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auto merge of #9039 : singingboyo/rust/update-for-expr-docs, r=thestinger

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The old documentation for for loops/expressions has been quite wrong since the change to iterators.  This updates the docs to make them relevant to how for loops work now, if not very in-depth.  There may be a need for updates giving more depth on how they work, such as detailing what method calls they make, but I don't know enough about the implementation to include that.
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bors 2013-09-11 07:46:04 -07:00
commit 49eb7bd271
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@ -2459,25 +2459,12 @@ do k(3) |j| {
### For expressions
~~~~~~~~{.ebnf .gram}
for_expr : "for" expr [ '|' ident_list '|' ] ? '{' block '}' ;
for_expr : "for" pat "in" expr '{' block '}' ;
~~~~~~~~
A _for expression_ is similar to a [`do` expression](#do-expressions),
in that it provides a special block-form of lambda expression,
suited to passing the `block` function to a higher-order function implementing a loop.
In contrast to a `do` expression, a `for` expression is designed to work
with methods such as `each` and `times`, that require the body block to
return a boolean. The `for` expression accommodates this by implicitly
returning `true` at the end of each block, unless a `break` expression
is evaluated.
In addition, [`break`](#break-expressions) and [`loop`](#loop-expressions) expressions
are rewritten inside `for` expressions in the same way that `return` expressions are,
with a combination of local flag variables,
and early boolean-valued returns from the `block` function,
such that the meaning of `break` and `loop` is preserved in a primitive loop
when rewritten as a `for` loop controlled by a higher order function.
A `for` expression is a syntactic construct for looping
over elements provided by an implementation of
`std::iterator::Iterator`.
An example of a for loop over the contents of a vector: