Haskell usage of null -

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I Real World Haskell and copy me, on page 59, It says:

We do not have a blank in Haskell, we can probably use ... instead we have decided to use a negative argument to use an empty constructor.

Then, in the next section, there is a hascal function on errors:

Now, I do not understand that "null" In this function definition Refers not, because it is not the same as was clearly stated that the null Haskell (Java).

Any help appreciated.

zero is the function that tests if only one list is empty. There is nothing to do with other tapable values ​​in other languages. 

  null :: [a] - & gt; Bull Null [] = True Null (_: _) = Incorrect

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