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

By Mena A.S.

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Using recursion, you can call (+++) to append the tail of the first list and the second one. The return value from this call will be the list you need, but without the first element. To solve this problem, you can just plug the head of the first list using the (:) operator. 27 Chapter 2 ■ DeClaring the Data MoDel When this definition is translated into code, the result is: lst1 +++ lst2 = if null lst1 {- check emptyness -} then lst2 -- base case else (head lst1) : (tail lst1 +++ lst2) This example also showcases for the first time the use of comments in Haskell code.

DataTypes> GovOrgR "NATO" GovOrgR {clientRName = "NATO"} Field names are also used to create special functions that access those particular fields. DataTypes> :t duty duty :: ClientR -> String As these functions will be automatically created, Haskell enforces two extra restrictions on field names: • They must not clash with any other field or function name. • As I mentioned before, you are allowed to use the same field name in more than one of the alternatives of your data type. However, if you do so, all those fields must have the same type.

Records have been introduced as a better syntax for building, accessing, and updating fields in a Haskell value. You have seen the “default values” design pattern, which uses records at its very core. 45 Chapter 3 Reusing Code Through Lists The first chapter explained that a functional language like Haskell is characterized by its profuse use of functions as parameters or return values. However, in the previous chapter nothing was mentioned about this topic. This will be solved in this chapter, where you will focus on not one, but three different ways in which Haskell allows for a great amount of re-use.

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