Compute mathematical constant `e`.

e-convergent.hs

e n = 1 + (foldr1 (\ a x -> a + 1 / x) $ take n [i ^ j | i <- [2,4..], j <- [0,0,1]])


-- sample:
--   e 1   -> 2.0
--   e 2   -> 3.0
--   e 6   -> 2.71875
--   e 100 -> 2.7182818284590455
-- see more:
--   <http://en.wikipedia.org/wiki/E_(mathematical_constant)#Representations>

e-infinite-series.hs

e n = (\(e:es) -> (e:'.':es)) $ show $ (\(a,l) -> (10^n * sum l) `div` a) $ (\(a,l) -> (a, [a `div` i | i <- l])) $ (\l -> (product l, l)) [factorial n | n <- [0..4*n`div`3]] where factorial 0 = 1; factorial n = n * factorial (n-1)


-- sample:
--   e 100 -> "2.7182818284590452353602874713526624977572470936999595749669676277240766303535475945713821785251664274"