*sigh* Never mind, I am being dumb.
factorial : Int -> Int
factorial x =
Trampoline.evaluate (fac 1 x)
fac : Int -> Int -> Trampoline Int
fac acc n =
if n <= 0 then
Trampoline.done acc
else
Trampoline.jump (\_ -> fac (n * acc) (n - 1))
On Tue, Jul 12, 2016 at 9:17 AM, Nick H <[email protected]> wrote:
> ... and I have no idea how this is supposed to work. Even for primitive
> recursion
>
> factorial : Int -> Int
> factorial x =
> Trampoline.evaluate (fac x)
>
>
> fac : Int -> Trampoline Int
> fac x =
> if x <= 0 then
> Trampoline.done 1
> else
> Trampoline.jump
> (\_ ->
> ???
> )
>
>
>
> On Tue, Jul 12, 2016 at 7:25 AM, Nick H <[email protected]> wrote:
>
>> Ian, that is a great thought, but even if we split the list exactly in
>> half, the call stack fills up when the list length gets to around 4500.
>>
>> partition _ list =
>> let
>> n = List.length list // 2
>> in
>> (List.take n list, List.drop n list)
>>
>> In my specific problem, the "wrap" function finds the smallest
>> rectangular prism that encloses a set of 3D points. The "partition"
>> function splits the list of points in half, with a plane that is aligned
>> with one of the box's axes. I have a lot of tests to ensure the function
>> doesn't break in weird cases, like all of the points being identical. So I
>> am pretty sure the partition is doing a reasonable split.
>>
>> I am taking a look at the trampoline library now...
>>
>> On Tue, Jul 12, 2016 at 12:03 AM, Janis Voigtländer <
>> [email protected]> wrote:
>>
>>> Maybe the Trampoline library can help you?
>>>
>>> http://package.elm-lang.org/packages/elm-lang/trampoline/1.0.0/Trampoline
>>>
>>>
>>> Am Dienstag, 12. Juli 2016 schrieb Nick H :
>>>
>>>> Here is a puzzle for you:
>>>>
>>>> For a large enough list, the code below will fill up the call stack and
>>>> crash. It makes sense that the Elm compiler can't optimize it with tail
>>>> calls, since the recursive function gets called more than once per
>>>> iteration.
>>>>
>>>> What is the best way to rewrite this without using recursion?
>>>>
>>>>
>>>>
>>>> type Tree = Leaf Foo | Node Bar Tree Tree
>>>>>
>>>>> create : List Foo -> Tree
>>>>> create faces =
>>>>> case faces of
>>>>> [] ->
>>>>> dummyFoo
>>>>>
>>>>> foo :: [] ->
>>>>> Leaf f
>>>>>
>>>>> list ->
>>>>> let
>>>>> box = wrap list
>>>>>
>>>>> ( left, right ) = partition box list
>>>>> in
>>>>> Node box (create left) (create right)
>>>>>
>>>>> wrap : List Foo -> Bar
>>>>>
>>>>> partition : Bar -> List Foo -> (List Foo, List Foo)
>>>>>
>>>>> --
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>>
>>
>
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