Re: [Haskell-cafe] Efficiency/Evaluation Question
There's a very good StackOverflow question which covers this: "When is
memoization automatic in GHC?"[1]. I found it really cleared up the issue
for me.
[1]:
http://stackoverflow.com/questions/3951012/when-is-memoization-automatic-in-ghc-haskell
On Sat, Jun 15, 2013 at 9:13 PM, Clark Gaebel wrote:
> Yes. In general, GHC won't CSE for you.
>
> - Clark
>
>
> On Saturday, June 15, 2013, Christopher Howard wrote:
>
>> On 06/15/2013 04:39 PM, Tommy Thorn wrote:
>> >
>> >
>> > There's not enough context to answer the specific question,
>> > but lazy evaluation isn't magic and the answer is probably "no".
>> >
>> > Tommy
>> >
>>
>> Perhaps to simplify the question somewhat with a simpler example.
>> Suppose you have
>>
>> code:
>>
>> let f x = if (x > 4) then f 0 else (sin x + 2 * cos x) : f (x + 1)
>>
>>
>> After calculating at x={0,1,2,3}, and the cycle repeats, are sin, cos,
>> etc. calculated anymore?
>>
>> --
>> frigidcode.com
>>
>>
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Re: [Haskell-cafe] Efficiency/Evaluation Question
Yes. In general, GHC won't CSE for you.
- Clark
On Saturday, June 15, 2013, Christopher Howard wrote:
> On 06/15/2013 04:39 PM, Tommy Thorn wrote:
> >
> >
> > There's not enough context to answer the specific question,
> > but lazy evaluation isn't magic and the answer is probably "no".
> >
> > Tommy
> >
>
> Perhaps to simplify the question somewhat with a simpler example.
> Suppose you have
>
> code:
>
> let f x = if (x > 4) then f 0 else (sin x + 2 * cos x) : f (x + 1)
>
>
> After calculating at x={0,1,2,3}, and the cycle repeats, are sin, cos,
> etc. calculated anymore?
>
> --
> frigidcode.com
>
>
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Re: [Haskell-cafe] Efficiency/Evaluation Question
On 06/15/2013 05:02 PM, Christopher Howard wrote:
> On 06/15/2013 04:39 PM, Tommy Thorn wrote:
>
> Perhaps to simplify the question somewhat with a simpler example.
> Suppose you have
>
> code:
>
> let f x = if (x > 4) then f 0 else (sin x + 2 * cos x) : f (x + 1)
>
>
> After calculating at x={0,1,2,3}, and the cycle repeats, are sin, cos,
> etc. calculated anymore?
That might have been ambiguous. What I meant was:
code:
let f x = if (x > 4) then f 0 else (sin x + 2 * cos x) : f (x + 1)
If I calculate (f 0), and the cycle repeats after four values, are sin,
cos, etc. calculated anymore?
--
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Re: [Haskell-cafe] Efficiency/Evaluation Question
On 06/15/2013 04:39 PM, Tommy Thorn wrote:
>
>
> There's not enough context to answer the specific question,
> but lazy evaluation isn't magic and the answer is probably "no".
>
> Tommy
>
Perhaps to simplify the question somewhat with a simpler example.
Suppose you have
code:
let f x = if (x > 4) then f 0 else (sin x + 2 * cos x) : f (x + 1)
After calculating at x={0,1,2,3}, and the cycle repeats, are sin, cos,
etc. calculated anymore?
--
frigidcode.com
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Re: [Haskell-cafe] Efficiency/Evaluation Question
I expect you'll get many replies... > row (Grid s l) n = if (n >= s) then [] else l !! n > > col g@(Grid s l) n = if (n >= s) then [] else col_ g n 0 >where col_ (Grid s l) n i = if (i >= s) then [] else (head l !! n) : > col_ (Grid s (tail l)) n (i + 1) While such low-level approach (focus on the element) can certainly be made to work, but Haskell encourages you to think in higher level constructs. I haven't seen this problem before but I would map the original array from [[Int]] -> [(Int, (Int, Int), Int, Int)], that is: a list of tuples consisting of the original element, its coordinate, the row maximum and the column minimum. From there its a trivial filter to find your results. (I'm sure there's a more elegant solution). > My question: With the way Haskell works (thunks, lazy evaluation, and > all that mystery), is it actually worth the trouble of /precalculating/ > the maximum row values and minimum column values, to compare cell values > against? Or will, for example, something like (smallest_list_value (col > array 1)) definitely only evaluate once? There's not enough context to answer the specific question, but lazy evaluation isn't magic and the answer is probably "no". Tommy ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
