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

[Brandon Allbery]

Generated code, sometimes including "deriving"-generated code.

On Mon, 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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-- 
brandon s allbery kf8nh                               sine nomine associates
allbery.b at gmail.com                                  ballbery at sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net
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