What are the functional equivalents of imperative break statements and other loop checks?

Question

Let's say, I've the below logic. How to write that in Functional Programming?

    public int doSomeCalc(int[] array)
    {
        int answer = 0;
        if(array!=null)
        {
            for(int e: array)
            {
                answer += e;
                if(answer == 10) break;
                if(answer == 150) answer += 100;
            }
        }
        return answer;
    }

The examples in most blogs, articles... I see just explains the simple case of one straight forward math function say 'Sum'. But, I have a logic similar to the above written in Java and would like to migrate that to functional code in Clojure. If we can't do the above in FP, then the kind of promotions for FP doesn't state this explicitly.

I know that the above code is totally imperative. It was not written with the forethought of migrating it to FP in future.

Answer 1

The closest equivalent to looping over an array in most functional languages is a fold function, i.e. a function that calls a user-specified function for each value of the array, passing an accumulated value along the chain. In many functional languages, fold is augmented by a variety of additional functions that provide extra features, including the option to stop early when some condition arises. In lazy languages (e.g. Haskell), stopping early can be achieved simply by not evaluating any further along the list, which will cause additional values to never be generated. Therefore, translating your example to Haskell, I would write it as:

doSomeCalc :: [Int] -> Int
doSomeCalc values = foldr1 combine values
  where combine v1 v2 | v1 == 10  = v1
                      | v1 == 150 = v1 + 100 + v2
                      | otherwise = v1 + v2

Breaking this down line by line in case you're not familiar with Haskell's syntax, this works like this:

doSomeCalc :: [Int] -> Int

Defines the type of the function, accepting a list of ints and returning a single int.

doSomeCalc values = foldr1 combine values

The main body of the function: given argument values, return foldr1 called with arguments combine (which we'll define below) and values. foldr1 is a variant of the fold primitive that starts with the accumulator set to the first value of the list (hence the 1 in the function name), then combines it using the user specified function from left to right (which is usually called a right fold, hence the r in the function name). So foldr1 f [1,2,3] is equivalent to f 1 (f 2 3) (or f(1,f(2,3)) in more conventional C-like syntax).

  where combine v1 v2 | v1 == 10  = v1

Defining the combine local function: it receives two arguments, v1 and v2. When v1 is 10, it just returns v1. In this case, v2 is never evaluated, so the loop stops here.

                      | v1 == 150 = v1 + 100 + v2

Alternatively, when v1 is 150, adds an extra 100 to it, and adds v2.

                      | otherwise = v1 + v2

And, if neither of those conditions is true, just adds v1 to v2.

Now, this solution is somewhat specific to Haskell, because the fact that a right fold terminates if the combining function doesn't evaluate its second argument is caused by Haskell's lazy evaluation strategy. I don't know Clojure, but I believe it uses strict evaluation, so I would expect it to have a fold function in its standard library that includes specific support for early termination. This is often called foldWhile, foldUntil or similar.

A quick look at the Clojure library documentation suggests that it is a little different from most functional languages in naming, and that fold isn't what you're looking for (it's a more advanced mechanism aimed at enabling parallel computation) but reduce is the more direct equivalent. Early termination occurs if the reduced function is called within your combining function. I'm not 100% sure I understand the syntax, but I suspect what you're looking for is something like this:

(reduce 
    (fn [v1 v2]
        (if (= v1 10) 
             (reduced v1)
             (+ v1 v2 (if (= v1 150) 100 0))))
    array)

NB: both translations, Haskell and Clojure, are not quite right for this specific code; but they convey the general gist of it -- see discussion in the comments below for specific problems with these examples.

Answer 2

You could easily convert it to recursion. And it has nice tail-optimized recursive call.

Pseudocode :

public int doSomeCalc(int[] array)
{
    return doSomeCalcInner(array, 0);
}

public int doSomeCalcInner(int[] array, int answer)
{
    if (array is empty) return answer;

    // not sure how to efficiently implement head/tails array split in clojure
    var head = array[0] // first element of array
    var tail = array[1..] // remainder of array

    answer += head;
    if (answer == 10) return answer;
    if (answer == 150) answer += 100;

    return doSomeCalcInner(tail, answer);
}

Answer 3

I really like Jules' answer, but I wanted to additionally point out something people often miss about lazy functional programming, which is that everything doesn't have to be "inside the loop." For example:

baseSums = scanl (+) 0

offsets = scanl (\offset sum -> if sum == 150 then offset + 100 else offset) 0

zipWithOffsets xs = zipWith (+) xs (offsets xs)

stopAt10 xs = if 10 `elem` xs then 10 else last xs

result = stopAt10 . zipWithOffsets . baseSums

result [1..]         -- 10
result [11..1000000] -- 500000499945

You can see that each part of your logic can be calculated in a separate function then composed together. This allows for smaller functions which are usually much easier to troubleshoot. For your toy example, perhaps this adds more complexity than it removes, but in real world code the split apart functions are often much simpler than the whole.

