Rust Syntax: When is it appropriate to use "=>"

When would a rust programmer use to following fat arrow operator => .

a) Is it only for match and macros?

b) Why is the lexeme on the left operand as apposed to being on the right operand?

';' => Semi,
',' => Comma,
'.' => Dot,
'(' => OpenParen,

VS

Semi => ';'  ,
Comma => ','  ,

The above is scanning lexemes to validate a token which is then passed on to the next phase of the lexical analysis process.

Thank you kindly

=> is not an operator as such, it's part of the syntax of match and macro_rules!. The difference is there's no foo => bar meaning in isolation for the compiler, only when on the context of a match block, etc. (Technically this is true of all syntax, but the general assumption is that you're in a function body or the like)

You can read it in both those cases as meaning "when you match the pattern on the left, output the right": for match this is pattern matching a runtime value, for macro_rules! it's syntax. The direction is technically arbitrary, but the pattern is generally far smaller, and it's the established convention from other languages with similar features.

3 Likes

Awesome. Thank you kindly! Im going through Rust by example now. So a good bit of reading. Thanks for explaining that.

Both of these are valid match arms. The difference is in whether you want to convert from a char to a token enum, or from the enum to the char.

1 Like

Thank you kindly.

This topic was automatically closed 90 days after the last reply. We invite you to open a new topic if you have further questions or comments.