Add a foreach() method to std::iter::Iterator

text/0000-foreach.md

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+- Feature Name: foreach
+- Start Date: 16 April 2015
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+
+Add a `foreach()` method to `std::iter::Iterator`, which eagerly calls a function on
+each item the Iterator contains and returns `()`.
+
+# Motivation
+
+By design, iterators are lazy, and will not be evaluated until they are consumed
+in some way. The design of the `std::iter` library, and the language in general,
+strongly encourages chaining Iterator method calls one after another. This idiom
+is a very positive attribute of Rust: things that are hard to do by chaining
+iterators are usually things that make code less comprehensible (e.g. it is
+usually clearer code to `filter()` than to `continue` and to `take()` than to
+`break`).
+
+The `foreach()` method proposed by this RFC is fundamentally sugar; it does not
+enable programmers to write any instruction not already possible with existing
+language features. The most common recommendation is to write a `for` loop, but
+it is also possible to chain one of several consuming iterator methods after a
+map, to write a fold which returns `()`, or to add a dependency to the itertools
+crate and use the foreach method defined there. A
+[prior RFC](https://github.com/rust-lang/rfcs/pull/582) to the same effect was
+closed without merging for these reasons.
+
+However, if the inclusion of `foreach()` is syntactic sugar, then the exclusion of
+`foreach()` is syntactic salt. Several consumers are currently 'blessed' by std as
+special methods of Iterator, but no generic consumer method exists. For this
+reason, anyone who needs to consume an iterator by some mean not blessed by std
+needs to either depend on a crate whose tools are otherwise _more specific_,
+rather than _more general_, or do something that feels unidiomatic and wrong.
+Compare these three examples (the first being semantically different from the
+others, and the last being enabled by this RFC):
+
+```rust
+let vec = my_collection.iter()
+                       .filter_map(|x| { ... })
+                       .take_while(|x| { ... })
+                       .collect::<Vec<_>>()
+```
+```rust
+let tmp = my_collection.iter()
+                       .filter_map(|x| { ... })
+                       .take_while(|x| { ... });
+for x in tmp {
+    tx.send(x).ok();
+}
+```
+```rust
+my_collection.iter()
+             .filter_map(|x| { ... })
+             .take_while(|x| { ... })
+             .foreach(|x| { tx.send(x).ok(); });
+```
+
+Surely it is not intended that consuming an iterator by sending each element
+across a thread boundary (or by any other means not blessed by std) seem
+unidiomatic, but that is the effect of the current design. This is not about
+making std 'batteries included', it is about defining the limits of idiomatic
+Rust.
+
+# Detailed design
+
+Implementing this RFC is quite simple. Since this is, in implementation, just
+sugar, it involves adding a very small predefined method to the
+`std::iter::Iterator` trait, which would look something like this:
+
+```
+fn foreach<F>(&mut self, mut f: F) where F: FnMut(Self::Item) {
+    for item in self { f(item); }
+}
+```
+
+# Drawbacks
+
+The main reason not to add this method is that it is sugar.
+
+It could be argued that the Itertools crate implements `foreach()`, and that the
+Itertools crate is not very popular, making this uncommonly used sugar. To this
+there are several responses:
+  * The common way to get around this salt is probably not to add a dependency,
+    but to do the salty thing and write a `for` loop or add a `.all(|_| true)` and
+    so on. Thus, the popularity of this method cannot be found by analyzing the
+    popularity of the Itertools crate.
+  * Even if this method were not popularly used (because the blessed consumers
+    satisfy the majority of cases), that does not mean that a flexible method
+    for consuming an iterator should not be made available by std; even if its
+    use is a corner case, the blessed consumers are all degenerate
+    implementations of foreach and the only reason to make available degenerate
+    cases without the general case is if the general case is intended to be
+    unidiomatic.
+
+# Alternatives
+
+### Do nothing
+
+The main alternative is to do nothing.
+
+### Implement `each()` or some other method with similar but distinct semantics.
+
+Alternatively, Rust could have a similar method with different semantics, such
+as a method like Ruby's `each()`, which is essentially an eager version of Rust's
+`inspect`. I think maintaining the consistency of iterators return iterators
+being lazy is valuable, and that this method should return `()`.
+
+# Unresolved questions
+
+Should this method be named `foreach()` or `for_each()`? I think of __foreach__ as
+an existing PL keyword (used mainly by languages with non-iterative __for__
+loops), so my preference is foreach(), but it doesn't much matter to me. It also
+could be called `each()`, like Ruby's method, though its semantics are different.