Thursday, March 19, 2015

Y Combinator

There is this blog post by Caltech computer scientist, Mike Vanier. The code in Mike's article uses the Scheme programming language. This one uses Python.

A $Y$ combinator is a higher-order function which, given argument $f$ (say,) satisfies the equation $Y f = f\;(Y f)$. They are said to compute a "fixed point" $f^{'}$ of their argument $f$ since $f^{'} = Y\;f = f\;(Y\;f) = f \;f^{'}$.

A $Y$ combinator takes a function that isn't recursive and returns a version of the function that is. That is, $Y$ is a function that takes $f$ and returns $f (f (f (\cdots)))$.

The existence of $Y$ combinators is amazing in that it tells us that it is possible to write recursive functions even in a programming language that says nothing about recursion!

The goal here is to derive a $Y$.

Start with the classic recursive definition of the factorial function.

  def fact (n) :
    if n == 0 : 
      return 1 
      return n * fact (n - 1)

We are trying to eliminate explicit recursion. To that end, factor out the recursive call and make it an application of a function argument.

def part_fact (this, n):
  if n == 0 :
    return 1
     return n * this (this, (n - 1))

fact = functools.partial(part_fact, part_fact)
That's sufficient to get rid of the explicit recursion but we mean to push on in search of a "library" solution to this problem, that is, some general result that can be re-used.

Next let's get this down to a function in one argument as is this way in the $\lambda$ calculus.

  def part_fact (this):
    return lambda n : 1 if n == 0 else n * (this (this)) (n - 1)

  fact = part_fact (part_fact)

We'd recover something tantalizingly close to the original factorial function if we factored out this (this) into a function of one argument.

def part_fact (this):
  f = this (this)
  return lambda n : 1 if n == 0 else n * f (n - 1)
fact = part_fact(part_fact)

This would be fine in a lazy language but Python is a strict language and exhibits infinite recursion because to evaluate part_fact (part_fact) requires evaluating f = part_fact (part_fact) and so on. The solution is to delay the evaluation until it's needed.

def part_fact (this):
  f = lambda y : (this (this)) y
  return lambda n : 1 if n == 0 else n * f (n - 1)
fact = part_fact(part_fact)

Refactor this into two parts.

def almost_fact (f):
  return lambda n : 1 if n == 0 else f (n - 1)

def part_fact (this):
  f = lambda y : (this (this))(y)
  return almost_fact (f)

fact = part_fact(part_fact)

Rephrase part_frac as a lambda and change the argument name to x.

def almost_fact (f):
  return lambda n : 1 if n == 0 else f (n - 1)

part_fract = lambda x : almost_fact (lambda y : (x (x))(y))

fact = part_fact (part_fact)

Eliminate 'part_fact'.

def almost_fact (f):
  return lambda n : 1 if n == 0 else f (n - 1)

fact = (lambda x : almost_fact (lambda y : (x (x))(y))) \
          (lambda x : almost_fact (lambda y : (x (x))(y)))

That's it! There's the $Y$ combinator. Generalize.

def Y (f):
 return (lambda x : f (lambda y : (x (x))(y))) \
           (lambda x : f (lambda y : (x (x))(y)))

def almost_fact (f):
  return lambda n : 1 if n == 0 else f (n - 1)

fact = Y (almost_fact)
That is, $Y = \lambda f.(\lambda x. f\;(\lambda y. (x\;x)\;y))\;(\lambda x. f\;(\lambda y. (x\;x)\;y))$. This $Y$ combinator is known s Curry's paradoxical combinator (in its "applicative order" form to account for the fact that Python is strict).

Try this on another function. Fibonacci numbers say.

def almost_fib (f) :
  return lambda n : 1 if n <= 2 else f (n - 1) + f (n - 2)

fib = Y (almost_fib)

print (str (fib (6))+"\n") #Prints '8'

Saturday, March 14, 2015

Labeled and optional arguments

I don't know why this is, but of all the features of the OCaml language, somehow remembering how to make use of labeled or optional arguments is difficult for me. I find when I need to use them, I have to go back and "relearn" the syntax every time. For that reason, I've written these summary notes for myself (based mostly on Jacques Garrigue's "Labels and Variants" chapter of the OCaml reference) and I hope they might be of help you too!


