haskell problems 001 -> 006

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+-- Even Fibonacci numbers
+
+-- Problem 2
+-- Each new term in the Fibonacci sequence is generated by adding the previous two terms. By starting with 1 and 2, the first 10 terms will be:
+
+-- 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...
+
+-- By considering the terms in the Fibonacci sequence whose values do not exceed four million, find the sum of the even-valued terms.
+
+
+main = do
+    print (sum (filter even (fib_until 4000000)))
+
+fib_until threshold = fib_from_index_until 0 threshold
+
+fib_from_index_until :: Int -> Int -> [Int]
+fib_from_index_until i threshold
+    | fib_i < threshold = fib_i:(fib_from_index_until (i + 1) threshold)
+    | otherwise = []
+        where fib_i = fibonacci i
+
+fibonacci :: Int -> Int
+fibonacci 0 = 0
+fibonacci 1 = 1
+fibonacci 2 = 1
+fibonacci x =  fibonacci (x - 1) + fibonacci (x - 2)