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https://github.com/roppa/funcker

Functional programming learning exercise
https://github.com/roppa/funcker

curry functional-programming javascript library notation

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Functional programming learning exercise

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# Funcker



This is my little foray into the world of functional programming. I mean, I was sold when Curry was mentioned. I called it funcker - 'func' in functional, plus getting your head around fp can be a bit of a 'funcker'.



Functional programming is one of the original paradigms of computing. Then things moved more towards a physical representation of the world - Object Oriented. Like the circle of life, we are moving back towards the past.



Functions are fast and make good use of disc space. Bruce Lee said be like water. Well, the body is 76% water, yet we are so solid. The idea then is to loosen up, to flow, to become pure and dynamic.



If you have done much TDD (please do!) you will know that as the tests get more specific, the code gets more generic. Same principle here - make the functions generic.



Benefits of fp:



 - modularise code, making it more reusable

 - reduce the complexity of components

 - data flows along a pipeline

 - things less likely to break because there is no state or side effects



## Nomenclature or Terminology  or Symbolic Representations of Concepts





  
Function(al)


  
An expression related to an input or inputs. Of or relating to this


  
Mutable


  
Liable to change, from latin 'to change'


  
Immutable


  
Not changing


  
Pure


  
Not mixed, sound, free of contamination


  
Curry


  
Converting 1 function with n arguments into n functions each with 1 argument


  
Higher order function


  
A function that takes other functions as parameters


  
Reactive programming


  
Using the 'observable' pattern and treating things as streams and reacting to events in them


  
Declarative


  
Opposite of imperative, not specifying the exact procedure, using expressions, more like 'map, then filter, then reduce'


  
Imperative


  
'specially ordered', opposite of declarative, procedural, writing out everything specifically 'for loop over array, put result in another array, loop again and copy certain values, then loop again and calculate total'


  
Side effect


  
Changing something outside of itself


  
Decomposition


  
Decay, but more like breaking into component parts


  
Referential transparency


  
Given parameters should always return the same values. transparency because it is clear you get the same results, referential because of the parameters used


  
Expression


  
Anything that results in a value


  
State


  
The condition something is in at a particular time


  
Stateless


  
Not in a particular condition that persists


  
Arity


  
The number of arguments passed to a function. myFunc(1, 2, 3) has an arity of 3


  
List comprehension


  
Encapsulates the functionality of map and filter into for...of syntax


  
Chain(ing)


  
Being able to chain functions like a().b().c()


  
:: (Has type)


  
Standard notation for fp: function_name :: input_type -> value






## Notation



There is a standard notation for functional programming worth covering first:



```

f :: input -> output

```



The 'f' is a function, the parameters for that function come after the '::' (or sometimes a single ':'). The '::' means 'has type'. The return value of the function comes after the comma, and the '->' is a function with its returning value. So this notation:



```

f :: number -> number

```



Becomes:



```javascript

const add10 = (x) => x + 10;

```



## Higher order functions



Functions are essentially objects, first class citizens, and can move around as such. Hence we have 'higher order' functions - functions that take other functions as parameters and/or return functions.



## Arity



It is best practice that the number of parameters a function takes needs to be a maximum of two. A function really should do one thing and one thing only. Imagine how many branches there would be if a function got 3, 4, 5 or more parameters? That could indicate a complex function - not good.



Functions should do one thing and one thing only. They should not modify anything other than in its own scope.



## Closure



The definition of a closure is 'a function object that has a reference to the variables in the context it was created in'.



## Mutability



Functions should be pure. This could lead to problems when parameters are references to an object. If that object is changed within a function then a side effect results. Not good.



## Monads



A monad is an object.



## Functions in the 'funker' example



### .curry



If you have ever used bind, then you are half way there. Currying is the same as bind, but applied in a different way. With bind a new function is returned with the 'this' parameter of the original function set to the first parameter of the bind function. This is partial application, and that is what currying is - partial application.



Currying takes 1 function with n arguments, and converts it to n functions with 1 argument. We can then create a function that already has most of the data it needs held in a closure, and then we can reuse it.



So instead of:



```javascript

add(1, 2, 3, 4, 5, 6);

```



it becomes:



```javascript

add(1)(2)(3)(4)(5)(6)

```



That looks a bit weird and not very useful. So to convert a function that usually takes multiple params:



```javascript

let addToFifteen = funcker.curry(add, 1, 2, 3, 4, 5);

let addToThirty = funcker.curry(add, 10, 10, 10);

console.log(addToFifteen(15));  //30

console.log(addToThirty(15));  //45

```



### .filter



Curried Array.filter

### .get



So with imperative coding, to get an attribute for each object in an array we would have to write a for loop, then access the property. Then when we need to do it somewhere else we would write another for loop ...



Not with higher order functions. We simply want to 'get' a property. So we can pass this to a map or forEach like so:



```javascript

[{ name: 'Billy', age: 42 }, { name: 'Bob', age: 24 }].map(funker.get('name'));

```



### .only



Get a new object with only the fields specified in a space delimited string.



```javascript

const getNameAge = only('name age');

const userObject = { name: 'Tad', age: 40, secret: 'shhh, don\'t tell anyone' };

console.log(getNameAge(userObject)); // { name: 'Tad', age: 40 }

```



### .compose



Function composition: s: f1 => f2 => f3 => f1(f2(f3(s)))



Takes an array of functions and applies them in reverse order to the supplied value.



```javascript

compose([f1, f2, f3], value) => f1(f2(f3(value)))

```