There are well-known rules for indenting Lisp code, but it's sometimes convenient to break the rules when the code has semantic structure not reflected in the S-expressions. This sometimes happens when there are alternating sequences, as in some versions of let and cond and setf. It's also common to leave forms unindented when they're supposed to be top-level but something has been wrapped around them anyway, such as eval-when, or the library or unit forms required in some Scheme module systems. In either case, the indentation reflects the structure a human reader understands the program by, rather than the structure of its S-expressions.
JRM recently posted a particularly extreme example of irregular indentation:
(define (polynomial-erf z)
;; Numerical Recipies 6.2
;; fractional error < 1.2e-7
(let* ((t (/ 1.0 (+ 1.0 (* .5 (abs z)))))
(ans (- 1.0 (* t
(exp (+ (- (* z z))
(+ -1.26551223
(* t (+ 1.00002368
(* t (+ .37409196
(* t (+ .09678418
(* t (+ -.18628806
(* t (+ .27886807
(* t (+ -1.13520398
(* t (+ 1.48851587
(* t (+ -.82215223
(* t .17087277))))))))))))))))))))))))
(if (>= z 0) ans (- ans))))Note the big polynomial written in sequential multiply-and-add style. It has two semantic structures: it's a tree of arithmetic expressions, but it's also a sequence of polynomial terms. The latter interpretation is more useful, and the indentation reflects this: it's intended to be read as a sequence, so it's indented as a sequence. If it were indented normally, according to its tree structure, its semantic sequentiality would be less obvious, not to mention it would be impractically wide:
(+ -1.26551223
(* t (+ 1.00002368
(* t (+ .37409196
(* t (+ .09678418
(* t (+ -.18628806
(* t (+ .27886807
(* t (+ -1.13520398
(* t (+ 1.48851587
(* t (+ -.82215223
(* t .17087277))))))))))))))))))Either way, that's an imposing pile of right-parens. (You could avoid this with a simple polynomial function or macro, but it's not worth the trouble for one polynomial.)
One of the reasons expression-based languages are so convenient is that their program structure reflects the semantic structure humans understand them by. Irregular indentation is a reminder that they don't always do so perfectly.
Honestly, I think that a polynomial macro is the Right Thing here, not necessarily written by the author of the code in question.
ReplyDeleteBut then as much as I love fully-featured standard libraries, the call of "minimalism" still resonates with me as a worthy goal... even if perhaps it shouldn't as much as it does.