Am 05.03.2018 um 13:40 schrieb Li-yao Xia:
On 03/05/2018 07:13 AM, Ben Franksen wrote:
Okay, okay, I got it. I did not think about strictness when I asked. The funny thing is that the two fusion rules combined, as explained by Josef, seem to cause this shortcut to be taken. But that can't be true because (++) really is non-strict, I tested that, with -O2. How do you explain that?
Rewrite rules apply at compile time and don't force any computation. The second rule fires only if the second argument of (++) is syntactically []. Otherwise the code doesn't change, and strictness is preserved.
Thanks, yet another thing learned. So let ys = [] in xs ++ ys will traverse the spine of xs but xs ++ [] will not. Interesting. (But who writes something like "xs ++ []" in a real program?) Cheers Ben