[erlang-questions] Erlang shows its slow face!

Edmond Begumisa <>
Wed Nov 17 16:00:29 CET 2010


Aaaahhh,

> The compiler *could* make a special case of lists:seq/[2,3]
> and generate
>
> 	<foo>(1, N, ...)
>
> But it doesn't.

OK. I erroneously thought that this *was* the case.

> The compiler can't inline the definition and
> use whether it is now, because it's a remote call (even
> if -imported), so it might change at run time.

I see it now :)

- Edmond -


On Wed, 17 Nov 2010 16:45:09 +1100, Richard O'Keefe <>  
wrote:

>
> On 17/11/2010, at 4:34 AM, Edmond Begumisa wrote:
>
>> On Wed, 17 Nov 2010 01:20:51 +1100, Kostis Sagonas <>  
>> wrote:
>>
>>> There are two basic issues here:
>>> (1) The Erlang compiler doesn't really handle list comprehension as
>>>    native loops; it compiles list comprehensions to recursive
>>>    functions.  lists:seq(1, N) is not special syntax -- unlike Haskell  
>>> --
>>>    and almost surely builds an actual list.
>>> Yes.  This is an issue.  Actually, it's more general: in my experience  
>>> most Erlang programmers do not understand the difference between list  
>>> comprehensions and lists:foreach/2. Many of them find list  
>>> comprehension notation so nice that they often use it in places where  
>>> building the list is not only is not their intention but is completely  
>>> unnecessary.
>>> The compiler is not sufficiently clever to perform deforestation and  
>>> avoid construction of intermediate lists.
>>>
>>
>> First I'm hearing of this. It really ought to be in one of those green  
>> boxes in the documentation.
>
> Let's go around this one more time.
>
> The current Erlang compiler turns
>
> 	[f(X) || X <- lists:seq(1, N)]
>
> into
> 	<foo>(lists:seq(1, N), ...)
> where
> 	<foo>
> is a function generated by the compiler.
>
> The compiler *could* make a special case of lists:seq/[2,3]
> and generate
>
> 	<foo>(1, N, ...)
>
> But it doesn't.  The actual definition of lists:seq/2 not
> only *could* change, in the not too distant past it *has*
> changed.  The compiler can't inline the definition and
> use whether it is now, because it's a remote call (even
> if -imported), so it might change at run time.
>
> Haskell compilers and the Mercury compiler *do* perform
> deforestation because they are given an unchanging program
> to work with and can safely do cross-module inlining.
> Erlang can't.
>
> This would be a potentially serious problem if iterating
> over integer ranges were something Erlang spent a lot of
> time doing.
>
> I just checked an Erlang/OTP release I keep handy for such
> things.
>     ~ 1,500,000 raw lines
>     ~   950,000 SLOW
>              34 list:seq iterations
> 	     18 in two EUNIT tests
> 	      3 in other test cases (at least)
> 	     13 list:seq not in an ?_assert
>     ~      3200 Pat <- Exp
>
> The unit tests don't matter for performance.  We're left
> with about one SLOC in every 72,000 being an iteration of
> this kind, or about <- in every 250 being an integer
> iteration.
>
> The Erlang/OTP team probably feel they have other maddened
> grizzly bears to stun before they bother with something that
> seems to turn up mainly in toy benchmarks, and who could blame them?
>
>
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