)
(**) http://en.wikipedia.org/wiki/Tacit_programming
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