How to create dispatch style lambda function using Python?

Question

If I have the given dispatch-style function to represent a pair, how can I implement the same using a one-line lambda function?

# ADT Level 0 (dispatch-style pair)
def make_pair(x, y):
"""Dispatch-style pair"""
    def dispatch(m):
        if m == 0:
            return x
        elif m == 1:
            return y
    return dispatch

Why do you want to do this? It will lead to bad readability. — , Dec 15 '14 at 09:11
@LutzHorn This is common in lambda calculus exercises. It shows that functions (and more specifically anonymous functions that aren't bound) can represent pairs (or in general: data constructs). — Reut Sharabani, Dec 15 '14 at 09:18
@Medvednic if you have an assignment to do this, should you *be doing it yourself*? — jonrsharpe, Dec 15 '14 at 09:19

score 3 · Accepted Answer · answered Dec 15 '14 at 09:09

You could do something like:

make_pair = lambda x,y: lambda m: x if m == 0 else y if m == 1 else None

The outer lambda returns an (inner) lambda waiting for an argument m that returns the bound variables in the scope created by the outer.

>>>one_two = make_pair(1, 2)
>>>one_two(1)
2
>>> one_two(2)
>>>

score 2 · Answer 2 · answered Dec 15 '14 at 09:11

You can use imbricated lambdas to return a lambda expression:

>>> make_pair=lambda x,y:lambda m:x if m==0 else y
>>> a=make_pair(2,4)
>>> a(1)
4
>>> a(0)
2

lambda m:x if m==0 else y is (more or less) equivalent to your dispatch function (a(2) will return 4 in my case)

How to create dispatch style lambda function using Python?

2 Answers2