7 lines
170 B
Haskell
7 lines
170 B
Haskell
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import Data.Function (fix)
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fib :: Integer -> Maybe Integer
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fib n
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| n < 0 = Nothing
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| otherwise = Just $ fix (\f -> (\n -> if n > 1 then f (n-1) + f (n-2) else 1)) n
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