Cannot infer type for `B` for filter_map().sum()

Question

The code below reads numbers, sums them, then prints the sum. I\'ve tried few annotations, but it didn\'t work. I must be missing something. How could I make it work?

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Answer 1

Let's look up the signature of filter_map to see, what the complain is about:

fn filter_map(self, f: F) -> FilterMap where F: FnMut(Self::Item) -> Option, 

Okay, so Option is the result, which means he cannot infer what w.parse().ok() will be.

Let's try to give him a hint

.filter_map(|w| w.parse::().ok())

Let's compile an see.... Hurray!

So, lesson learned: Look up the signature and try to figure out, which part the compiler cannot infer and try to specify it.