Solve the programming task below in a Python markdown code block.
You get an array of numbers, return the sum of all of the positives ones.

Example `[1,-4,7,12]` => `1 + 7 + 12 = 20`

Note: if there is nothing to sum, the sum is default to `0`.
Also feel free to reuse/extend the following starter code:
```python
def positive_sum(arr):
```