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https://github.com/oubiwann/maintaining-state-in-clojure

Examples of stateful data in Clojure
https://github.com/oubiwann/maintaining-state-in-clojure

channels clojure closures code-exploration code-play data light-weight-processes lisp objects protocols state

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Examples of stateful data in Clojure

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README 

        
# Maintaining State in Clojure



## Caveat



These examples are here to help those new to Clojure in establishing

analogies with other languages (or Lisp dialects). Please keep in mind

the travesties committed in the majority of object-oriented software over

the decades since the 80s: data and behaviour should not be conflated.



To put a finer point on this, if you are looking for proper design patterns

for working with state in Clojure applications, be sure to purchase any

number of the Clojure books dedicated to real-world applications. Please do

not treat this project as an application reference.



One note on real-world applications: I highly recommend Stuart Sierra's

[component library][component] for separating your apps into components

and sharing only that state which each needs between components.



This is an old repo, but since it comes up in search results, I try to keep

it updated.



## Background



Learning how to work with state in such a way as fits the language of one's

choice is critical for building production-ready applications.



Learning how to _play_ with state in different ways is actually a fun topic

of exploration. Different languages often have widely diverging features

that allow one to maintain state in all sorts of unique ways.



The examples in this repo explore some of this, from the perspective of

Clojure. They include:



* Using closures

* Using a data structure

* Using Clojure protocols

* Using `core.async` channels



Enjoy!



## Preparations



### Getting the Code



To play with the examples in this repo, you'll need to clone it to the

machine that you're working on:



```shell

$ git clone https://github.com/oubiwann/maintaining-state-in-clojure.git

$ cd maintaining-state-in-clojure

```



### Firing up the REPL



If you don't have `lein` installed, you'll need to [download it][lein-dl]. When

`lein` is on your system, make sure you're in the cloned directory for this

repository and start up the REPL:



```

$ cd maintaining-state-in-clojure

$ lein repl



state-examples.dev=>

```



At this point, you will have access to the following, which referencce the

namespaces of the different examples in this repository:



* `channels`

* `closures`

* `data`

* `protocols`



We will use each of these below.



## Examples



The examples below are adapted from an example given by Peter Norvig in

Chapter 13 of his famous Lisp/AI book, [PAIP][paip]. The

[Chapter 13 PAIP source code][paip-ch13] is available online at the

[Peter Norvig site][norvig-site].



### Using Closures



Once upon a time (before CLOS), if you wanted to maintain state in Lisp, you

used closures. As a nod to this savory history, we start with a closure

example.



This example uses nested closures to:



1. dispatch based upon a passed keyword, and



1. return a dispatched function that has access to the top-level function's

   variables as well as variables that are passed in to the nested functions.



This sort of construction provides some of the basic functionality of an

object system (mostly just state data).



Here is a link to the [state-via-closures example code][closures].

Take a look, then let's see how this works in action:



```clj

state-examples.dev=> (def acc (closures/new-account "savings" 1000 0.05))

#'state-examples.dev/acc

state-examples.dev=> (closures/get-name acc)

"savings"

state-examples.dev=> (closures/get-balance acc)

1000

```



If we call any functions that make any changes to state data, a new account

object gets returned. As such, in those cases we'll need to reasign the new

object to our account variable:



```clj

state-examples.dev=> (def acc (closures/deposit acc 150.50))

#'state-examples.dev/acc

state-examples.dev=> (closures/get-balance acc)

1150.5

state-examples.dev=> (def acc (closures/apply-interest acc))

#'state-examples.dev/acc

state-examples.dev=> (closures/get-balance acc)

1208.025

state-examples.dev=> (def acc (closures/withdraw acc 25.25))

#'state-examples.dev/acc

state-examples.dev=> (closures/get-balance acc)

1182.775

state-examples.dev=> (closures/withdraw acc 2000)



Exception : Insufficient funds.  state-examples.closures/new-account/fn--28/fn--38 (closures.clj:28)

```



### Data Structures as a Counter Example



Most of these examples are using fairly elaborate means of doing something

quite simple: tracking data. What simpler way to do that than a data

structure? None, that's what way.



