[Haskell-cafe] Why is Haskell so slow (comparing to Java/Scala)?

[Brandon Allbery]

Briefly, because the 'spine' of IO (and indeed most monads) imposes a
boundary on laziness, so when the IO action happens, the list *will* be
built. It can only be bypassed by laziness if the IO action itself is.

Hypothetically one could have RULES that rewrite to the non-list-building
variants (trailing _) if the result is seen to be discarded --- but it
would probably be unreliable, much as stream fusion can only happen if
things go just right.

On Wed, Sep 20, 2017 at 12:12 PM, Saurabh Nanda <saurabhnanda at gmail.com>
wrote:

> Intresting. Wont using "void" or "_ <- forM blah" have the same effect?
> Why not?
>
> On 20-Sep-2017 9:37 PM, "Nick Smallbone" <nick at smallbone.se> wrote:
>
>
> Hi Stanislav,
>
> Станислав Черничкин <schernichkin at gmail.com> writes:
> > I've wrote simple Haskell benchmark program, which populated primitive
> vector
> > from vector package:
> >
> > ...
> > void $ for [0..1000000 - 1] $ flip (P.unsafeWrite v) (1 :: Int)
>
> 'for' is a variant of 'map' - it returns a list of results (in this case
> a list of a million () values). Instead you should use 'forM_' (note the
> underscore) from Control.Monad, which discards the result - that cuts
> the runtime by a huge amount when I try it. (And make sure to compile
> with -O too.)
>
> Nick
>
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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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