在Haskell中实现此monad/type? [英] Implementing this monad/type in Haskell?

问题描述

我真的无法弄清楚这样做的必要语法，这可能是由于我对类型的工作原理缺乏了解.

I really cannot figure out the syntax necessary for this, and it probably comes from my lack of understanding of how types work.

我想要一个类型DataPoint，它存储一个元组(x, dataval)或两个字段x和dataval(其中x是Double而dataval是Complex Double.

I want a type DataPoint, which stores either a tuple (x, dataval) or two fields x and dataval (where x is a Double and dataval is a Complex Double.

我想要一个Monad实例，它的位置类似于:

I want a Monad instance where it goes something like:

instance Monad (DataPoint x dataval) where
    return dataval = DataPoint 0.0 dataval
    DataPoint x dataval >>= f = DataPoint x (f dataval)

基本上，monad的值"是dataval，而x只是容器的一部分.

Basically, the "value" of the monad is dataval, and x is just a part of the container.

我认为我的语法不正确.如果我将DataPoint定义为

I don't think my syntax is correct though. If i define DataPoint as

data DataPoint x dataval = DataPoint { x       :: Double
                                     , dataval :: Complex Double }

那它应该工作，对吧?

只有我遇到不匹配的情况"

Only I get a "kind mismatch"

The first argument of `Monad' should have kind `* -> *',
but `DataPoint x dataval' has kind `*'

有人可以帮助我获得我要实现的功能/单子吗?

Can anyone help me get the functionality/monad I am trying to acheive?

推荐答案

就语法而言，

instance Monad (DataPoint x) where
    -- etc

尽管我同意hammar的担忧，并认为您应该尝试使其成为Functor:

Although I share hammar's concerns and think you should be trying to make it a Functor instead:

instance Functor (DataPoint x) where
    -- etc

您得到的同类错误

The first argument of `Monad' should have kind `* -> *',
but `DataPoint x dataval' has kind `*'

是因为Monad和Functor是适用于高阶类型的类型类(与Monoid(适用于简单类型的类型类比较)相比).

is because Monad and Functor are typeclasses that apply to higher order types (compare with Monoid, a typeclass that applies to simple types).

例如IO Int不是 一元； IO ()不是 一元； IO 是一个单子.

e.g. IO Int is not a monad; IO () is not a monad; IO is a monad.

我想要一个类型DataPoint，它存储一个元组(x, dataval)或两个字段x和dataval(其中x是Double，而dataval是Complex Double. /p>

I want a type DataPoint, which stores either a tuple (x, dataval) or two fields x and dataval (where x is a Double and dataval is a Complex Double.

data DataPoint a = DataPoint {x :: Double,
                              dataval :: a}

instance Functor DataPoint where
    fmap f dataPoint = DataPoint {x = x dataPoint,
                                  dataval = f (dataval dataPoint)}

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