Take - Haskell problems within lists

Question

I am in the process of learning Haskell. I have a function that looks as follows:

takeN :: Integral a => a -> [a]
takeN n = take n [x | x<-[0..]]

All I want this to do, is return n amount of elements in an infinite list, and I am unaware of why this is not working. Any explanations of how to fix it without abandoning my binding (?)

Notice `takeN n = [0 .. n - 1]` also works and has the same meaning - the fact that `[0..]` is potentially infinite is immaterial. — Thomas M. DuBuisson, Mar 04 '16 at 21:10

score 3 · Accepted Answer · answered Mar 04 '16 at 20:52

3

The reason this doesn't work is that take has the type Int -> [a] -> [a]. The number must be an Int, and can't be any Integral.

You can address the issue with fromIntegral:

takeN :: Integral a => a -> [a]
takeN n = take (fromIntegral n) [x | x<-[0..]]

answered Mar 04 '16 at 20:52

Mark Seemann

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@Carsten I'm not sure what you mean... The above implementation works on my machine™. – Mark Seemann Mar 04 '16 at 20:54

score 0 · Answer 2 · answered Mar 04 '16 at 20:52

0

You can do it without making your own function: input>> take 3 [1..] output>> [1,2,3]

answered Mar 04 '16 at 20:52

MugenTwo

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Take - Haskell problems within lists

2 Answers2