Confusing function application and function composition in Haskell

The operation

(filter (`notElem` "'\"").[(1,'a','%',"yes"开发者_StackOverflow)])

gives an error. How can apply this filter on that list properly?

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You've got a couple of serious problems. First, your syntax is wacky (. definitely shouldn't be there). But the bigger problem is that what you're trying to filter is of the type [(Int,Char,Char,[Char])] (that is, a list containing a 4-tuple).

And your list has only one element, which is (1,'a','%',"yes"). So filtering that is useless anyway. When function you provide for filtering must be of type a -> Boolean, where a is the type of all the elements of the list.

Seems like you wanted some sort of wonky heterogenous list or something.

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The . operator in Haskell is function composition -- it composes two functions together.

So your code,

(`notElem` "'\"") . [(1,'a','%',"yes")]

looks like the composition of the notElem function and some list. That's just wrong.

Remove the ., and make sure to show the list first:

> filter (`notElem` "'\"") (show [(1,'a','%',"yes")])
"[(1,a,%,yes)]"