Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <p>I haven't got any experience with reactive programming under F#, but the problem of global state in purely functional systems is quite common and has a quite elegant solution: <strong>Monads</strong>.</p> <p>While monads themselves are primarily used in Haskell, the underlying concept made it into F# as <a href="http://en.wikibooks.org/wiki/F_Sharp_Programming/Computation_Expressions" rel="noreferrer">computation expressions</a>.</p> <p>The idea is that you don't actually change states but just describe the transitions of the state, i.e. how to produce new states. The state itself can be completely hidden in the programm. By using special monadic syntax, you can write pure but stateful programms almost imperatively.</p> <p>Taking a (modified) implementation from <a href="http://www.navision-blog.de/2009/10/23/using-monads-in-fsharp-part-i-the-state-monad/" rel="noreferrer">this source</a>, the <code>State</code> monad could look like this</p> <pre><code>let (&gt;&gt;=) x f = (fun s0 -&gt; let a,s = x s0 f a s) let returnS a = (fun s -&gt; a, s) type StateBuilder() = member m.Delay(f) = f() member m.Bind(x, f) = x &gt;&gt;= f member m.Return a = returnS a member m.ReturnFrom(f) = f let state = new StateBuilder() let getState = (fun s -&gt; s, s) let setState s = (fun _ -&gt; (),s) let runState m s = m s |&gt; fst </code></pre> <p>So let's have an example: We want to write a function that can write values into a log (just a list) while proceeding. We therefore define</p> <pre><code>let writeLog x = state { let! oldLog = getState // Notice the ! for monadic computations (i.e. where the state is involved) do! setState (oldLog @ [x]) // Set new state return () // Just return (), we only update the state } </code></pre> <p>Within <code>state</code>, we can now use this in an imperative syntax without having to handle the log list manually.</p> <pre><code>let test = state { let k = 42 do! writeLog k // It's just that - no log list we had to handle explicitly let b = 2 * k do! writeLog b return "Blub" } let (result, finalState) = test [] // Run the stateful computation (starting with an empty list) printfn "Result: %A\nState: %A" result finalState </code></pre> <p>Still, everything is purely functional here ;)</p>
    singulars
    1. This table or related slice is empty.
    1. POFunctional Reactive F# - Storing States in Games
      singulars
      1. PTQuestion
    1. PTAnswer
    1. This table or related slice is empty.
    1. USDario
    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. VTDownMod
    1. COThis is mostly about a state monad. Functional reactive programming often involves monads, but not usually this simple kind of state monad.
      singulars
      1. PO
      1. USRD1
    2. COAs I said, I haven't got experience in FRP. Nevertheless the state monad (or monads at all) seems to be the concept that has been asked for - storing and modifying context data conveniently without losing referential transparency. If FTP already uses a monadic infrastructure, so much the better. A State monad transformer should make the thing (do you mean that with *simple kind*?). But without having explained the fundamentals, this information would be useless!
      singulars
      1. PO
      1. USDario
    3. COThe main point of FRP is allowing behaviours to be defined as continuous functions of time - e.g. you can define the z-position of a ball under gravity as z(t)=9.8*t*t . Monadic state is only relevant for state which makes discrete changes - discrete changes are also allowed in FRP, but they are less central and often they don't fit the exact form of a monad.
      singulars
      1. PO
      1. USRD1
 

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