在 Haskell 中混合 Monad

发布于 2024-11-03 19:20:25 字数 989 浏览 1 评论 0原文

我正在尝试在 haskell 中使用 Ubigraph，但我相信我的问题更通用。我正在尝试编译：

import Graphics.Ubigraph
import Control.Monad
import System.Posix.Unistd

main = do
    h <- initHubigraph "http://127.0.0.1:20738/RPC2"
    runHubigraph op h

op = do
  clear
  vs <- mapM (const newVertex) [0..200]
  mapM_ (setVAttr (VShape Sphere)) vs
  putStrLn "something"
  let bind i = zipWithM (\a b -> newEdge (a,b)) vs (drop i vs ++ take i vs)
  mapM_ bind [1..15]
  mapM_ (removeVertex) vs
  return ()

并且我得到了

无法匹配预期类型`Control.Monad.Trans.Reader.ReaderT
                                Ubigraph IO a0'
            与实际类型“IO ()”
在“putStrLn”调用的返回类型中
在 'do' 表达式的 stmt 中： putStrLn "something"
在表达式中：
  做{清除;
       vs <-mapM (const newVertex) [0 .. 200];
       mapM_(setVAttr(VShape Sphere)) vs;
       putStrLn“某事”；
       ....}

我可以看到 op 的类型如何隐含为与 putStrLn 的返回类型不同的东西，但我不确定如何重新设计此代码以正确编译。我可以简单地更改 op 函数的返回类型吗？

谢谢

原文

I'm trying to work with Ubigraph in haskell, but I believe my problem is more generic. I'm trying to compile:

import Graphics.Ubigraph
import Control.Monad
import System.Posix.Unistd

main = do
    h <- initHubigraph "http://127.0.0.1:20738/RPC2"
    runHubigraph op h

op = do
  clear
  vs <- mapM (const newVertex) [0..200]
  mapM_ (setVAttr (VShape Sphere)) vs
  putStrLn "something"
  let bind i = zipWithM (\a b -> newEdge (a,b)) vs (drop i vs ++ take i vs)
  mapM_ bind [1..15]
  mapM_ (removeVertex) vs
  return ()

and I am getting

Couldn't match expected type `Control.Monad.Trans.Reader.ReaderT
                                Ubigraph IO a0'
            with actual type `IO ()'
In the return type of a call of `putStrLn'
In a stmt of a 'do' expression: putStrLn "something"
In the expression:
  do { clear;
       vs <- mapM (const newVertex) [0 .. 200];
       mapM_ (setVAttr (VShape Sphere)) vs;
       putStrLn "something";
       .... }

I can see how the type of op is being implied as something different from the return type of putStrLn, but I am not sure how I would reengineer this code to compile properly. Can I simply change the return type of the op function?

Thanks

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您的好友蓝忘机已上羡 2024-11-10 19:20:26

您对埋藏在 op 中的 putStrLn 的调用位于 IO monad 中。您需要将其提升到 Ubigraph monad 中，使用

 liftIO $ putStrLn "foo"

这样的提升函数可以帮助您访问堆栈中较低的 Monadic 函数。在这种情况下，Ubigraph是一个由IO monad组成的Reader monad，IO monad位于底部。所以IO里面的东西一定要解除。

liftIO 来自 MonadIO 类，在变压器包中。

Your call to putStrLn buried down in op is in the IO monad. You will need to lift it into the Ubigraph monad, using

 liftIO $ putStrLn "foo"

Such lifting functions help you access monadic functions lower in the stack. In this case, Ubigraph is a Reader monad composed with the IO monad, and the IO monad is at the bottom. So things in IO must be lifted.

liftIO is from the MonadIO class, in the transformers package.

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