RFC: Add Option::filter to the standard library

text/2124-option-filter.md

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+- Feature Name: option_filter
+- Start Date: 2017-08-21
+- RFC PR: https://github.com/rust-lang/rfcs/pull/2124
+- Rust Issue: https://github.com/rust-lang/rust/issues/45860
+
+# Summary
+[summary]: #summary
+
+Add the method `Option::filter<P>(self, predicate: P) -> Self` to the
+standard library. This method makes it possible to easily throw away a `Some`
+value depending on a given predicate. The call `opt.filter(p)` is equivalent
+to `opt.into_iter().filter(p).next()`.
+
+```rust
+assert_eq!(Some(3).filter(|_| true)), Some(3));
+assert_eq!(Some(3).filter(|_| false)), None);
+assert_eq!(None.filter(|_| true), None);
+```
+
+# Motivation
+[motivation]: #motivation
+
+The `Option` type has plenty of methods, every single one intended to help the
+user write short code dealing with this ubiquitous type. If we would not care
+about convenience when dealing with `Option`, the type would not have nearly
+as many methods.
+
+Just like other methods, `filter()` is a useful method in *certain*
+situations. While it is not nearly as important as `map()`, it is very handy
+in many situations. The feedback on the [corresponding `rfcs`-issue][issue]
+clearly shows that many people encountered a situation in which `filter()`
+would have been helpful.
+
+Consider this tiny example:
+
+```rust
+let api_key = std::env::arg("APIKEY").ok()
+    .filter(|key| key.starts_with("api"));
+```
+
+Here is another example showing tree traversal with a queue:
+
+```rust
+let mut queue = VecDeque::new();
+queue.push_back(tree.root());
+
+// We want to visit all nodes in breadth first search order, but stop
+// immediately once we found a leaf node.
+while let Some(node) = queue.pop_front().filter(|node| !node.is_leaf()) {
+    queue.extend(node.children());
+}
+```
+
+Additionally, adding `filter()` would make the interfaces of `Option` and
+`Iterator` more consistent. Both types already shared a handful of methods
+with identical names and functions, most importantly `map()`. Adding another
+such method would make the whole interface feel more consistent.
+
+In the following example the programmer can easily swap `nth()` and `filter()`
+statements, if they decide they want to allow the `-j` parameter at any
+position.
+
+```rust
+let num_threads = std::env::args()
+    .nth(1)
+    .filter(|arg| arg.starts_with("-j"))
+    .and_then(|arg| arg[2..].parse().ok());
+
+```
+
+`filter()` can be especially useful for integration into existing method-
+chains. Here is a slightly more complicated example which is taken from an
+existing, real web app's session management. Note that each line introduces a
+new reason to reject the session.
+
+```rust
+// Check if there is a session-cookie
+let session = cookies.get(SESSION_COOKIE_NAME)
+    // Try to decode the cookie's value as hexadecimal string
+    .and_then(|cookie| hex::decode(cookie.value()).ok())
+    // Make sure the session id has the correct length
+    .filter(|session_id| session_id.len() == SESSION_ID_LEN)
+    // Try to find the session with the given ID in the database
+    .and_then(|session_id| db.find_session_by_id(session_id));
+```
+
+All these examples would be less easy to read without `filter()`. There are
+two main ways to achieve something equivalent to `filter(p)`:
+
+- `opt.into_iter().filter(p).next()`: notably longer and the `next()` feels
+  semantically wrong.
+- `opt.and_then(|v| if p(&v) { Some(v) } else { None })`: notably longer and a
+  questionable single-line `if-else`.
+
+
+[issue]: https://github.com/rust-lang/rfcs/issues/1485
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+A possible documentation of the method:
+
+> ```rust
+> fn filter<P>(self, predicate: P) -> Self
+>     where P: FnOnce(&T) -> bool
+> ```
+>
+> Returns `None` if the option is `None`, otherwise calls `predicate` with the
+> wrapped value and returns:
+>
+> - `Some(t)` if `predicate` returns `true` (where `t` is the wrapped value),
+>    and
+> - `None` if `predicate` returns `false`.
+>
+> This function works similar to `Iterator::filter()`. You can imagine the
+> `Option<T>` being an iterator over one or zero elements. `filter()` lets
+> you decide which elements to keep.
+>
+> # Examples
+>
+> ```rust
+> fn is_even(n: i32) -> bool {
+>     n % 2 == 0
+> }
+>
+> assert_eq!(None.filter(is_even), None);
+> assert_eq!(Some(3).filter(is_even), None);
+> assert_eq!(Some(4).filter(is_even), Some(4));
+> ```
+>
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+It is hopefully sufficiently clear how `filter()` is supposed to work from the
+explanations above. Here is one example implementation:
+
+```rust
+impl<T> Option<T> {
+    pub fn filter<P>(self, predicate: P) -> Self
+        where P: FnOnce(&T) -> bool
+    {
+        match self {
+            Some(x) => {
+                if predicate(&x) {
+                    Some(x)
+                } else {
+                    None
+                }
+            }
+            None => None,
+        }
+    }
+}
+```
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+It increases the size of the standard library by a tiny bit.
+
+# Rationale and Alternatives
+[alternatives]: #alternatives
+
+- Don't do anything.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+### Maybe `filter()` wouldn't be used a lot.
+
+The feature proposed in this RFC is already implemented in the
+[`option-filter` crate][crate]. This crate hasn't been used a lot (only
+around 1500 downloads at the time of writing this). Thus, it makes sense to ask whether people would actually use the `filter()` method. However, there
+are many other reasons for not using this crate:
+
+- The programmer doesn't know about the crate
+- The programmer knows about the crate, but doesn't want to have too many tiny
+  dependencies in their project
+- The programmer knows about the crate, but they decided it's too much work to
+  use the crate.
+
+  A simple calculation: using the crate would require around 80 new characters
+  (`option-filter = "*"` + `extern crate option_filter;` +
+  `use option_filter::OptionFilterExt;`) in at least 2, probably 3, files. On
+  the other hand, using the `.and_then()` workaround shown above would only
+  need 39 more characters than `filter()` and wouldn't require opening other
+  files.
+
+According to the assessment of this RFC's author, the mentioned crate is not
+used for reasons independently of `filter()`'s usefulness.
+
+Reading the comments and looking at the feedback in [this thread][rfcs-issue],
+it's clear that there are at least some people openly requesting this feature.
+And to give a specific example: this RFC's author wanted to use `filter()` a
+whole lot more often than he used some of the other methods of `Option` (like
+`map_or_else()` and `ok_or_else()`).
+
+
+[crate]: https://crates.io/crates/option-filter
+[rfcs-issue]: https://github.com/rust-lang/rfcs/issues/1485