[Haskell-cafe] Efficiency/Evaluation Question

Tommy Thorn tt1729 at yahoo.com
Sun Jun 16 02:39:18 CEST 2013

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

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".


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