Drinkable Chicken

I did some long overdue Chicken hackery yesterday, and by accident I found out that Chicken's (or rather, SRFI-26's) cut/cute macros are not the same as Arc's [ ] syntax after all.

Scheme/Arc aficionados already knew this, of course, but to me it was news since I've never really used cut much. Here's a short explanation.

Scheme's cut produces an anonymous function that takes as many arguments as there are <> symbols at the "top level". E.g.

> (cut + 1 <>)
#<procedure (? g3)>
; has one argument

> (cut + 1 <> <>)
#<procedure (? g4 g5)>
; has two arguments

; etc...

However, any <> that is found in a nested expression, is not considered as a parameter. Therefore:

;this works...
> (map (cut + 1 <>) '(1 2 3))
(2 3 4)

; but this does not:
> (map (cut + 1 (* 2 <>)) '(1 2 3))
Error: bad argument count - received 1 but expected 0: #<procedure (?)>

By contrast, Arc's [ ] syntax produces an anonymous function that expects one and only one argument, which is represented by a single underscore. As a result, it's simultaneously more and less limited than cut:

arc> (map [+ 1 _] '(1 2 3))
(2 3 4)
arc> (map [+ 1 (* 2 _)] '(1 2 3))
(3 5 7)
; no problem

; but two parameters doesn't work:
arc> (map [cons _ _] '(1 2 3) '(4 5 6))
Error: "#<procedure>: expects 1 argument, given 2: 1 4"

Of course, for anonymous functions that are more complex than this, it's probably better to just use lambda... :-/ Or a named function.