x = constant 3 ^+^ time
If I understand the rest of your mail, Wire defines an Applicative
instance, so why not:
x = (+3) <$> time
??
2011/12/12 Ertugrul Söylemez
Hello fellows,
after a few discussions on IRC and via private mail I feel obligated to point out that arrows and in particular AFRP do not force you to use an imperative style in any way. You can use a style very similar to SHE's idiom brackets. I will demonstrate this using the Netwire library. The following code has a very imperative feel to it and also looks quite ugly:
myWire = proc _ -> do fps <- avgFpsInt 1000 100 -< () t <- time -< () let x = 3 + t y <- integral 0 -< t returnA -< printf "%8.2f %8.2f %8.2f" fps x y
Let's improve this code. The magic lies in identifying behaviors from classic FRP. The arrow variables from the above code can be seen as the behaviors, but that's not very useful for getting rid of the imperative style. A better way to look at it is that every wire that ignores its input (i.e. has a fully polymorphic input type) is a behavior, so let's find them.
First of all it is impossible to write a proper Num instance for wires. The underlying problem is the same as for writing a Num instance for functions. However, the Wire type forms a vector space, and the next release of Netwire will include the corresponding instances (see the vector-space package by Conal Elliot). With them we can write:
x = constant 3 ^+^ time
The x wire is our first behavior. Passing x to a generic wire is simply regular arrow composition, giving you behaviors as functions of other behaviors, hence:
y = integral 0 <<< x
Also fps is just a simple behavior:
fps = avgFpsInt 1000 100
To get to the final output there are multiple ways. Perhaps the nicest way is to exploit the Applicative instance, giving you:
myWire = liftA3 (printf "%8.2f %8.2f %8.2f") fps x y
where fps = avgFpsInt 1000 100 x = constant 3 ^+^ time y = integral 0 <<< x
Looks much more declarative, no? For more complicated compositions or generic wires use banana brackets or the combinator syntax in arrow notation. Examples:
myWire = proc x -> (someWire -< x) ^+^ (otherWire -< 15)
myWire = proc x -> (| f (someWire -< x) (otherWire -< 15) |)
This closely resembles idiom bracket notation, but allows you to be explicit about inputs, takes care of creating side channels and gives you the full power of arrows, including ArrowChoice and ArrowLoop.
Greets, Ertugrul
-- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/
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