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The language was prepared for that

Aug 10, 2015 - 2 minute read - Comments - clojurehaskelljavalanguage-comparisonprefix-notationoperatoroverloading

Many times I’ve written this function:

public boolean between(int lowerBound, int n, int upperBound){
	return lowerBound <= n &&
		n <= upperBound;
}

It may depend on the case, whether it is [], [), (] or (), to use mathematical terms.

When the two comparisons are the same ([] and ()), there is duplication in the comparisons.

Investigating a little bit on this in clojure, I’ve found this function:

<=

And its clojuredocs: Returns non-nil if nums are in monotonically non-decreasing order, otherwise false.

A sample usage:

(<= 1 2)
; true

(<= 1 2 1)
; false

The last part is the most interesting one. As this function is prepared to receive more than two parameters, it is very easy for the programmer to use it. We could say that the language was prepared for that.

The implementation:

(defn <=
  ([x] true)
  ([x y] (. clojure.lang.Numbers (lte x y)))
  ([x y & more]
   (if (<= x y)
     (if (next more)
       (recur y (first more) (next more))
       (<= y (first more)))
     false)))

Inspired by this, I’ve implemented the same function in haskell (for the repl):

let isBigger acc ele = (snd acc) && (fst acc) < ele in
   foldl (\acc ele -> (ele, isBigger acc ele)) (1, True) [1,2,1,3] 

and a simpler solution I’ve found on Stack Overflow:

isSorted :: (Ord a) => [a] -> Bool
isSorted xs = all (\(x, y) -> x <= y) $ zip xs (tail xs)

or

isSorted :: (Ord a) => [a] -> Bool
isSorted xs = and $ zipWith (<=) xs (tail xs)

Conclusion

Unless a more elegant, language-provided solution exists in haskell, the clojure one is way simpler. This is one of the benefits of prefix notation, that operators (e.g., +, -, *, <=) are overloaded to take more arguments than before.

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