Fibonacci secuence 无法将预期类型 'a0 -> t' 与实际类型 '[Integer]' 匹配

Question

Down two functions. One that calculates fibonacci of a certain number and the other that tries to show the fibonacci sequence up to a certain number using map. The error occurs when I type fibonacci and a number, for example fibonacci 8 and throws error. What am I doing wrong?

fib :: Integer -> Integer
fib 0 = 0
fib 1 = 1
fib n = fib (n-1) + fib (n-2)

fibonacci :: [Integer]
fibonacci = map fib [0..]

Answer 1

When you use fibonacci , you're writing:

> fibonacci 8
(error)

But, fibonacci :: [Integer] - fibonacci isn't a function and you can't call it! fibonacci is an infinite list, so you must use list-based operations to get specific numbers.

The list indexing operator is (!!) :: [a] -> Integer -> a , so use that:

> fibonacci !! 8
21

You could also use take :: Integer -> [a] -> [a] to get the first n numbers:

> take 8 fibonacci
[0,1,1,2,3,5,8,13]

You can, however do this:

> fib 8
21

Because fib :: Integer -> Integer is a function.