The Fibonacci sequence is defined by the recurrence relation:

Fn = Fn1 + Fn2, where F1 = 1 and F2 = 1.

Hence the first 12 terms will be:

F1 = 1

F2 = 1

F3 = 2

F4 = 3

F5 = 5

F6 = 8

F7 = 13

F8 = 21

F9 = 34

F10 = 55

F11 = 89

F12 = 144

The 12th term, F12, is the first term to contain three digits.

What is the first term in the Fibonacci sequence to contain 1000 digits? 

import Control.Arrow
fib = :1:[i+j|(i,j)<-zip fib $tail fib]
p025 = length.takeWhile((<1000).snd).map(id&&&length.show)$ fib

じつはArrowなど使わなくてよかった罠 

fib = :1:[i+j|(i,j)<-zip fib $tail fib]
p025 = length.takeWhile(<1000).map(length.show)$ fib