[Haskell-cafe] Unnecessarily strict implementations
jac at informatik.uni-kiel.de
Thu Sep 2 03:25:59 EDT 2010
On 02.09.2010, at 01:35, Daniel Fischer wrote:
> It's not that it's not as non-strict as possible per se. (Sorry, had
> to :)
> It's that intersperse's current definition (in GHC at least) can
> cause a
> space leak. In this case, making the function less strict can cure
> it, in
> other cases, more strictness might be the solution.
I would be very happy if you would share this example with me. I am
looking for an example where the current implementation of intersperse
or inits causes a space leak for quite a while now.
> On the other hand, we currently have
> intersect  _|_ = 
> and one of intersect _|_  and intersect  _|_ must give _|_.
> Which one is a matter of choice.
I am sorry for not being precise. You are right. But right now we have
intersect xs  = _|_ for every list xs terminated by _|_. But I
suffices to evaluate xs to head normal to decide that the result
should be . That is, we could have
intersect  _|_ =  and intersect (_|_:_|_)  = 
intersect  (_|_:_|_) =  and intersect _|_  = 
and the current implementation satisfies neither.
> And before that, the rule intersect  _ =  if the current
> behaviour of
> intersect  should be retained.
That's a deal.
>> The implication (<=) :: Bool -> Bool -> Bool is too strict as well.
>> have False <= _|_ = _|_ as well as _|_ <= True = _|_ while one of
>> these cases could yield True.
> I'm not convinced either should (nor that they shouldn't).
I think this is a matter of elegance rather than a matter of
efficiency. In the same way as I prefer
False && _|_ = False
False && _|_ = _|_
False <= _|_ = True
False <= _|_ = _|_
> The last slide lists among the problems
> "proposes undesirably inefficient functions (reverse)".
> I wouldn't equate 'not minimally strict' with 'too strict'.
> Minimal strictness also can have negative effects, one must look at
> case individually.
I second this but in my opinion the minimally strict implementation
should be the default if there is no reason against it.
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