Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <p>The most obvious way:</p> <pre><code>type Num = { def +(a: Num): Num def *(a: Num): Num } def pyth[A &lt;: Num](a: A, b: A)(sqrt: A=&gt;A) = sqrt(a * a + b * b) // usage pyth(3, 4)(Math.sqrt) </code></pre> <p>This is horrible for many reasons. First, we have the problem of the recursive type, <code>Num</code>. This is only allowed if you compile this code with the <code>-Xrecursive</code> option set to some integer value (5 is probably more than sufficient for numbers). Second, the type <code>Num</code> is structural, which means that any usage of the members it defines will be compiled into corresponding reflective invocations. Putting it mildly, this version of <code>pyth</code> is obscenely inefficient, running on the order of several <em>hundred thousand</em> times slower than a conventional implementation. There's no way around the structural type though if you want to define <code>pyth</code> for <em>any</em> type which defines <code>+</code>, <code>*</code> and for which there exists a <code>sqrt</code> function.</p> <p>Finally, we come to the most fundamental issue: it's over-complicated. Why bother implementing the function in this way? Practically speaking, the only types it will ever need to apply to are real Scala numbers. Thus, it's easiest just to do the following:</p> <pre><code>def pyth(a: Double, b: Double) = Math.sqrt(a * a + b * b) </code></pre> <p>All problems solved! This function is usable on values of type <code>Double</code>, <code>Int</code>, <code>Float</code>, even odd ones like <code>Short</code> thanks to the marvels of implicit conversion. While it is true that this function is technically less flexible than our structurally-typed version, it is <em>vastly</em> more efficient and eminently more readable. We may have lost the ability to calculate the Pythagrean theorem for unforeseen types defining <code>+</code> and <code>*</code>, but I don't think you're going to miss that ability.</p>
    singulars
    1. This table or related slice is empty.
    1. POHow does one write the Pythagoras Theorem in Scala?
      singulars
      1. PTQuestion
    1. PTAnswer
    1. This table or related slice is empty.
    1. USDaniel Spiewak
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    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. CODoes the "simple" solution work with BigNum or Rational? Can I define a whole library of math theorems and have them used by either double, integer, bignum or rational?
      singulars
      1. PO
      1. USDaniel C. Sobral
    2. CO+1 for both the implementation and the reasoning why it should not be done like this. :-)
      singulars
      1. PO
      1. USAndrzej Doyle
    3. CONow that I'm much more informed about Scala, I see that another solutione exists. Defining an abstract Num class, subclasses for any desired type, implicit conversions from the desired types to the corresponding subclass, and making pyth[A] accept "a" and "b" of A, plus an implicit from A => Num[A]. Would you mind adding this solution to your answer? I'd like to accept it, but I'd prefer for it to be more complete.
      singulars
      1. PO
      1. USDaniel C. Sobral
 

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