#incrementalbackup

[djacales]

List of Lists Processing with Haskell

Input: lastBackupTime = 461620205

changes = [[461620203, 1],           [461620204, 2],           [461620205, 6],           [461620206, 5],           [461620207, 3],           [461620207, 5],           [461620208, 1]]

Introduction

Haskell is a pure functional programming language with a lot of great features, one of them is the simplicity that offers for handling with list manipulation.

In this example, found at code signal, given a lastBackupTime timestamp and a list of process ( each process is a list where index 0 is the timestamp of the last change to the file, and index 0 is the file id ). The program has to return a list of unique file ids for those who last modification occurs after the lastBackupTime.

Solution

import Data.List

import Data.Ord

incrementalBackups lastBackupTime changes = sort $ nub $ map ( !! 1 ) $ filter (\xs -> xs !! 0 > lastBackupTime) changes

Explanation

This problem can be divided into 4 small problems. First we need to filter those files where lastModification happen after the backup timestime.

Luckly Haskell has exaclty a filter function:

filter :: (a -> Bool) -> [a] -> [a]

The filter signature applies a comparison function to a list and for those elements where the comparison is true are part of the result.

filter (\xs -> xs !! 0 > lastBackupTime) changes

For our problem whe use a lambda function that takes the index 0, timestamp, for each element on the changes list and compares it to the lastBackupTime value.

Our next task is to select only the file 1, this can be done with the map function

map :: ( a -> a ) [a]

map applies a function to each element of a list and return a list of the same size where each element is a transformation of the element after aplying the function.

In Haskell to access elements on a list at specific index whe can use the ( list !! index) operator.

map ( !! 1 ) $ filter (\xs -> xs !! 0 > lastBackupTime) changes

f $ g is equivalent to f ( g ())

Good to notice here is the $ operator what this does is explicit indicates to the Haskell compiler than whe apply the map function after we apply the filter to our changes list.

After applying the map function we need to select only unique file ids,

The Data.List module has a nub function that does this.

After removing duplicates the last thing to do is to sort the file ids.

For this we can use the sort function available at the Data.Ord module.

With only 4 function we were able to solve this code challenge . If you are interested in learn more details about Haskell you can refer to this link.

[wpstaging-blog]

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