This example provides a convenience function which creates a simple map. This

allows us to use it just like we did the previous example. The functions,

instead of extracting info from nested closures, simply operate on the

provided data structure.



Note that this approach is not thread-safe.



Here is a link to the [state-via-data-structures example code][data].



For this example, we've used an identical set of functions as the closures

example, with no fancy-pants. Just data. We'll start it off like we did

before:



```clj

state-examples.dev=> (def acc (data/new-account "savings" 1000 0.05))

#'state-examples.dev/acc

state-examples.dev=> (data/get-name acc)

"savings"

state-examples.dev=> (data/get-balance acc)

1000

```



Let's walk through the same steps:



```clj

state-examples.dev=> (def acc (data/deposit acc 150.50))

#'state-examples.dev/acc

state-examples.dev=> (data/get-balance acc)

1150.5

state-examples.dev=> (def acc (data/apply-interest acc))

#'state-examples.dev/acc

state-examples.dev=> (data/get-balance acc)

1208.025

state-examples.dev=> (def acc (data/withdraw acc 25.25))

#'state-examples.dev/acc

state-examples.dev=> (data/get-balance acc)

1182.775

state-examples.dev=> (data/withdraw acc 2000)



Exception : Insufficient funds.  state-examples.data/withdraw (data.clj:27)

```



### Using Protocols and Records



We now take a look at Clojure's wrapping around Java interfaces and classes

as a means of maintaining state.



Again, we've set things up so that the usage is almost identical to the

previous examples.



Here is a link to the [state-via-protocols example code][protocols].



```clj

state-examples.dev=> (def acc (protocols/new-account "savings" 1000 0.05))

#'state-examples.dev/acc

state-examples.dev=> (protocols/get-name acc)

"savings"

state-examples.dev=> (protocols/get-balance acc)

1000

```



And now for some operations on our data:



```clj

state-examples.dev=> (def acc (protocols/deposit acc 150.50))

#'state-examples.dev/acc

state-examples.dev=> (protocols/get-balance acc)

1150.5

state-examples.dev=> (def acc (protocols/apply-interest acc))

#'state-examples.dev/acc

state-examples.dev=> (protocols/get-balance acc)

1208.025

state-examples.dev=> (def acc (protocols/withdraw acc 25.25))

#'state-examples.dev/acc

state-examples.dev=> (protocols/get-balance acc)

1182.775

state-examples.dev=> (protocols/withdraw acc 2000)



Exception : Insufficient funds.  state-examples.protocols.Account (protocols.clj:22)

```



### Using `core.async` Channels



This example is a completely different animal ... on the surface.

Generalizing to core concepts, this is very simiilar to using closures.



Here is a link to the [state-via-channels example code][channels].



```clj

state-examples.dev=> (def acc (channels/new-account "savings" 1000 0.05))

#'state-examples.dev/acc

state-examples.dev=> (channels/get-name acc)

"savings"

state-examples.dev=> (channels/get-balance acc)

1000

```



Let's walk through the same steps as the other examples:



```clj

state-examples.dev=> (channels/deposit acc 150.50)

:ok

state-examples.dev=> (channels/get-balance acc)

1150.5

state-examples.dev=> (channels/apply-interest acc)

:ok

state-examples.dev=> (channels/get-balance acc)

1208.025

state-examples.dev=> (channels/withdraw acc 25.25)

:ok

state-examples.dev=> (channels/get-balance acc)

1182.775

state-examples.dev=> (channels/withdraw acc 2000)

:insufficient-funds

```



[lein-dl]: https://github.com/technomancy/leiningen#installation

[closures]: src/state_examples/closures.clj

[data]: src/state_examples/data.clj

[protocols]: src/state_examples/protocols.clj

[channels]: src/state_examples/channels.clj

[paip]: http://www.amazon.com/dp/B003VWBY1I/

[paip-ch13]: http://norvig.com/paip/clos.lisp

[norvig-site]: http://norvig.com/

[component]: https://github.com/stuartsierra/component