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pythonpython-3.xlambda

Nested lambdas working logic needs explanation

I got code from here: https://news.ycombinator.com/item?id=25869596

>>> (
      lambda n: (lambda fact: fact(n, fact))(
          lambda n, inner: 1 if n == 0 else (n * inner(n - 1, inner))
      )
  )(5)
  120

How does this work? I particularly cannot understand how inner gets value.

Solution

  • This can be written as

    def func(n):

    def func1(fact):
        return fact(n, fact)

    def func2(n, inner):
        return 1 if n == 0 else (n * inner(n - 1, inner))

    return func1(func2)

func(5)

    inner is func2, or lambda n, inner: 1 if n == 0 else (n * inner(n - 1, inner)) in your example.

    lambda fact: fact(n, fact) get the lambda as a parameter and call it recursively.