Book Image

Haskell Data Analysis cookbook

By : Nishant Shukla
Book Image

Haskell Data Analysis cookbook

By: Nishant Shukla

Overview of this book

Step-by-step recipes filled with practical code samples and engaging examples demonstrate Haskell in practice, and then the concepts behind the code. This book shows functional developers and analysts how to leverage their existing knowledge of Haskell specifically for high-quality data analysis. A good understanding of data sets and functional programming is assumed.
Table of Contents (14 chapters)
13
Index

Implementing a Foldable instance for a tree


The idea of traversing a tree can be generalized by implementing a Foldable instance. Usually, folds are used on lists; for example, foldr1 (+) [1..10] traverses a list of numbers to produce a grand sum. Similarly, we can apply foldr1 (+) tree to find the sum of all nodes in a tree.

Getting ready

We will be folding through the following tree to obtain a sum of all node values.

How to do it...

  1. Import the following built-in packages:

    import Data.Monoid (mempty, mappend)
    import qualified Data.Foldable as F
    import Data.Foldable (Foldable, foldMap)
  2. The tree from Data.Tree already implements Foldable, so we will define our own tree data type for demonstration purposes:

    data Tree a = Node { value :: a
                       , children :: [Tree a] }
                       deriving Show
  3. Implement the foldMap function for the Foldable instance. This implementation will give us a post-order traversal of the tree:

    instance Foldable Tree where
      foldMap f Null = mempty
      foldMap...