Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <p>Partial answer to the first part of your question:</p> <p>Passing in <code>intCMP</code> to <code>sort3</code> lets you control the way the sorting is done. Presumably, <code>sort3 intCMP [7,3,6]</code> will return <code>[3,6,7]</code>, whereas <code>sort3 intCMPRev [7,3,6]</code> will return <code>[7,6,3]</code>. You could even make your own weird sorting functions like first all the even numbers and then all the odd ones in descending order, like so:</p> <pre><code>intCMPWeird :: Int -&gt; Int -&gt; Ordering intCMPWeird a b | even a &amp;&amp; even b = intCMP a b -- even numbers compare as usual | odd a &amp;&amp; odd b = intCMPRev a b -- odd numbers compare in reverse order | even a &amp;&amp; odd b = LT -- evens are all 'smaller' than odds | odd a &amp;&amp; even b = GT -- odds are all 'greater' than evens </code></pre> <p>Using this, <code>sort3 intCMPWeird [7,3,6]</code> should give <code>[6,7,3]</code>.</p> <p>What happens during typechecking when you pass one of the <code>intCMP...</code> functions into <code>sort3</code> is (in a very small nutshell) that the compiler tries to match the type of <code>sort3</code>'s first argument <code>(a -&gt; a -&gt; Ordering)</code> with the type of the supplied value <code>(Int -&gt; Int -&gt; Ordering)</code> and succeeds in doing that, by making <code>a</code> equal to <code>Int</code>. Then it needs to check whether the constraint <code>Ord a</code> is satisfied for <code>Int</code>, which works! Finally the compiler can figure out that the type of <code>sort3 intCMP</code> is <code>[Int] -&gt; [Int]</code>. Similarly, <code>sort3 floatCMP</code> has type <code>[Float] -&gt; [Float]</code>.</p> <p>I'm not 100% sure why the <code>Ord</code> constraint is on the type of <code>sort3</code>, since you can get the needed information from the passed-in comparison function - are you sure you've typed that correctly?</p> <p>EDIT: Hint on how to use a where clause to get a readable definition, you still have to fill in some bits and add some more clauses:</p> <pre><code>sort3 cmp [x,y,z] | x &lt;= y &amp;&amp; somethingElse1 = listWithXyzInSomeOrder1 | x &lt;= z &amp;&amp; somethingElse2 = listWithXyzInSomeOrder2 where a &lt;= b = cmp a b /= GT </code></pre>
    singulars
    1. This table or related slice is empty.
    1. POHaskell: Polymorphic functions explanation
      singulars
      1. PTQuestion
    1. PTAnswer
    1. USyatima2975
    1. USyatima2975
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. POHaskell: Polymorphic functions explanation
      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. COYea, i copied it straight from the assignment. I understand how that works now. But how would i use the comparison function now? do i just do some if then statements or guards and just do cmp x y == GT && cmp y z == GT = [z,x,y] for example or is there some way to get what was passed in pretty easily? Thanks.
      singulars
      1. PO
      1. USMatt
    2. COYou're on the right track! (Although your example seems a bit strange to me; cmp x y == GT should mean x > y, and y > z should give sort order [z,y,x]).
      singulars
      1. PO
      1. USyatima2975
    3. COyea sorry. I have it written down. But i just wrote it out for an example real quick. Ok thanks. I think i have an idea on how to do it.
      singulars
      1. PO
      1. USMatt
 

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