Answer 4

Most list processing examples you will see use functions like map, filter, sum etc. which operate on the list as a whole. But in your case you have a conditional early exit - a rather uncommon pattern which is not supported by the usual list operations. So you need to drop down an abstraction level and use recursion - which is also closer to how the imperative example looks.

This is a rather direct (probably not idiomatic) translation into Clojure:

(defn doSomeCalc 
  ([lst] (doSomeCalc lst 0))
  ([lst sum]
    (if (empty? lst) sum
        (if (= sum 10) sum
            (let [sum (+ sum (first lst))]
                 [sum (if (= sum 150) (+ sum 100) sum)]
               (recur (rest lst) sum))))))) 

Edit: Jules point out that reduce in Clojure do support early exit. Using this is more elegant:

(defn doSomeCalc [lst]  
  (reduce (fn [sum val]
    (if (= sum 10) (reduced sum)
        (let [sum (+ sum val)]
             [sum (if (= sum 150) (+ sum 100) sum)]
           sum))
   lst)))

In any case, you can do anything in functional languages as you can in imperative languages, but you often have to change your mindset somewhat to find an elegant solution. In imperative coding you think of processing a list step by step, while in functional languages you look for an operation to apply to all elements in the list.

Answer 5

As pointed out by other answers, Clojure has reduced for stopping reductions early:

(defn some-calc [coll]
  (reduce (fn [answer e]
            (let [answer (+ answer e)]
               (case answer
                 10  (reduced answer)
                 150 (+ answer 100)
                 answer)))
          0 coll))

This is the best solution for your particular situation. You can also get a lot of mileage from combining reduced with transduce, which lets you use transducers from map, filter etc. However it is far from a complete answer to your general question.

Escape continuations are a generalized version of break and return statements. They are directly implemented in some Schemes (call-with-escape-continuation), Common Lisp (block + return, catch + throw) and even C (setjmp + longjmp). More general delimited or undelimited continuations as found in standard Scheme or as continuation monads in Haskell and Scala can also be used as escape continuations.

For example, in Racket you could use let/ec like this:

(define (some-calc ls)
  (let/ec break ; let break be an escape continuation
    (foldl (lambda (answer e)
             (let ([answer (+ answer e)])
               (case answer
                 [(10)  (break answer)] ; return answer immediately
                 [(150) (+ answer 100)]
                 [else  answer])))
           0 ls)))

Many other languages also have escape continuation -like constructs in the form of exception handling. In Haskell you could also use one of the various error monads with foldM. Because they are primarily error handling constructs using exceptions or error monads for early returns is usually culturally unacceptable and possibly quite slow.

You can also drop down from higher-order functions to tail calls.

When using loops, you enter the next iteration automatically when you reach the end of the loop body. You can enter the next iteration early with continue or exit the loop with break (or return). When using tail calls (or Clojure's loop construct which mimics tail recursion), you must always make an explicit call to enter the next iteration. To stop looping you just don't make the recursive call but give the value directly:

(defn some-calc [coll]
  (loop [answer 0, [e es :as coll] coll]
    (if (empty? coll)
      answer
      (let [answer (+ answer e)]
        (case answer
          10 answer
          150 (recur (+ answer 100) es)
          (recur answer es))))))

Answer 6

The intricate part is the loop. Let us start with that. A loop is usually converted to functional style by expressing the iteration with a single function. An iteration is a transformation of the loop variable.

Here is a functional implementation of a general loop :

loop : v -> (v -> v) -> (v -> Bool) -> v
loop init iter cond_to_cont = 
    if cond_to_cont init 
        then loop (iter init) iter cond
        else init

It takes (an initial value of the loop variable, the function that expresses a single iteration [on the loop variable]) (a condition to continue the loop).

