is that useful?

["Walter C. Pelissero" <walter@xxxxxxxxxxxxx>]

Maybe somebody can explain me what is the use of this SRFI.

The original curry, as explained by some book,

    (define (curry f x)
      (lambda (y)
	(f x y)))

is a suggestive trick to produce a specialization of f without an
explicit lambda expression.  A special case of partial application,
one might argue.

While having an educational value the curry higher-order function is
not very useful as its specialization is only on the first parameter
and therefore its applicability is marginal as it doesn't address
common cases like:

    (lambda (y) (/ y 2))		; where x=2
    (lambda (z) (< 0 z 1))		; where x=0, y=1
    (lambda (y) (hash-store table y y))	; where x=table
    (lambda (y z) (list 1 z 2 y 3))

I see that this SRFI aims at solving the first and second cases, with
the introduction of a place-holder notation, but still doesn't address
the swapping of two arguments and the repetition of one argument.
Indeed, pretty common cases.

The original curry is elegant, because the declaration of the missing
argument is implicit.  This SRFI, to cope with the positional
arguments, departs from that idea.  Making explicit the position of
the parameters, it goes toward the vanilla lambda expression with the
only benefit that you don't have to invent and declare names for the
place-holders.  In short, it looks to me like a lambda form with the
limitations of curry.

If all you want is a shortcut for the lambda expression, there are a
couple of things that come to mind.  In Scheme 48, with the macro:

    (define-syntax f
      (lambda (form rename compare)
	`(lambda (_)
	   ,(cdr form))))

you can write closures like:

    (define halve (f / _ 2))
    (define in-range? (f < 0 _ 1))
    (define store (f hash-store table _ _))

but, of course, this goes far beyond curry, because you are free to
write subforms:

    (define quadratic (f + (* _ _ 2) (* _ 3) 4))

It's, indeed, a replacement notation for lambda with the imaginable
impact on readability (not so easy sometime).

The underscore notation is not by chance.  It's taken from the Paul
Graham's wish list for his brainchild, Arc.  Actually, he proposes the
use of a square bracket shortcut:

    (define halve [/ _ 2])
    (define in-range? [< 0 _ 1])
    (define store [hash-store table _ _])
    (define quadratic [+ (* _ _ 2) (* _ 3) 4])

Which is trivial to implement in Common Lisp and Scheme 48, but not
necessarily in other Lisp dialects lacking read macros.

This macro, while flexible, still doesn't address the multiple (and
swapped) arguments issue.  This is a bit more work:

    (define-syntax fn
      (lambda (form rename compare)
	(letrec ((arg-number
		  (lambda (x)
		    (if (symbol? x)
			(let ((s (symbol->string x)))
			  (if (char=? #\$ (string-ref s 0))
			      (or (string->number (substring s 1 (string-length s)))
				  0)
			      0))
			0)))
		 (number-of-arguments
		  (lambda (l)
		    (let loop ((l l))
		      (if (null? l)
			  0
			  (let ((x (car l)))
			    (max (if (pair? x)
				     (loop x)
				     (arg-number x))
				 (loop (cdr l))))))))
		 (make-argument-list
		  (lambda (n)
		    (do ((n n (- n 1))
			 (l '() (cons (string->symbol (string-append "$" (number->string n))) l)))
			((zero? n) l)))))
	  (list 'lambda (make-argument-list (number-of-arguments form))
		(cdr form)))))

With that you can write things like:

    (define foo (fn list "(" $2 "." $1 ")"))	; swap 1st and 2nd arg
    (define bar (fn list $2))			; ignore 1st arg
    (define acf (fn an $2 (arbitrary $3 'complex) $1 (form $2)))

Once again, this has nothing to do with curry as it lets you write
horrible syntactic closures resembling more Perl than Scheme in
anything with more than two or three parameters.

-- 
walter pelissero
http://www.pelissero.org