fmap进入do块失败,并显示打印错误 [英] fmap into a do block fails with a print error
问题描述
我试图理解为什么我用do-block编写的函数不能被重写以在列表上fmap一个类似的lambda表达式.
I'm trying to understand why a function I have written with a do-block can't be rewritten to fmap a similar lambda expression over a list.
我有以下内容:
-- This works
test1 x = do
let m = T.pack $ show x
T.putStrLn m
test1 1
生产
1
但是
-- This fails
fmap (\x -> do
let m = T.pack $ show x
T.putStrLn m
) [1..10]
-- And this also fails
fmap (\x -> do
T.putStrLn $ T.pack $ show x
) [1..10]
有错误:
<interactive>:1:1: error:
• No instance for (Show (IO ())) arising from a use of ‘print’
• In a stmt of an interactive GHCi command: print it
我的putStrLn在正常工作和无效工作之间是一致的.进口是相同的.我需要打印的show-pack-putstrln舞步在正常工作和无效工作之间也是一致的.
My putStrLn is consistent between the working and the non-working. The imports are the same. My show-pack-putstrln dance required to print is also consistent between the working and the non-working.
正在使用的打印纸和正在使用的打印纸正在改变,这是怎么回事?
What is happening that the use of print is changing between the working and non-working?
-- I was also surprised that this fails
fmap (T.putStrLn $ T.pack $ show) [1..10]
-- it seemed as similar as possible to the test1 function but mapped.
<interactive>:1:7: error:
• Couldn't match expected type ‘Integer -> b’ with actual type ‘IO ()’
• In the first argument of ‘fmap’, namely ‘(T.putStrLn $ pack $ show)’
In the expression: fmap (T.putStrLn $ pack $ show) [1 .. 10]
In an equation for ‘it’: it = fmap (T.putStrLn $ pack $ show) [1 .. 10]
• Relevant bindings include it :: [b] (bound at <interactive>:1:1)
<interactive>:1:29: error:
• Couldn't match type ‘() -> String’ with ‘String’
Expected type: String
Actual type: () -> String
• Probable cause: ‘show’ is applied to too few arguments
In the second argument of ‘($)’, namely ‘show’
In the second argument of ‘($)’, namely ‘pack $ show’
In the first argument of ‘fmap’, namely ‘(T.putStrLn $ pack $ show)’
更新2
-- This lambda returns x of the same type as \x
-- even while incidentally printing along the way
fmap (\x -> do
let m = T.pack $ show x
T.putStrLn $ m
return x
) [1..10]
但也失败了:
<interactive>:1:1: error:
• No instance for (Show (IO Integer)) arising from a use of ‘print’
• In a stmt of an interactive GHCi command: print it
推荐答案
fmap f [1..10]
的类型是[T]
,其中T
是f
的返回类型.
The type of fmap f [1..10]
is [T]
where T
is the return type of f
.
在您的情况下为T = IO ()
,因此完整表达式的类型为[IO ()]
.
In your case, T = IO ()
, so the type of the full expression is [IO ()]
.
无法打印IO操作,因此,当您尝试打印该列表时,GHCi会抱怨.您可能想使用sequence_ (fmap f [1..10])
之类的方法来运行这些操作而不是打印它们.
IO actions can not be printed, so GHCi complains when you try to print that list. You might want to run those actions instead of printing them, using something like sequence_ (fmap f [1..10])
.
或者,考虑放弃fmap
,而是使用类似的
Alternatively, consider ditching fmap
and instead using something like
import Data.Foldable (for_)
main = do
putStrLn "hello"
for_ [1..10] $ \i -> do
putStrLn "in the loop"
print (i*2)
putStrLn "out of the loop"
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