Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <p>As mentioned in the comments, you can't have a function to which you'd pass the arity of the function you want to flip. Parameters are computed at runtime, while you need the value at compile time so that you can determine the correct type.</p> <p>Neither you can make it without passing it the arity somehow. For example <code>a -&gt; b -&gt; c -&gt; d</code> could be viewed as a function of three arguments returning <code>d</code>, or as a function if two arguments returning <code>c -&gt; d</code>.</p> <p>Probably the easiest solution would be to define the functions explicitly, like <code>flip2</code>, <code>flip3</code> etc. I know this isn't what you're looking for, but it's the most practical solution.</p> <p>Another option would be to use Template Haskell. Then, the situation is different, because Template Haskell executes (I'd say "meta-") code at compile time. With TH you can create a function that takes a natural number and produces a TH expression that can be compiled into another module. The meta-function could be defined as </p> <pre><code>{-# LANGUAGE TemplateHaskell #-} module GenFlipTH where import Language.Haskell.TH pull :: Int -&gt; Q Exp pull 0 = varE 'flip pull n | n &lt; 0 = fail "Negative arity" | otherwise = [| fmap $(pull (n - 1)) . flip |] -- Here we use the fact that `(-&gt;) r` is a functor. </code></pre> <p>and used in another module to generate the appropriate expression like</p> <pre><code>{-# LANGUAGE TemplateHaskell #-} import GenFlipTH flip3 :: (a -&gt; b3 -&gt; b2 -&gt; b1 -&gt; b -&gt; c) -&gt; (b3 -&gt; b2 -&gt; b1 -&gt; b -&gt; a -&gt; c) flip3 = $( pull 3 ) </code></pre> <p>This is probably closes to your requirements I can get - you determine the function by a number and get compile-time guarantees that it's created and used correctly.</p>
    singulars
    1. This table or related slice is empty.
    1. POI want to write a function which is similar to `flip` in Haskell to get rid of lambda expressions. But I can't deal with it's type
      singulars
      1. PTQuestion
    1. PTAnswer
    1. USPetr Pudlák
    1. USPetr Pudlák
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. POI want to write a function which is similar to `flip` in Haskell to get rid of lambda expressions. But I can't deal with it's type
      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. VTAcceptedByOriginator
    1. COBy the way, can we define a class which is the complement of an existing class. If that's possible I can tell whether a variable is a function or not or even the function's number of parameters.
      singulars
      1. PO
      1. USTorosFanny
    2. CO@TorosFanny What do you mean by the complement of a class? I believe you can't tell the number of parameters automatically. Like, does `a -> b -> c -> d` have one, two or three parameters?
      singulars
      1. PO
      1. USPetr Pudlák
    3. COCurrently, I could only tell the number of parameters automatically for functions whoes "final result" is of types I defined specially as zero. see:http://stackoverflow.com/questions/17318107/is-there-a-general-way-to-tell-the-number-of-parameters-of-a-function-in-haskell
      singulars
      1. PO
      1. USTorosFanny
 

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