Haskell打印字符串stdout [英] Haskell printing strings stdout

问题描述

我需要在powerset中打印元素。现在我的代码的输出是这样的：

 a
ab
b
x
xy
xyz
xz
y
yz
z
  

但是，我需要输出不带引号，如下所示：

  a 
 ab 
b 
x 
 xy 
 xyz 
 xz 
y 
 yz 
z 
  

这就是我所拥有的。

  import Data.List 
 powerset = foldr（\ x acc  - > acc ++ map（x :) acc）[[]] 
 
 main = do 
 numCases<  -  getLine 
 repl $（读取numCases :: Int）
 
 repl num = do 
 if（num == 0）then return（）
 else do 
 size<  -  getLine 
 input< ;  -  getLine 
 let ret = tail $ sort $ powerset input $ b $ mapM（\ x  - > print x）ret 
 repl $ num-1 
 
 
 
解决方案
第一（\x  - > fx）相当于普通的 f （在几乎所有情况下）通过eta-reduction。因此，您可以将 mapM（\ x  - > print x）重写为 mapM print 。 p> 
 
 
要删除引号，您应该使用函数 putStrLn 而不是 print 函数。  print 中的引号来自 print = putStrLn。显示。  show 是一个函数，它以一种可以（如果定义了一个合适的实例）的方式打印出值，并用读入。因此，字符串上的引号，您不希望（或需要）用于您的用例。
 
I need to print the elements in a powerset. Right now my code's output is this:
"a"
"ab"
"b"
"x"
"xy"
"xyz"
"xz"
"y"
"yz"
"z"
However, I need the output to not have quotation marks, like this:
a
ab
b
x
xy
xyz
xz
y
yz
z
This is what I have. How do I fix it do to get the right output?
import Data.List
powerset = foldr (\x acc -> acc ++ map (x:) acc) [[]]

main = do
    numCases <- getLine
    repl $ (read numCases :: Int)

repl num = do
    if(num == 0) then return ()
    else do
        size <- getLine
        input <- getLine
        let ret = tail $ sort $ powerset input
        mapM (\x -> print x) ret
        repl $ num-1

解决方案
First (\x -> f x) is equivalent to plain f (in almost all cases) by eta-reduction. So, you can re-write mapM (\x -> print x) as mapM print.

To remove the quotation marks, you should use the function putStrLn instead of the print function. The quotation marks in print come from print = putStrLn . show. show is a function that prints out values in a way that can (if a suitable instance is defined) be read back in with read. Thus, quotation marks on strings, which you don't want (or need) for your use-case.

                        
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