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User Error Matching Error Using getArgs

import Control.Concurrent (forkIO)
import System.Environment (getArgs)

main= do
    [a,b]<- getArgs
    putStrLn $ "command line arguments: " ++ show [a,b]

When I compiled it, everything was fine, but when I started it,
he said "user error (pattern matching error in expression)", what is wrong here?

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2 answers

The problem is that you are matching the pattern [a, b]with the return value getArgs. If you run your program with something other than two arguments, then the return value will not match the pattern [a, b]. Therefore, if you do not run this program as

$ ./xie 1 2
command line arguments: ["1","2"]

He gives an error. Instead, if you wrote your code

main = do
    args <- getArgs
    case args of
        [a, b] -> putStrLn $ "command line arguments: " ++ show [a, b]
        _      -> putStrLn "Invalid number of arguments"

then you will never fail when matching with a pattern.

+6

[a,b] 2- , , getArgs , .

do, , fail, IO userError.

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