On Dec 1, 7:37 pm, Timothy Pratley <[EMAIL PROTECTED]> wrote:
> Local variables are good in these situations
>
> 1) You want to accumulate some changes eg:
> // sum odds
> int x = 0;
> for (int i=0; i<100; i++) {
> if ( i%2==1 ) x+=i; }
>
> ;It is relatively easy to rearrange these sort of things into a form
> that requires no variables
> user=> (reduce #(+ %1 (if (= 1 (rem %2 2)) %2 0)) (range 100))
> 2500
>
> 2) You need to save a result and use it multiple times:
> Can work around this with let:
> (let [g (rand 0.8)]
> (Color3f. (+ 0.2 g) (+ 0.2 g) (+ 0.2 g)))
>
> 3) Java interop:
> I find using java libraries often forces you into a situation where
> you want to use a local variable.
> You can always get around it by using nested lets though it makes the
> code ugly. However keep in mind doto.
> The great thing about doto is that it returns the object after
> applying several calls, so it allows you to avoid nesting lets in a
> lot of situations:
> (doto (MainFrame. (doto (Applet.)
> (setLayout (BorderLayout.))
> (add "Center" (.getCanvas *universe*)))
> 1024 512)
> (setTitle "2D Plotting in Clojure"))
>
> 4) Mutating permanent state variables
> Use refs with some simple wrappers to reduce the syntax requirements
>
> I find that if I follow those four principles I can usually figure out
> how to translate from variables to non-variables.
>
> I don't fully understand why local variables are such a Bad Thing...
> or if it is just a style enforcement... but I remember reading
> somewhere that Clojure doesn't use the stack hence no local variables.
Clojure does use the stack, it just doesn't support mutable locals.
The reason is simple - plain mutable variables have no concurrency
semantics. What if you closed over a mutable local? Now you have an
object with no synchronization, a concurrency mess.
All of the mutable Clojure references have concurrency semantics, so
they have dependable behavior and promises, and require no locks.
Rich
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