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Question

I\'ve been trying to define a function which, given a list of Integers and an Integer n, returns a Boolean indicating whether n occurs exactly once in the list.

I have t

Answer 1

Well, you can filter the list, then see how many elements are in the resulting filter, right?

To get you started:

 > filter (== 2) [1,2,3,4,5]
 [2]

 > filter (== 2) [1,2,3,4,5,2,2]
 [2,2,2]

And to fold your list down to a Bool value, here, an example where we test if a list has three elements, returning a Bool:

 > isThree (a:b:c:[]) = True
 > isThree _          = False 

So it is just a short matter of composing such functions:

 > isThree . filter (==2)

or your variant (e.g. matching for lists of length 1).