Haskell: unexpected "Not in scope" error for simple function

Question

This is very ad-hoc example describes Not in scope: isOne error that I have:

ignoreFirstOnes :: [Int] -> [Int]
ignoreFirstOnes (1:xs) = dropWhile isOne xs
ignoreFirstOnes xs     = xs
  where isOne = (== 1)

Strange that isOne function was defined in where, however compiler keeps complaining. I can rewrite that using guards or even to dropWhile (== 1) but I would like to understand how to make work the current example.

This is part of the reason I prefer case for most functions: I often want to use a where clause definition in multiple branches, and equational style doesn’t allow it. — Jon Purdy
– Jon Purdy, Commented May 14, 2016 at 19:27

Benjamin Hodgson · Accepted Answer · 2016-05-14 11:14:40Z

5

Names defined in a where clause are only in scope over the branch that the where clause is attached to.

This version of your definition will compile, because I attached the where clause to the branch of ignoreFirstOnes that uses isOne.

ignoreFirstOnes :: [Int] -> [Int]
ignoreFirstOnes (1:xs) = dropWhile isOne xs
    where isOne = (== 1)
ignoreFirstOnes xs = xs

Though note that this definition is equivalent to ignoreFirstOnes = dropWhile (==1), which I think is simpler.

answered May 14, 2016 at 11:14

Benjamin Hodgson

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1 Comment

franza Over a year ago

Yes, I have already mentioned about dropWhile (==1) in question, meaning that this example is only to describe my concern. However, I have never known about this feature, so thank you.

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