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Undefined

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This is a very useful value: undefined :: a You can put it anywhere and it will compile. When evaluated, it throws an exception "undefined". But thanks to laziness you can put it anywhere you've not written yet:

   Haskell> let (x,y) = (undefined,"hello!") in y
   "hello!"

If you just want to get a function to compile, but you've not finished writing half of it, just stick an undefined in there. For example, say I'm writing a function to read from a file, print it out and parse it with a given parser; I haven't thought of the parse bit yet, but I want to check the rest of the function's okay, so I just stick an undefined in there:

   readFromFile :: Parser a -> SourceName -> IO (Either ParseError a)
   readFromFile parser path = do
     contents <- readFile path
     putStrLn $ contents
     return undefined

Now I come back to it, see the undefined and I know what needs to be written:

   readFromFile :: Parser a -> SourceName -> IO (Either ParseError a)
   readFromFile parser path = do
     contents <- readFile path
     putStrLn $ contents
     return $ parse parser path contents

It's also good for initialising record values:

   MyRecord { rec_name = "Foo", rec_other = undefined }

This function is the same: error :: String -> a You can use it in the same way:

   MyRecord { rec_name = "Foo", rec_other = error "rec_other is empty" }
   readFromFile :: Parser a -> SourceName -> IO (Either ParseError a)
   readFromFile parser path = do
     contents <- readFile path
     putStrLn $ contents
     return $ error "implement the return part of readfromfile"
   Haskell> (2*2,error "failure!")
   (4,*** Exception: failure!