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| -rw-r--r-- | doc/rust.md | 28 |
1 files changed, 16 insertions, 12 deletions
diff --git a/doc/rust.md b/doc/rust.md index 9839e9e8afd..a115f41ae86 100644 --- a/doc/rust.md +++ b/doc/rust.md @@ -2250,6 +2250,14 @@ do_expr : "do" expr [ '|' ident_list '|' ] ? '{' block '}' ; A _do expression_ provides a more-familiar block-syntax for a [lambda expression](#lambda-expressions), including a special translation of [return expressions](#return-expressions) inside the supplied block. +Any occurrence of a [return expression](#return-expressions) +inside this `block` expression is rewritten +as a reference to an (anonymous) flag set in the caller's environment, +which is checked on return from the `expr` and, if set, +causes a corresponding return from the caller. +In this way, the meaning of `return` statements in language built-in control blocks is preserved, +if they are rewritten using lambda functions and `do` expressions as abstractions. + The optional `ident_list` and `block` provided in a `do` expression are parsed as though they constitute a lambda expression; if the `ident_list` is missing, an empty `ident_list` is implied. @@ -2296,19 +2304,15 @@ 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. -Like a `do` expression, a `return` expression inside a `for` expresison is rewritten, -to access a local flag that causes an early return in the caller. - -Additionally, any occurrence of a [return expression](#return-expressions) -inside the `block` of a `for` expression is rewritten -as a reference to an (anonymous) flag set in the caller's environment, -which is checked on return from the `expr` and, if set, -causes a corresponding return from the caller. -In this way, the meaning of `return` statements in language built-in control blocks is preserved, -if they are rewritten using lambda functions and `do` expressions as abstractions. +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. -Like `return` expressions, any [`break`](#break-expressions) and [`loop`](#loop-expressions) expressions -are rewritten inside `for` expressions, with a combination of local flag variables, +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. |