从箭头符号转换 [英] Convert from arrow notation

问题描述

我仍然试图弄清楚箭头符号和在Haskell中定义的箭头类型的语义之间的相似之处。特别是，这个问题似乎有一个很小的计数器的典型例子用箭头符号表示：

  counter :: ArrowCircuit a => Bool Int 
 counter = proc reset  - >做
 rec输出<  -  returnA  - <如果重置，那么0 else else 
 next<  - 延迟0  - <输出+ 1 
 returnA  - <输出
  

有人可以告诉我如何将其转换回Haskell2010而不使用箭头符号吗？

解决方案

  { -  | 
 + --------- + 
> Bool> --------------> | 
 | > ------------------>诠释> 
 + --------- + | arr f | 
 / ---->延迟0>  - > > --------- \ 
 | + --------- + | | | 
 | + --------- + | 
 | | 
 \ -------------------------------------- / 
 
  - } 
 counter':: ArrowCircuit a => Bool Int 
 counter'= loop $ second（delay 0）>>> arr f 
其中
f（reset，next）= let output = if reset 0 then else next 
 next'= output + 1 
 in（output，next'）
  

递归 rec 部分使用环。使用下一个（（code>））将 reset 转换为输出的内部部分并产生新的下一个值）只是一个具有两个输入和两个输出的纯函数。

I'm still trying to get a hang of the parallels between arrow notation and the semantics of the Arrow typeclasses defined in Haskell. In particular, this question seems to have a very canonical example of a small counter written with arrow notation:

counter :: ArrowCircuit a => a Bool Int
counter = proc reset -> do
        rec     output <- returnA -< if reset then 0 else next
                next <- delay 0 -< output+1
        returnA -< output

Can someone show me how to convert this back into Haskell2010 without arrow notation?

解决方案

{- |
                     +---------+
 >Bool>-------------->         |
                     |         >------------------>Int>
       +---------+   |  arr f  |
  /----> delay 0 >--->         >---------\
  |    +---------+   |         |         |
  |                  +---------+         |
  |                                      |
  \--------------------------------------/ 

 -}
counter' :: ArrowCircuit a => a Bool Int
counter' = loop $ second (delay 0) >>> arr f
  where
    f (reset, next) = let output = if reset then 0 else next
                          next' = output + 1
                       in (output, next')

The recursive rec part is implemented using loop. The inner part that converts reset to output using next (and producing new next value) is just a pure function with two inputs and two outputs.

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