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Pure functions using Javascript.

Leon Maia
Leon Maia

Basic introduction to pure functions using Javascript as the language. Introduction to some concepts as side effects, referential transparency and shared state.

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Pure Functions Leon Maia https://leonmaia.com
Agenda Whats a function? Whats a pure function? Whats a side effect? Referential transparency. Keep it simple. Examples. The trouble with shared state. 2
Whats a function? 3 A function is something that takes an input, and produces some output. function sum(a, b) { return a+b; } For example, this is a function that takes two numbers and returns the sum.
Whats a pure function? 4 A pure function is a function which: Given the same input, will always return the same output. Produces no side effects. Relies on no external state.
Whats a side effect? 5 Side effects may include, but are not limited to: Changing the file system. Inserting a record into a database. Making an http call. Mutations. Printing to the screen / logging. Obtaining user input. Accessing system state.
Referential transparency. 6 A function in algebra: f(x) = 2x > const double = x => x * 2; Since `double` is a pure function we can replace `double(5)` with 10 without changing the meaning. So, `console.log(double(5));` is the same as `console.log(10);`
Keep it simple. 7 Pure functions take some input and return some output based on that input. They are the simplest reusable building blocks of code in a program: KISS (Keep It Simple, Stupid) Pure functions are stupid simple in the best possible way.
Examples: 8
Examples: 9
In summary. Extremely independent. Easy to move around. Refactor. Easy to reorganize in your code. Gives more flexibility. Adaptable to future changes. Easy to test. 10
The trouble with shared state 11
The trouble with shared state Reliant on sequences which vary depending on indeterministic factors. The output is impossible to predict. Impossible to properly test. Impossible to fully understand. 12
| Confidential There is no non-determinism without mutable states. There is no race condition or deadlock without mutable states. Mutable states are the root of the problem. 13 Martin Odersky
| Confidential non-determinism = parallel processing + mutable state 14
Recipe for bugs. A single threaded JS engine does not imply that there is no concurrency. API I/O, event listeners, web workers, iframes, and timeouts. Combine that with shared state, and youve got a recipe for bugs. 15
| Confidential Shared mutable state and parallel processing doesn't go together at all. If you do not use proper synchronization you will create extremely difficult to find bugs and your programs are basically broken even if they appear to work just fine in most cases. 16
| Confidential Thanks! 17

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Pure functions using Javascript.

  • 2. Agenda Whats a function? Whats a pure function? Whats a side effect? Referential transparency. Keep it simple. Examples. The trouble with shared state. 2
  • 3. Whats a function? 3 A function is something that takes an input, and produces some output. function sum(a, b) { return a+b; } For example, this is a function that takes two numbers and returns the sum.
  • 4. Whats a pure function? 4 A pure function is a function which: Given the same input, will always return the same output. Produces no side effects. Relies on no external state.
  • 5. Whats a side effect? 5 Side effects may include, but are not limited to: Changing the file system. Inserting a record into a database. Making an http call. Mutations. Printing to the screen / logging. Obtaining user input. Accessing system state.
  • 6. Referential transparency. 6 A function in algebra: f(x) = 2x > const double = x => x * 2; Since `double` is a pure function we can replace `double(5)` with 10 without changing the meaning. So, `console.log(double(5));` is the same as `console.log(10);`
  • 7. Keep it simple. 7 Pure functions take some input and return some output based on that input. They are the simplest reusable building blocks of code in a program: KISS (Keep It Simple, Stupid) Pure functions are stupid simple in the best possible way.
  • 10. In summary. Extremely independent. Easy to move around. Refactor. Easy to reorganize in your code. Gives more flexibility. Adaptable to future changes. Easy to test. 10
  • 11. The trouble with shared state 11
  • 12. The trouble with shared state Reliant on sequences which vary depending on indeterministic factors. The output is impossible to predict. Impossible to properly test. Impossible to fully understand. 12
  • 13. | Confidential There is no non-determinism without mutable states. There is no race condition or deadlock without mutable states. Mutable states are the root of the problem. 13 Martin Odersky
  • 14. | Confidential non-determinism = parallel processing + mutable state 14
  • 15. Recipe for bugs. A single threaded JS engine does not imply that there is no concurrency. API I/O, event listeners, web workers, iframes, and timeouts. Combine that with shared state, and youve got a recipe for bugs. 15
  • 16. | Confidential Shared mutable state and parallel processing doesn't go together at all. If you do not use proper synchronization you will create extremely difficult to find bugs and your programs are basically broken even if they appear to work just fine in most cases. 16