Beginning Haskell. a project-based approach by Mena A.S.

By Mena A.S.

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However, in most cases this function can be extended in a sensible way to empty lists. For example, you can assign the value 0 to the sum of an empty list, because if you add that value to any number it does not change. Values like 0, which can be safely applied with respect to an operation, like sum, are called the neutral elements of that operation. We will look at such neutral elements in more detail in Chapter 3 when discussing folds, and again in Chapter 4 when discussing monoids. Matching on tuples is also easy: just use the syntax of a comma-separated list of components between parentheses.

For those cases, we can use view patterns. These patterns extend all of those previously seen with a new syntax element, (function -> pattern), which applies function to the value and then matches the result with the pattern. For example, remember the clientName function from the beginning of the chapter, and let’s add a responsibility one: responsibility :: Client -> String responsibility (Company _ _ _ r) = r responsibility _ = "Unknown" Now you can create a function returning whether a given client is special or not.

In this case defining a function by recursion boils down to considering two general cases: • What to do when the list is empty • What to do when the list has some initial element and some tail The basic skeleton is the same in both cases: if null list then else Let’s start with the concatenation function. First of all, due to its symbolic name of (+++) you have to write the name infix. So in the definition you shall write: lst1 +++ lst2 Remember our two general cases from the earlier list.

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