Added Forward Euler to Haskell (#215)

* Added Forward Euler in Haskell

Author: Thurii

chapters/differential_equations/euler/code/haskell/euler.hs

@@ -0,0 +1,20 @@
+solveEuler :: Num a => (a -> a) -> a -> a -> [a]
+solveEuler f ts = iterate (\x -> x + f x * ts)
+
+checkResult :: (Ord a, Num a, Num t, Enum t) => a -> (t -> a) -> [a] -> Bool
+checkResult thresh check =
+    and . zipWith (\i k -> abs (check i - k) < thresh) [0..]
+
+kinematics :: Double -> Double
+kinematics x = -3 * x
+
+main :: IO ()
+main =
+    let timestep = 0.01
+        n = 100
+        threshold = 0.01
+        checkResult' = checkResult threshold $ exp . (\x -> -3 * x * timestep)
+    in  putStrLn $
+        if checkResult' (take n $ solveEuler kinematics timestep 1)
+        then "All values within threshold"
+        else "Value(s) not in threshold"

chapters/differential_equations/euler/euler.md

@@ -121,6 +121,9 @@ Full code for the visualization follows:
 {% sample lang="py" %}
 ### Python
 [import, lang:"python"](code/python/euler.py)
+{% sample lang="hs" %}
+### Haskell
+[import, lang:"haskell"](code/haskell/euler.hs)
 {% endmethod %}
 
 <script>
@@ -145,4 +148,3 @@ $$
 \newcommand{\bfomega}{\boldsymbol{\omega}}
 \newcommand{\bftau}{\boldsymbol{\tau}}
 $$
-