Initial data commit

Task/Factorial/Haskell/factorial-1.hs

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+factorial n = product [1..n]

Task/Factorial/Haskell/factorial-2.hs

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+factorial = product . enumFromTo 1

Task/Factorial/Haskell/factorial-3.hs

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+factorial n = foldl (*) 1 [1..n]

Task/Factorial/Haskell/factorial-4.hs

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+factorials = scanl (*) 1 [1..]

Task/Factorial/Haskell/factorial-5.hs

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+factorial :: Integral -> Integral
+factorial 0 = 1
+factorial n = n * factorial (n-1)

Task/Factorial/Haskell/factorial-6.hs

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+fac n
+    | n >= 0    = go 1 n
+    | otherwise = error "Negative factorial!"
+        where go acc 0 = acc
+              go acc n = go (acc * n) (n - 1)

Task/Factorial/Haskell/factorial-7.hs

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+{-# LANGUAGE PostfixOperators #-}
+
+(!) :: Integer -> Integer
+(!) 0 = 1
+(!) n = n * (pred n !)
+
+main :: IO ()
+main = do
+  print (5 !)
+  print ((4 !) !)

Task/Factorial/Haskell/factorial-8.hs

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+-- product of [a,a+1..b]
+productFromTo a b =
+  if a>b then 1
+  else if a == b then a
+  else productFromTo a c * productFromTo (c+1) b
+  where c = (a+b) `div` 2
+
+factorial = productFromTo 1