自身数据类型的haskell随机实例 [英] haskell random instance of own datatype

问题描述

我正在使用Graphics.gloss开发Haskell制作的小行星游戏.现在，我已经为小行星定义了这样的数据类型:

I am working on an Asteroid game made in Haskell with Graphics.gloss. Now I have defined a datatype for the asteroids like this:

data Asteroid = Asteroid {   asteroidPos:: Point, 
                             asteroidVel   :: Vector,
                             asteroidSize  :: Float }

因此它具有一个由点定义的位置，由向量定义的速度及其大小.现在，我想知道如何为该数据类型编写一个Random实例，以便在随机时间以随机速度在随机位置出现一个新的小行星.有人知道我该怎么做吗?

So that it has a Position defined by a point, a velocity defined by a vector and it's size. Now I want to know how I could write an instance of Random for this datatype so that a new asteroid appears at a random time, on a random place with a random velocity. Does anyone know how I can accomplish this?

推荐答案

Float已有一个Random实例.假设您还有Vector和Point的Random实例，则可以使用它们为Asteroid定义Random实例:

There's already a Random instance for Float. Assuming that you also have Random instances for Vector and Point, you can use them to define a Random instance for Asteroid:

instance Random Asteroid where
  randomR (Asteroid pl vl sl, Asteroid ph vh sh) g =
    let (p, g1) = randomR (pl, ph) g
        (v, g2) = randomR (vl, vh) g1
        (s, g3) = randomR (sl, sh) g2
    in (Asteroid p v s, g3)
  random g =
    let (p, g1) = random g
        (v, g2) = random g1
        (s, g3) = random g2
    in (Asteroid p v s, g3)

randomR函数采用从最小(低)到最大(高)的值范围.如何定义诸如Asteroid之类的复杂值的有意义范围，我将任由您自己决定；在这里，我只是假设您可以传递一个高低的Asteroid值.

The randomR function takes a range of values, from minimum (low) to maximum (high). How you define a meaningful range of a complex value like Asteroid, I'll leave up to you; here, I'm simply assuming that you can pass a low and high Asteroid value.

第一步，通过调用randomR将基础Random实例用于Point. p是随机生成的Point值，而g1是下一个要使用的随机生成器值.

The first step uses the underlying Random instance for Point by calling randomR. p is a randomly generated Point value, and g1 is the next random generator value to use.

同样，v是随机生成的Vector值，由Vector的基础实例生成.

Likewise, v is a randomly generated Vector value, generated by the underlying instance for Vector.

最后，s是Float由randomR生成的Float.

返回值是一个新的Asteroid值，由p，v和s加上最新的生成器g3组成.

The return value is a new Asteroid value composed from p, v, and s, plus the most recent generator g3.

random的实现遵循相同的模板.

The implementation of random follows the same template.

例如，您可以使用更漂亮的形式编写此代码.使用MonadRandom，但是对于初学者来说，我将代码保留为原始格式以显示其工作原理.

You could write this code in a prettier form, using e.g. using MonadRandom, but for starters, I left the code in its raw form to show how it works.

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