Add test, and Cookbook example, of array-ref arg

doc/Inline/C/Cookbook.swim

@@ -332,6 +332,36 @@ How do I pass a variable-sized list of arguments to an Inline C function?
   ( http://www.microship.com ), but he is close ( http://bike-nomad.com ). Thanks
   Ned.
 
+== Taking an array-ref as an argument
+
+- Problem
+
+  How can I take a Perl array-ref as an argument in my C function?
+
+- Solution
+
+    SV *sum(SV *array) {
+        int total = 0;
+        int numelts, i;
+        SvGETMAGIC(array);
+        if ((!SvROK(array))
+            || (SvTYPE(SvRV(array)) != SVt_PVAV)
+            || ((numelts = av_len((AV *)SvRV(array))) < 0)
+        ) {
+            return &PL_sv_undef;
+        }
+        for (i = 0; i <= numelts; i++) {
+            total += SvIV(*av_fetch((AV *)SvRV(array), i, 0));
+        }
+        return newSViv(total);
+    }
+
+- Discussion
+
+  This example returns `undef` if given a non-ref, or a non-array-ref,
+  or a ref to an empty array. You can see how you might expand this to
+  take more than one array-ref.
+
 == Using Memory
 
 - Problem

test/29refargs.t

@@ -0,0 +1,32 @@
+use strict; use warnings;
+use lib -e 't' ? 't' : 'test';
+use diagnostics;
+use TestInlineSetup;
+use Inline Config => DIRECTORY => $TestInlineSetup::DIR;
+use Test::More;
+
+my $c_text = <<'EOC';
+SV *sum(SV *array) {
+    int total = 0;
+    int numelts, i;
+    SvGETMAGIC(array);
+    if ((!SvROK(array))
+        || (SvTYPE(SvRV(array)) != SVt_PVAV)
+        || ((numelts = av_len((AV *)SvRV(array))) < 0)
+    ) {
+        return &PL_sv_undef;
+    }
+    for (i = 0; i <= numelts; i++) {
+        total += SvIV(*av_fetch((AV *)SvRV(array), i, 0));
+    }
+    return newSViv(total);
+}
+EOC
+Inline->bind(C => $c_text);
+
+is sum([(1..4)]), 10, 'correct sum';
+is sum(undef), undef, 'return undef when given undef';
+is sum('hello'), undef, 'return undef when given non-ref';
+is sum([]), undef, 'return undef when given empty list';
+
+done_testing;