[Haskell-beginners] Double usage of a wire (arrow)

Ertugrul Söylemez es at ertes.de
Mon Nov 5 17:22:48 CET 2012

Nathan Hüsken <nathan.huesken at posteo.de> wrote:

> If I double use an arrow (in this example a wire from netwire) like
> this:
> objectWire :: WireP [Collision] Object
> objectWire = (Object <$> integral_ initPos) . speedWire <*> speedWire
> will it be double evaluated, or can the compiler optimize this to
> evaluate speedWire only once?

It will be double-evaluated.  To prevent this you can use the Arrow

    proc x' -> do
        x <- speedWire -< x'
        {- ... use x ... -}

However, in WireP it's guaranteed that you get the same result, so for
code conciseness you can still have speedWire twice, if sacrificing some
speed is not too big an issue.


Not to be or to be and (not to be or to be and (not to be or to be and
(not to be or to be and ... that is the list monad.
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