Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <pre><code>-- FoodOne.hs statusMessage :: Int -&gt; String statusMessage 30 = "BBQ ready" statusMessage 40 = "Beverages served" statusMessage rest = "Unknown Food Status" </code></pre> <p>Most compilers are will compile this to a linear search, so it will have linear runtime cost.</p> <pre><code>-- FoodTwo.hs data FoodStatus = FoodStatus { fstatusCode :: Int, fstatusMessage :: String } status30 = FoodStatus 30 "BBQ ready" status40 = FoodStatus 40 "Beverages served" </code></pre> <p>This approach is not comparable to the others: it doesn't provide status lookup given a (dynamically-known) <code>Int</code>. However, for statically-known <code>Int</code>s, it is the fastest: the lookup is done once at compile-time and therefore has constant runtime cost, regardless of how many different constants there are.</p> <pre><code>-- FoodThree.hs statusMessages = [(30,"BBQ ready"),(40,"Beverages served")] </code></pre> <p>Doing naive lookup in this list (e.g. by the built-in <code>lookup</code> function) will involve linear search, and therefore have linear runtime cost.</p> <pre><code>-- FoodFour.hs import Data.Map as M statusMessages = fromList [(30, "BBQ ready"),(40,"Beverages served")] message n = fromMaybe "Unknown Food Status" (M.lookup n statusMessages) </code></pre> <p>The <code>Data.Map</code> module implements balanced search trees, and each lookup takes logarithmic time in the number of different statuses. You might also consider <code>Data.IntMap</code>, which also takes logarithmic time, but has better constants.</p> <pre><code>-- FoodFive.hs import Data.Array as A (statusLo, statusHi) = (30, 40) statusMessages = listArray (statusLo, statusHi) [ Just "BBQ ready" , Nothing , Nothing , {- ... -} , Just "Beverages served" ] message' n = guard (statusLo &lt;= n &amp;&amp; n &lt;= statusHi) &gt;&gt; statusMessages ! n message = fromMaybe "Unknown Food Status" . message' </code></pre> <p>The <code>Data.Array</code> module implements immutable arrays, and each lookup takes constant time. If your array is sparse, however, this may have higher memory costs than the alternatives.</p> <p>These are <em>all</em> the Haskell way. Choose the one with the right tradeoffs between developer annoyance, speed, and memory consumption for <em>you</em>, just as you would in any other language.</p>
    singulars
    1. This table or related slice is empty.
    1. POHaskell beginner question (defining pairs, similar to Http statuses)
      singulars
      1. PTQuestion
    1. PTAnswer
    1. USDaniel Wagner
    1. USDaniel Wagner
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. POHaskell beginner question (defining pairs, similar to Http statuses)
      singulars
      1. PTQuestion
    1. This table or related slice is empty.
    1. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    3. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    1. COThat's very interesting, I was under the impression that once an expression was evaluated, it will be cached in Haskell and so the performance between variant 1 should be similar to variant 2. Also, with "status200", how similar is accessing the status message through "statusMessage status200" to the first "statusMessage 200"?
      singulars
      1. PO
      1. USkitsune
    2. CO@kitsune Most compilers only do caching when it's explicitly requested by the programmer -- e.g. when they create a data structure via let-binding, where-binding, or a top-level definition -- because it's too hard to analyze automatically when the memory cost of caching will pay off. As for your other question, in `FoodTwo.hs`, the `fstatusMessage` function is essentially a pointer dereference -- it's a constant time operation.
      singulars
      1. PO
      1. USDaniel Wagner
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload