Introduction To Functional Programming

Intro to Functional Programming ,[object Object],[object Object]

What is Functional Programming? ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

What’s the Point? Principle of Least Surprise Principle of Least Surprise ,[object Object],[object Object],[object Object],[object Object],Explicitly mention stateless. Might also talk about theoretical statelessness of the web, but also why it isn’t truly stateless.

No assignments! How do you loop? ,[object Object],[object Object],[object Object],[object Object],Need to mention that although you _can_ do recursive algorithms and modify state, that is usually the worst of both worlds.

Recursion Basics ,[object Object],[object Object],[object Object],[object Object]

Recursion Example: The Factorial Function The Factorial Function ,[object Object],[object Object],[object Object],def factorial(number) if number == 1 #Base Case -- compute answer return 1 else #Get us closer to the base case return number * factorial(number - 1) endend Note that we didn’t do ANY assignment in the whole program -- we just passed parameters. Function calls create new variables, so if we think we want to do assignment, in functional programming you usually want to call a function or create a different variable.

Selection Sort Example ,[object Object],[object Object],[object Object],[object Object]

Selection Sort Example def select_sort(values) select_sort_loop([], values)enddef select_sort_loop(sorted_values, unsorted_values) if unsorted_values.empty? #BASE CASE! return sorted_values else #FIND SMALLEST VALUE AND ITS INDEX smallest_value_index = find_smallest_value_index(unsorted_values) smallest_value = unsorted_values[smallest_value_index] #CREATE NEW SORTED AND UNSORTED ARRAYS new_sorted_values = add_element(sorted_values, smallest_value) new_unsorted_values = remove_element_index(unsorted_values, smallest_value_index) #LOOP WITH NEW VALUES return select_sort_loop(new_sorted_values, new_unsorted_values) endend select_sort([1,5,23,8,3,5,6,8,34,65,2,5,3])

Selection Sort: Utility Functions Utility Functions def add_element(ary, val) ary.dup.push(val)enddef remove_element_index(ary, idx) idx == 0 ? ary[1..-1] : (idx == (ary.size - 1)) ? ary[1..-2] : ary.slice(0, idx - 1).concat(ary.slice(idx + 1, ary.size - 1))enddef find_smallest_value_index(ary) smallest_idx = 0 ary.each_index do |idx| if ary[idx] < ary[smallest_idx] smallest_idx = idx smallest_val = ary[idx] end end return smallest_idxend

Basic Recursive Loop ,[object Object],[object Object],In this basic example, it looks like the standard loop is better. But as the loop gets more complicated, knowing exactly what defines your variables makes all the difference between an understandable and a totally incoherent loop.

Basic Recursive Loop ,[object Object],[object Object],Loop Definition with Parameter Declaration Note that the recursive loop explicitly names its inputs, while the standard loop does not, meaning that to truly know what contributes to the standard loops functionality requires full inspection of the whole loop. Loop Definition

Basic Recursive Loop ,[object Object],[object Object],Loop Initialization Loop Initialization

Basic Recursive Loop ,[object Object],[object Object],Loop Invocation An explicit invocation step is not needed for standard loops, as the while statement invokes it implicitly.

Basic Recursive Loop ,[object Object],[object Object],Loop Condition Loop Condition

Basic Recursive Loop ,[object Object],[object Object],Loop Body Loop Body

Basic Recursive Loop ,[object Object],[object Object],Modification of Loop Variables Modification of Loop Variables Note that in the standard loop assignments can occur anywhere, while in recursive loops they only occur by passing new parameters as part of the loop iteration process. Which do you think leads to easier bug detection in large loops? Recursive loops can create new definitions for new variables within the loop, but cannot modify already-assigned variables.

Basic Recursive Loop ,[object Object],[object Object],Control Variable Modification Control Variable Modification Note that since this is done as a function parameter, no assignment needs to be made. The new value is placed into a new variable (with the same name) in the next function call.

Basic Recursive Loop ,[object Object],[object Object],Loop Iteration Loop Iteration Not e that since the loop is represented as a function, to get back to the beginning of the loop, all we need to do is a function call. But remember that unless the function call is the last thing you do, the function will return back to the loop!

Basic Recursive Loop ,[object Object],[object Object],Loop Exit Note that the loop exit is explicit in the standard loop but implicit in the recursive loop.

Packaged Looping Constructs ,[object Object],[object Object],[object Object],[object Object],Should I give an example somewhere?

Packaged Looping Constructs ,[object Object],[object Object],[object Object],Include upto? Somewhere I need to talk about fold and unfold, but that is probably for a different talk.

Functional Looping Advantages ,[object Object],[object Object],[object Object],[object Object],[object Object],Note that these are the same ones we talked about for FP in general. Also, in general this makes the programs easier to reason about.

General Functional Programming Issues ,[object Object],[object Object],[object Object],[object Object],[object Object],Mention Monads briefly Do I need to talk about stateful/stateless somewhere?

Practical Tips ,[object Object],[object Object],[object Object],[object Object],[object Object],On state modification - might mention how ruby uses ! to mark non-functional versions of traditionally functional functions. When a task requires both computation and state modification, it is often best to separate them into two functions - one to perform the computation and one to actually perform the state change (usually with the state-change function calling the computational function, but not always).

References ,[object Object],[object Object],[object Object],[object Object]

Introduction To Functional Programming

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