Python’s iterools module provides 4 useful combinatoric functions for generating k-length sequences from n-length sequences:

• permutations(xs, k): All possible k-length sequences from xs without repeats
• product(xs, k): Cartesian product of xs with itself k times
• combinations(xs, k): All possible k-length sorted sequences from xs without repeats
• combinations_with_replacement(xs, k): All possible k-length sorted sequences from xs

I think it is clearer to combine these into one function: generate(xs, length, repeats=False, inorder=False). Think of this function as generating all possible tuples by repeatedly selecting elements from xs.

• length: The length of the tuples to generate
• repeats: Whether to allow selecting the same element of xs multiple times for an output tuple
• inorder: Whether to select elements in xs from left-to-right or in any order

The itertools functions are then given as:

• permutations(xs, k) = generate(xs, k, repeats=False, inorder=False)
• product(xs, k) = generate(xs, k, repeats=True, inorder=False)
• combinations(xs, k) = generate(xs, k, repeats=False, inorder=True)
• combinations_with_replacement(xs, k) = generate(xs, k, repeats=True, inorder=True)

## The twelvefold way

We can generalize this further to the Twelvefold way by adding two more parameters:

• missing: Whether elements from xs are allowed to be missing from an output tuple
• skip: Whether we are allowed to skip over elements of the input list while generating an output tuple

Note that repeats and missing together control how often each source element will be selected:

• repeats=True, missing=True: Each element can be selected any number of times
• repeats=True, missing=False: Each element must be selected at least once
• repeats=False, missing=True: Each element can be selected at most once
• repeats=False, missing=False: Each element must be selected exactly once

The skip parameter controls whether we can skip over elements of the input list while generating the output tuple. If skip=False, then every time we select a new element, we must either select the next element we haven’t yet picked, or any of the elements we have previously picked.

The skip and inorder parameters are intimately related: inorder determines whether we generate only tuples that are lexicographically minimal with respect to permutations of the output, and skip determines whether we generate only tuples that are lexicographically minimal with respect to permutations of the input. For instance, if the input is xs=[0,1,2] then the output (0,2,2) is not minimal with respect to permutations of the input, because we have skipped over element 1 (i.e, because (0,1,1) is lexicographically smaller). On the other hand, (0,2,0) is not minimal with respect to permutations of the output, because the tuple is not sorted (i.e., because (0,0,2) is lexicographically smaller).

To visualize this, consider that we generate an output tuple by having a pointer into xs. The inorder parameter determines whether the pointer is allowed to move backward, and the skip parameter determines whether the pointer is allowed to move forward over elements that have not yet been selected.

Whether these generalizations are practically useful is debatable, but I think it’s nice to have a single function that can generate all of these different types of tuples corresponding to the standard combinatoric classification of the twelvefold way.

## Further generalization

There are many more generalizations we could make. For instance, instead of specifying whether elements can be used at most once or a least once, we could specify a list of sets counts such that the set counts[i] specifies the allowed number of times that element xs[i] may be picked.

Similarly, we could generalize inorder and skip to other types of constraints on the movement of the pointer into xs.

However, at this point it becomes clear that we are just generating all possible tuples that satisfy some arbitrary constraint. This is a very general problem. One way to generalize it is to just generate all tuples and filter out the ones that satisfy some criterion. Unfortunately, this can be extremely slow. For instance, if we generate permutations this way, then we first generate all $$n^n$$ tuples, and then we filter out the $$n!$$ permutations. This is a huge waste of time.

A better way to generalize this problem is to use a higher-order function. One option is the following:

generate(f : list[T] → (bool, list[T])) → list[list[T]]


This function takes a predicate f(xs) that takes a list of elements and returns a pair (b, ys) where b is a boolean indicating whether the list xs satisfies the constraint and should be included in the output, and ys is a list of all possible extensions of xs that will potentially satisfy the constraint. The function generate then returns a list of all lists that satisfy the constraint.

The generate function starts with the empty list []. It calls f on the empty list to determine whether it should be included in the output, and to get a list of all possible extensions of the empty list. It then calls f on each of these extensions to determine whether they should be included in the output, and to get a list of all possible extensions of each of these extensions. It continues this process until it has generated all lists that satisfy the constraint.

This is a very general way to generate all lists that satisfy some constraint. For instance, we can generate all k-permutations of a list xs by using the predicate f(ys) = (len(ys) == k, set(xs) - set(ys)).

A way to further generalize is to allow f to use its own data type for its internal state, similar to unfold in Haskell:

generate[S,A](init : S, f : S → (list[A], list[S])) : list[A]


This function takes an initial state init and a function f that takes a state s and returns a pair (xs, ss) where xs is a list of elements to be included in the output, and ss is a list of all successor states to explore. The function generate then returns a list of all elements that can be reached from init by repeatedly applying f.

I’m sure that an experienced Haskell programmer could write this as a one-liner using a monadic function from Haskell’s standard library.

A way to further generalize this is to go all the way to constraint programming. In constraint programming, we can specify a constraint, and have the constraint solver be responsible for generating all solutions that satisfy the constraint. This is much more efficient than generating all possible solutions and then filtering out the ones that satisfy the constraint, but it requires the constraint to be formulated in a way that the constraint solver can understand.