I've seen people write this function from time to time.
let flip (f : 'a -> 'b -> 'c) = fun y x -> f x y

flip applied to a function f in two arguments, produces a new function that when presented reverses their order in the application of f. For example, if sub is defined by let sub x y = x - y, then sub 3 2 will give the result 1 whereas (flip sub) 3 2 will produce the value -1.

The reason you'd find yourself wanting a function like flip is because you want to partially apply f but the argument you want to bind is inconveniently in the 'wrong' position. For example, the function that subtracts 2 from it's argument can be written (flip sub 2) (whereas sub 2 is the function that subtracts its argument from 2).

Labeled arguments do away with the need for functions like flip. You see, arguments in function types can be annotated.

val sub : (x:int) -> (y:int) -> int
In the above, x and y are labels. When you define a function with labeled arguments, you prefix the labels with a tilde ~.
let sub : (x:int -> y:int -> int) = fun ~x ~y -> x - y

Labels and variables are not the same things. A label is a name associated with a variable e.g. let u = 3 and v = 2 in sub ~x:u ~y:v (u, v are variables x, y labels) and this also works when the variables are replaced with literals as in sub ~x:3 ~y:2.

The expression ~x on it's own is shorthand for ~x:x which means that you can conveniently write let x = 3 and y = 2 in sub ~x ~y for example. This is "punning" - meaning if the variable name and the label are the same, the variable name is permitted to be elided in the application.

Labels enable making function application commutative (changing the order of the arguments does not change the result); one can write sub ~y ~x and that will yield the same result as sub ~x ~y. Where this is useful of course is partial application of a function on any any argument. For example, we can create functions from sub by binding either of x or y such as sub ~x:3 or sub ~y:2 without having to resort to ad-hoc trickery like flip sub.

Some technical details:

  • If more than one argument of a function has the same label (or no label), position prevails and they will not commute among themselves;
  • If an application is total, labels can be omitted (e.g. like in sub 3 2). Be wary though - when a function is polymorphic in its return type, it will never be considered as totally applied. Consider for example , let app ~f ~x = f x. Then app has type f:('a -> 'b') -> x:'a -> 'b. Now we attempt total application but omit the required labels app (fun x -> x) 1 and this types to f:('a -> ('b -> 'b) -> int -> 'c) -> x:'a -> 'c. You can see that the arguments have been "stacked up" in anticipation of presentation of f and x and the result although it types, is not at all what we were going for!
  • When a function is passed to a higher order function, labels must match in both types, labels may not be added or removed.

Optional arguments

In a function type involving an optional argument, the annotation ? appears.
val range : ?step:int -> int -> int -> int list
The ? appears again in the value syntax of a function definition involving an optional argument. Additionally, at that time, optional parameters can be given default values. When calling a function with a value for an optional parameter, well, you use the fact it is a labeled argument and provide a ~ as normal.
let rec range ?(step:int = 1) (m : int) (n : int) : int list = 
    if m >= n then [] 
    else m :: (range ~step (m + step) n)

Now, this bit is important. A function can not take optional arguments alone. There must be some non-optional ones there too and it is when a non-optional parameter that comes after an optional one in the function type is encountered is it determined if the optional parameter has been omitted or not. For example, if we reorder the argument list as in the below, we find we can't 'erase' the optional argument anymore.

let rec range (m : int) (n : int) ?(step : int = 1): int list = ...
Warning 16: this optional argument cannot be erased.
That said, optional parameters may commute with non-optional or unlabeled ones, as long as they are applied simultaneously. That is, going back to this definition,
let rec range ?(step:int = 1) (m : int) (n : int) : int list = 
    if m >= n then [] 
    else m :: (range ~step (m + step) n)
(range 0 ~step:2) 100 is valid whereas, (range 0) ~step:2 100 is not.

Optional arguments are implemented as option types. In the event no default is given, you can treat them as exactly that by matching on None or Some x to switch behaviors. Here's a schematic of what that looks like.

let f ?s y =
    match s with
      | None -> ...
      | Some x -> ...  