Your example uses a loop on an array, which also breaks. This capability in your imperative language is baked into the language itself. In functional programming such capability is usually implemented at the library level. Here is a possible implementation

module Array (foldlc) where

foldlc : v -> (v -> e -> v) -> (v -> Bool) -> Array e -> v
foldlc init iter cond_to_cont arr = 
    loop 
        (init, 0)
        (λ (val, next_pos) -> (iter val (at next_pos arr), next_pos + 1))
        (λ (val, next_pos) -> and (cond_to_cont val) (next_pos < size arr))

In it :

I use a ((val, next_pos)) pair which contains the loop variable visible outside and the position in the array, which this function hides.

The iteration function is slightly more complex than in the general loop, this version makes it possible to use the current element of the array. [It is in curried form.]

Such functions are usually named "fold".

I put an "l" in the name to indicate that the accumulation of the elements of the array is done in a left-associative manner; to mimic the habit of imperative programming languages to iterate an array from low to high index.

I put a "c" in the name to indicate that this version of fold takes a condition that controls if and when the loop to be stopped early.

Of course such utility functions are likely to be readily available in the base library shipped with the functional programming language used. I wrote them here for demonstration.

Now that we have all the tools that are in the language in the imperative case, we can turn to implement the specific functionality of your example.

The variable in your loop is a pair ('answer', a boolean that encodes whether to continue).

iter : (Int, Bool) -> Int -> (Int, Bool)
iter (answer, cont) collection_element = 
  let new_answer = answer + collection_element
  in case new_answer of
    10 -> (new_answer, false)
    150 -> (new_answer + 100, true)
    _ -> (new_answer, true)

Note that i used a new "variable" 'new_answer'. This is because in functional programming i can not change the value of an already initialized "variable". I do not worry about performance, the compiler may get to reuse the memory of 'answer' for 'new_answer' via life-time analysis, if it thinks that is more efficient.

Incorporating this into our loop function developed earlier :

doSomeCalc :: Array Int -> Int
doSomeCalc arr = fst (Array.foldlc (0, true) iter snd arr)

"Array" here is the module name which exports function foldlc is.

"fist", "second" stand for functions that returns the first, second component of its pair parameter

fst : (x, y) -> x
snd : (x, y) -> y

In this case "point-free" style increases the readability of the implementation of doSomeCalc:

doSomeCalc = Array.foldlc (0, true) iter snd >>> fst

(>>>) is function composition : (>>>) : (a -> b) -> (b -> c) -> (a -> c)

It is the same as above, just the "arr" parameter is left out from both sides of the defining equation.

One last thing : checking for case (array == null). In better designed programming languages, but even in badly designed languages with some basic discipline one rather uses an optional type to express non-existence. This does not have much to do with functional programming, which the question is ultimately about, thus i do not deal with it.

Answer 7

First, rewrite the loop slightly, such that each iteration of the loop either early-exits, or mutates answer exactly once:

    public int doSomeCalc(int[] array)
    {
        int answer = 0;
        if(array!=null)
        {
            for(int e: array)
            {
                if(answer + e == 10) return answer + e;
                else if(answer + e == 150) answer = answer + e + 100;
                else answer = answer + e;
            }
        }
        return answer;
    }

It should be clear that the behavior of this version is exactly the same as before, but now, it's much more straightforward to convert to recursive style. Here's a direct Haskell translation:

doSomeCalc :: [Int] -> Int
doSomeCalc = recurse 0
  where recurse :: Int -> [Int] -> Int
        recurse answer [] = answer
        recurse answer (e:array)
          | answer + e == 10 = answer + e
          | answer + e == 150 = recurse (answer + e + 100) array
          | otherwise = recurse (answer + e) array

Now it's purely functional, but we can improve it from both an efficiency and readability standpoint by using a fold instead of explicit recursion:

import Control.Monad (foldM)

doSomeCalc :: [Int] -> Int
doSomeCalc = either id id . foldM go 0
  where go :: Int -> Int -> Either Int Int
        go answer e
          | answer + e == 10 = Left (answer + e)
          | answer + e == 150 = Right (answer + e + 100)
          | otherwise = Right (answer + e)

In this context, Left early-exits with its value, and Right continues the recursion with its value.

---

This could now be simplified a bit more, like this:

import Control.Monad (foldM)

doSomeCalc :: [Int] -> Int
doSomeCalc = either id id . foldM go 0
  where go :: Int -> Int -> Either Int Int
        go answer e
          | answer' == 10 = Left 10
          | answer' == 150 = Right 250
          | otherwise = Right answer'
          where answer' = answer + e

This is better as final Haskell code, but it's now a bit less clear how it maps back to the original Java.