Haskell-函数中的单词交换

Question

I'm trying to create a function which swaps a string (w1) in a sentence with another string (w2). Here is what I have so far:

swapwords :: String -> String -> String -> String 
swapwords w1 w2 [] = [] 
swapwords w1 w2 (x:xs) 
    | length (x:xs) < n = (x:xs) 
    | otherwise = do 
if w1 == take n (x:xs) then w2 ++ swapwords w1 w2 (drop n (x:xs)) 
else x:swapwords w1 w2 xs 
where 
    n = length w1 

However, I'm getting an 'undefined variable "main"' error, and 'Warning: Defined but not used: `w1'' and 'w2'. What am I doing wrong?

Answer 1

The error is due to you trying to build an executable from a module that does not define a function main . The warnings are caused by your code being somewhat awkwardly indented (I am assuming that you are using a Haskell 98 compiler; GHC 7.8.3 accepts your code if you compile it in Haskell 2010 mode.)

Although you can use do -notation to construct expressions of type String (as strings are lists and the type constructor for lists is a monad), it is a bit strange to do it in this context (you are not really doing anything monadic with the string you are constructing, aren't you?).

Slightly reformatting your code and adding a function main will make your code compile (also as Haskell 98) to an executable:

swapwords :: String -> String -> String   -> String 
swapwords    w1        w2        []       =  [] 
swapwords    w1        w2        (x : xs)
  | length (x:xs) < n                     = x : xs  
  | otherwise                             =
      if   w1 == take n (x:xs)
      then w2 ++ swapwords w1 w2 (drop n (x:xs)) 
      else x : swapwords w1 w2 xs 
  where 
    n = length w1

main :: IO ()
main =  putStrLn (swapwords "bar" "qux" "foo bar baz")

Running this executable then allows you to test your code:

$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 7.8.3

$ ghc -XHaskell98 Test.hs -o test.exe
[1 of 1] Compiling Main             ( Test.hs, Test.o )
Linking test.exe ...

$ ./test.exe 
foo qux baz

Answer 2

While this has nothing to do with your compile problems, here is another way you could implement your function using Text.Regex.PCRE :

module Swap where

import Text.Regex.PCRE ((=~))

swap :: String -> String -> String -> String
swap orig toSwap ws =
  case ws =~ orig :: (String,String,String) of
   (_,"","") -> ws
   (b,_,a) -> b ++ toSwap ++ a

Although this text-icu library is probably better for this by now, as Text.Regex doesn't support Text.Data.Text