[Haskell-cafe] missing optimization for (++)

[Andrew Martin]

Actually, it is likely that neither of the examples you gave will end up traversing the spine of the list. The definition of ys would almost certainly be inlined, and then the rule would fire.

Sent from my iPhone

> On Mar 5, 2018, at 5:18 PM, Ben Franksen <ben.franksen at online.de> wrote:
> 
>> 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
> 
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