September Morn Update

Task/First-class-functions/Python/first-class-functions-2.py

@@ -0,0 +1,59 @@
+'''First-class functions'''
+
+from math import (acos, cos, asin, sin)
+from inspect import signature
+
+
+# main :: IO ()
+def main():
+    '''Composition of several functions.'''
+
+    pwr = flip(curry(pow))
+
+    fs = [sin, cos, pwr(3.0)]
+    ifs = [asin, acos, pwr(1 / 3.0)]
+
+    print([
+        f(0.5) for f in
+        zipWith(compose)(fs)(ifs)
+    ])
+
+
+# GENERIC FUNCTIONS ------------------------------
+
+
+# compose (<<<) :: (b -> c) -> (a -> b) -> a -> c
+def compose(g):
+    '''Right to left function composition.'''
+    return lambda f: lambda x: g(f(x))
+
+
+# curry :: ((a, b) -> c) -> a -> b -> c
+def curry(f):
+    '''A curried function derived
+       from an uncurried function.'''
+    return lambda a: lambda b: f(a, b)
+
+
+# flip :: (a -> b -> c) -> b -> a -> c
+def flip(f):
+    '''The (curried or uncurried) function f with its
+       two arguments reversed.'''
+    if 1 < len(signature(f).parameters):
+        return lambda a, b: f(b, a)
+    else:
+        return lambda a: lambda b: f(b)(a)
+
+
+# zipWith :: (a -> b -> c) -> [a] -> [b] -> [c]
+def zipWith(f):
+    '''A list constructed by zipping with a
+       custom function, rather than with the
+       default tuple constructor.'''
+    return lambda xs: lambda ys: [
+        f(a)(b) for (a, b) in zip(xs, ys)
+    ]
+
+
+if __name__ == '__main__':
+    main()