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

Am 05.03.2018 um 09:02 schrieb Sven Panne:

2018-03-04 10:42 GMT+01:00 Clinton Mead :

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Adding that case will require one to evaluate the second argument to check it's empty before allowing one to examine the result.

Consider `x ++ some_list_that_takes_a_long_time_to_produce_its_first_ element`.

In the extreme, evaluating the 2nd argument might not even terminate or it could throw an exception.

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In this case your proposal will not be an optimisation.

I would go even a step further: The proposed additional case would not just be worse for some cases, it would be completely wrong: The Prelude part of the Haskell Report specifies among other things the strictness of function arguments. [...]

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? Cheers Ben