Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <p>Sorry, you can't<sup>1</sup>. There are fundamental differences between tuples and lists:</p> <ul> <li>A tuple always have a finite amount of elements, that is known <em>at compile time</em>. Tuples with different amounts of elements are actually different types.</li> <li>List an have as many elements as they want. The amount of elements in a list doesn't need to be known at compile time.</li> <li>A tuple can have elements of arbitrary types. Since the way you can use tuples always ensures that there is no type mismatch, this is safe.</li> <li>On the other hand, all elements of a list have to have the same type. Haskell is a statically-typed language; that basically means that all types are known at compile time.</li> </ul> <p>Because of these reasons, you can't. If it's not known, how many elements will fit into the tuple, you can't give it a type.</p> <p>I guess that the input you get from your user is actually a string like <code>"(1,2,3)"</code>. Try to make this directly a list, whithout making it a tuple before. You can use pattern matching for this, but here is a slightly sneaky approach. I just remove the opening and closing paranthesis from the string and replace them with brackets -- and voila it becomes a list.</p> <pre><code>tuplishToList :: String -&gt; [Int] tuplishToList str = read ('[' : tail (init str) ++ "]") </code></pre> <h2>Edit</h2> <p>Sorry, I did not see your latest comment. What you try to do is not that difficult. I use these simple functions for my task:</p> <ul> <li><p><code>words str</code> splits <code>str</code> into a list of words that where separated by whitespace before. The output is a list of <code>String</code>s. <strong>Caution</strong>: This only works if the string inside your tuple contains no whitespace. Implementing a better solution is left as an excercise to the reader.</p></li> <li><p><code>map f lst</code> applies <code>f</code> to each element of <code>lst</code></p></li> <li><p><code>read</code> is a magic function that makes a a data type from a String. It only works if you know before, what the output is supposed to be. If you really want to understand how that works, consider implementing <code>read</code> for your specific usecase.</p></li> </ul> <p>And here you go:</p> <pre><code>tuplish2List :: String -&gt; [(String,Int)] tuplish2List str = map read (words str) </code></pre> <hr> <p><sup>1</sup> As some others may point out, it may be possible using templates and other hacks, but I don't consider that a real solution.</p>
    singulars
    1. This table or related slice is empty.
    1. POHow do I add x tuples into a list x number of times?
      singulars
      1. PTQuestion
    1. PTAnswer
    1. USfuz
    1. USfuz
    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
    1. CO@user988050 It's really difficult to guess how to generalize your program without having the source. Consider editing the question to add some sourcecode. Please indent source with foru whitespace (or highlight it and press the `{}` button for more convenience)
      singulars
      1. PO
      1. USfuz
    2. CO@user988050 Consider `fst :: (a,b) -> a`. You can use `map fst` to get what you want. BTW. I suggest you to read Learn You A Haskell, since it is the best introduction out there.
      singulars
      1. PO
      1. USfuz
 

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