Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <p>This excerpt from the book <em>Programming in Lua</em> shows <a href="http://www.lua.org/pil/6.3.html" rel="noreferrer">how to make a proper tail recursion</a> (in Lua, but should apply to Lisp too) and why it's better.</p> <blockquote> <p>A <em>tail call</em> [tail recursion] is a kind of goto dressed as a call. A tail call happens when a function calls another as its last action, so it has nothing else to do. For instance, in the following code, the call to <code>g</code> is a tail call:</p> <pre class="lang-lua prettyprint-override"><code>function f (x) return g(x) end </code></pre> <p>After <code>f</code> calls <code>g</code>, it has nothing else to do. In such situations, the program does not need to return to the calling function when the called function ends. Therefore, after the tail call, the program does not need to keep any information about the calling function in the stack. ...</p> <p>Because a proper tail call uses no stack space, there is no limit on the number of "nested" tail calls that a program can make. For instance, we can call the following function with any number as argument; it will never overflow the stack:</p> <pre class="lang-lua prettyprint-override"><code>function foo (n) if n &gt; 0 then return foo(n - 1) end end </code></pre> <p>... As I said earlier, a tail call is a kind of goto. As such, a quite useful application of proper tail calls in Lua is for programming state machines. Such applications can represent each state by a function; to change state is to go to (or to call) a specific function. As an example, let us consider a simple maze game. The maze has several rooms, each with up to four doors: north, south, east, and west. At each step, the user enters a movement direction. If there is a door in that direction, the user goes to the corresponding room; otherwise, the program prints a warning. The goal is to go from an initial room to a final room.</p> <p>This game is a typical state machine, where the current room is the state. We can implement such maze with one function for each room. We use tail calls to move from one room to another. A small maze with four rooms could look like this:</p> <pre class="lang-lua prettyprint-override"><code>function room1 () local move = io.read() if move == "south" then return room3() elseif move == "east" then return room2() else print("invalid move") return room1() -- stay in the same room end end function room2 () local move = io.read() if move == "south" then return room4() elseif move == "west" then return room1() else print("invalid move") return room2() end end function room3 () local move = io.read() if move == "north" then return room1() elseif move == "east" then return room4() else print("invalid move") return room3() end end function room4 () print("congratulations!") end </code></pre> </blockquote> <p>So you see, when you make a recursive call like:</p> <pre class="lang-lua prettyprint-override"><code>function x(n) if n==0 then return 0 n= n-2 return x(n) + 1 end </code></pre> <p>This is not tail recursive because you still have things to do (add 1) in that function after the recursive call is made. If you input a very high number it will probably cause a stack overflow.</p>
    singulars
    1. This table or related slice is empty.
    1. POWhat is tail recursion?
      singulars
      1. PTQuestion
    1. PTAnswer
    1. USAir
    1. USHoffmann
    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. This table or related slice is empty.
 

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