[FIXED] What is the meaning of (n:ns) in Haskell?

Issue

In the first chapter of the book, Programming in Haskell, it has the following function definition:

summ [] = 0
summ (n:ns) = n + summ ns

What is the meaning of (n:ns), I guess from the function ns is a list and n is the first element of the original list, but what does the (n:ns) notation actually tell Haskell? What part of the notation makes it clear what happens in the function?

Solution

The sequence (n:ns) is a shorthand for head - tail. Quite literally, the first value, the head, is called n and the remained are the other, potentially plural, ns, which is why it is called ns.

Haskell has pattern matching. So, if I say (n:ns) = [1,2,3] then Haskell will pattern match n to 1, and ns to match [2,3]. Effectively, n:ns salami slices the first value off the front of the list.

The algorithm for calculating the sum of a list in Haskell is recursive. If the list is empty, [ ], then zero is returned. Otherwise, we slice off the first value from the list, n, and add it to the result.

Haskell has a REPL, called ghci, and using this is fundamental to getting the hang of the language.

Answered By - Francis King

Answer Checked By - Mildred Charles (PHPFixing Admin)