Another bug: 'indent-pairs can't handle being presented with a list containing only one element. For an example in the wild, see the source of 'rand-choice:
arc> src.rand-choice
(from "arc.arc")
(mac rand-choice exprs
`(case (rand ,(len exprs))
Error: "car: expects argument of type <pair>; given 1"
More succinctly:
arc> (ppr '(case foo ,@(bar baz quux xyzzy plugh very long)))
(case foo
Error: "car: expects argument of type <pair>; given 1"
The error lies in 'indent-pairs:
(def indent-pairs (xs (o col 0))
(let l (apply max (map len:tostring:print:car (keep [cdr _] pair.xs)))
(on x pair.xs
(if (~is index 0)
(do (prn)
(sp col)))
(let str (tostring:print car.x)
(if cdr.x
(do pr.str
(sp:- l len.str -1)
(ppr cadr.x (+ col 1 l) t))
(do (sp (+ l 1)) ;offending expression
pr.str))))))
If 'xs is a list with only one element, then (pair xs) returns a list whose sole element is itself a list of one element. 'l is then the result of applying 'max to an empty list, which is nil. (+ l 1) then becomes (+ nil 1), which errors. One simple solution is to add a preceding 0 to the application of max.
--- ppr-old.arc 2009-07-28 21:10:49.000000000 +0200
+++ ppr.arc 2009-07-28 21:10:52.000000000 +0200
@@ -54 +54 @@
- (let l (apply max (map len:tostring:print:car (keep [cdr _] pair.xs)))
+ (let l (apply max 0 (map len:tostring:print:car (keep [cdr _] pair.xs)))
