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Update "let is used to introduce variables" paragraph

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York Xiang 2015-04-08 10:29:11 +08:00
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src/doc/complement-design-faq.md
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@ -163,13 +163,17 @@ This is to make the language easier to parse for humans, especially in the face
of higher-order functions. `fn foo<T>(f: fn(int): int, fn(T): U): U` is not of higher-order functions. `fn foo<T>(f: fn(int): int, fn(T): U): U` is not
particularly easy to read. particularly easy to read.
## `let` is used to introduce variables ## Why is `let` used to introduce variables?
`let` not only defines variables, but can do pattern matching. One can also We don't use the term "variable", instead, we use "variable bindings". The
redeclare immutable variables with `let`. This is useful to avoid unnecessary simplest way for binding is the `let` syntax, other ways including `if let`,
`mut` annotations. An interesting historical note is that Rust comes, `while let` and `match`. Bindings also exist in function arguments positions.
syntactically, most closely from ML, which also uses `let` to introduce
bindings. Bindings always happen in pattern matching positions, and it's also Rust's way
to declare mutability. One can also redeclare mutability of a binding in
pattern matching. This is useful to avoid unnecessary `mut` annotations. An
interesting historical note is that Rust comes, syntactically, most closely
from ML, which also uses `let` to introduce bindings.
See also [a long thread][alt] on renaming `let mut` to `var`. See also [a long thread][alt] on renaming `let mut` to `var`.