Brian Ginsburg

A Computer Science student at Portland State University and a QA tester at FREDAsoft.

Haskell Merge Sort

11 Sep 2017 » haskell

One of the many things I love about Haskell is its expressiveness. What is fraught with process and detail in an imperative language is often a simple description in Haskell. As an example, consider my Haskell merge sort implementation.


The merge function merges two sorted lists. The heads of each list are compared, and the smaller element is prepended to a merge on the remaining lists. When one list is empty, the other list holds the largest elements in sorted order, and we can append these.

merge       :: Ord a => [a] -> [a] -> [a]
merge xs []  = xs
merge [] ys  = ys
merge (x:xs) (y:ys)
  | x <= y = x : merge xs (y:ys)
  | otherwise = y : merge (x:xs) ys

The resulting list is sorted regardless of the length of the input lists.

λ> merge [2,5,6] [1,3,4]
λ> merge [2,5,6] [1,3,4,7,8]
λ> merge [2,5,6,7,8] [1,3,4]
λ> merge [2,9,10] [1,3,4,7,8]

If one list is empty, the other list is immediately returned. If both lists are empty, an empty list is returned.

λ> merge [1,2,3] []
λ> merge [] [1,2,3]
λ> merge [] []


The halve function splits a list into two halves with the take and drop functions.

halve :: [a] -> ([a],[a])
halve xs = (take half xs, drop half xs)
     where half = length xs `div` 2

A couple of examples demonstrate halving.

λ> halve [1,2,3,4,5,6]
λ> halve [1,2,3,4,5,6,7]
λ> halve [1]
λ> halve []


The msort function implements merge sort by recursively halving a list and merging the sorted lists as they return. The merge starts by assembling zero- and one-element lists returned from the halve base cases.

msort :: Ord a => [a] -> [a]
msort [] = []
msort [x] = [x]
msort xs = merge (msort (fst halves)) (msort (snd halves))
     where halves = halve xs

The msort function can be tested on base cases and simple lists of even and odd length.

λ> msort []
λ> msort [3]
λ> msort [3,1,2]
λ> msort [3,1,5,2]
λ> msort [3,1,5,2,5]

In terms of speed, a good low-level implementation of merge sort will outperform the Haskell version. But Haskell provides another benefit.

Type Inference

Notice that our merge sort implementation has stayed general by only using type variables. This means we can sort Char, String, Float, and any other type in the Ord typeclass.

λ> msort ['a','c','f','b']
λ> msort ["erg","ech","ezh"]
λ> msort [2.3, 1.1, 9.9, 5.4]

Alternative representations can also be inferred. The largest 64-bit integer is 9,223,372,036,854,775,807. What happens if we want to sort larger values? Haskell will infer that we must be working with the arbitrary precision Integer type instead of the bounded Int type.

λ> msort [0,9223372036854775807,9223372036854775809,9223372036854775808]


Expressiveness and type inference are a couple of the benefits of a Haskell merge sort. The benefits should be weighed against the performance cost. A Haskell merge sort implementation is probably not the best choice in many cases, but it is a nice choice when you can make it.