Return function that modifies the value of the input function

问题

How can I make a function that is given a function as input and returns a function with the value tripled. Here is some pseudo code for what I'm looking for. Concrete examples in Python or Scala would be appreciated.

def f(int x):
    return x ** 2

def tipleFunc(Function g)
    return 3 * g

Function newFunc = tripleFunc(f)
print newFunc(5)

回答1:

def f(x):
    return x ** 2

def tripleFunc(g):
    return lambda *args: 3 * g(*args)

newFunc = tripleFunc(f)
print newFunc(5)

回答2:

In Scala:

def foo(x: Int) = x * x
def quadruple(f: Int => Int) = (x: Int) => 4 * f(x)
val quadfoo = quadruple(foo)
scala> quadfoo(3)
res0: Int = 36

回答3:

You can use the Python @decorator syntax to do this; @decorator is equivalent to assigning somefunc = decorator(somefunc), allowing an arbitrary wrapper function to be applied:

>>> from functools import wraps
>>> def multiplier(n):
    """Create a decorator to multiply a wrapped function's output by n."""
    def wrapper(f):
        @wraps(f) # retain the wrapped function's docstring
        def func(*args, **kwargs):
            return n * f(*args, **kwargs)
        return func
    return wrapper

>>> @multiplier(3) # apply a multiplier of 3 to the outputs of f
def f(x):
    """Return x squared."""
    return x ** 2

>>> f(5)
75

Note the *args, **kwargs syntax to handle arbitrary arguments (positional and keyword, respectively). You can also create the triple_func decorator directly, e.g.:

>>> triple_func = multiplier(3)
>>> @triple_func
def f(x):
    """Return x squared."""
    return x ** 2

>>> f(5)
75

or even apply it without the decorator syntax at all:

>>> def f(x):
    """Return x squared."""
    return x ** 2

>>> new_func = triple_func(f) # or 'new_func = multiplier(3)(f)'
>>> new_func(5)
75

来源：https://stackoverflow.com/questions/26221148/return-function-that-modifies-the-value-of-the-input-function

标签

python

scala

functional-programming

higher-order-functions