# Programming paradigms

One of the biggest hurdles to understanding functional programs when coming from
an imperative background is the shift in thinking. Imperative programs describe
**how** to do something, whereas declarative programs describe **what** to do.
Let’s sum the numbers from 1 to 10 to show this.

## Imperative

`#![allow(unused)] fn main() { let mut sum = 0; for i in 1..11 { sum += i; } println!("{}", sum); }`

With imperative programs, we have to play compiler to see what is happening.
Here, we start with a `sum`

of `0`

. Next, we iterate through the range from 1
to 10. Each time through the loop, we add the corresponding value in the range.
Then we print it out.

`i` | `sum` |
---|---|

1 | 1 |

2 | 3 |

3 | 6 |

4 | 10 |

5 | 15 |

6 | 21 |

7 | 28 |

8 | 36 |

9 | 45 |

10 | 55 |

This is how most of us start out programming. We learn that a program is a set of steps.

## Declarative

`#![allow(unused)] fn main() { println!("{}", (1..11).fold(0, |a, b| a + b)); }`

Whoa! This is really different! What’s going on here? Remember that with
declarative programs we are describing **what** to do, rather than **how** to do
it. `fold`

is a function that
composes functions. The
name is a convention from Haskell.

Here, we are composing functions of addition (this closure: `|a, b| a + b`

) with
a range from 1 to 10. The `0`

is the starting point, so `a`

is `0`

at first. `b`

is the first element of the range, `1`

. `0 + 1 = 1`

is the result. So now we
`fold`

again, with `a = 1`

, `b = 2`

and so `1 + 2 = 3`

is the next result. This
process continues until we get to the last element in the range, `10`

.

`a` | `b` | result |
---|---|---|

0 | 1 | 1 |

1 | 2 | 3 |

3 | 3 | 6 |

6 | 4 | 10 |

10 | 5 | 15 |

15 | 6 | 21 |

21 | 7 | 28 |

28 | 8 | 36 |

36 | 9 | 45 |

45 | 10 | 55 |