When you are just "passing through" an optional argument from one function call to another, you can prefix the applied argument with ? as in the following.

 let g ?s x = ...
 let h ?s x = g ?s x  (*[s] is passed through to [g]*)

Friday, March 6, 2015

Heap sort

Given the existence of a priority queue data structure, the heap sort algorithm is trivially implemented by loading the unsorted sequence into a queue then successively pulling of the minimum element from the queue until the queue is exhausted.

There are many ways to implement a priority queue and so we seek an expression for a function for heap sorting that is polymorphic over those choices.

To begin, a module type for priority queues.

(**Priority queues over ordered types*)
module type PRIORITY_QUEUE = sig

    (**Output signature of the functor [Make]*)
    module type S = sig
        exception Empty

        type element (*Abstract type of elements of the queue*)
        type t (*Abstract type of a queue*)

        val empty : t (*The empty queue*)
        val is_empty : t -> bool (*Check if queue is empty*)
        val insert : t -> element -> t (*Insert item into queue*)
        val delete_min : t -> t (*Delete the minimum element*)
        val find_min : t -> element (*Return the minimum element*)
        val of_list : element list -> t

    (**Input signature of the functor [Make]*)
    module type Ordered_type = sig
        type t
        val compare : t -> t -> int

   (**Functor building an implementation of the priority queue structure
      given a totally ordered type*)
    module Make : 
       functor (Ord : Ordered_type) -> S with type element = Ord.t

An implementation of this signature using "leftist heaps" is described for the interested in this Caltech lab but such details are omitted here.

module Priority_queue : PRIORITY_QUEUE = struct 
  module type S = sig .. end
  module type Ordered_type = sig .. end
  module Make (Elt : Ordered_type) : (S with type element = Elt.t) = struct .. end

What I really want to show you is this. We start with the following module type abbreviation.

type 'a queue_impl = (module Priority_queue.S with type element = 'a)
Then, the heap_sort function can be written such that it takes a module as a first class value and uses a locally abstract type to connect it with the element type of the list to be sorted.
let heap_sort (type a) (queue : a queue_impl) (l : a list) : a list =
  let module Queue = 
     (val queue : Priority_queue.S with type element = a) in
  let rec loop acc h =
    if Queue.is_empty h then acc
      let p = Queue.find_min h in
      loop (p :: acc) (Queue.delete_min h) in
  List.rev (loop [] (Queue.of_list l))
There we have it. The objective has been achieved : we have written a heap sorting function that is polymorphic in the implementation of the priority queue with which it is implemented.

Usage (testing) proceeds as in this example.

(*Prepare an [Priority_queue.Ordered_type] module to pass as argument
  to [Priority_queue.Make]*)
module Int : Priority_queue.Ordered_type with type t = int = struct 
  type t = int let compare = 

(*Make a priority queue module*)
module Int_prioqueue : (Priority_queue.S with type element = int) = Priority_queue.Make (Int)

(*Make a first class value of the module by packing it*)
let queue = (module Int_prioqueue : Priority_queue.S with type element = int)

(*Now, pass the module to [heap_sort]*)
let sorted = heap_sort queue [-1; -2; 2] (*Produces the list [-2; -1; 2]*)

Addendum :

These ideas can be pushed a little further yielding a simpler syntax for the parametric heapsort algorithm.
(*Type abbreviations*)

type 'a order_impl = (module Priority_queue.Ordered_type with type t = 'a)
type 'a queue_impl = (module Priority_queue.S with type element = 'a)

(*Module factory functions*)

let mk_ord : 'a. unit -> 'a order_impl =
  fun (type s) () ->
       type t = s 
       let compare = 
     end : Priority_queue.Ordered_type with type t = s

let mk_queue : 'a. unit -> 'a queue_impl =
  fun (type s) ord ->
    let module Ord = 
      (val mk_ord () : Priority_queue.Ordered_type with type t = s) in
    (module Priority_queue.Make (Ord) :
                               Priority_queue.S with type element = s)
For example, now we can write
# heap_sort (mk_queue ()) [-3; 1; 5] ;;
  - : int list = [-3; 1; 5]