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> Does it interact with the borrow checker somehow? Nope. The borrow checker cares about the control flow graph, expression vs statement doesn't matter. > why

> Does it interact with the borrow checker somehow? Nope. The borrow checker cares about the control flow graph, expression vs statement doesn't matter. > why does Rust make the expression/statement distinction? I am not 100% sure. If I had to guess, older Rust was much less expression oriented, and then, over time, got moreso. But also, I think it kinda just makes sense in general for the kind of language Rust is. Like, in Ruby, where everything truly is an expression, importing a file and then evaluating it has side effects. Whereas in Rust, 95% of statements are declarations, and of those 95%, the only ones you really use in a normal execution context are let statements. The rest are stuff like "declaring functions" and "declaring structs" and those don't really get evaluated in a language like Rust. let being a statement is nice because it means it can only happen at the "top level" of a block, and not say, inside a loop condition.

int_19h 2y ago
> Like, in Ruby, where everything truly is an expression, importing a file and then evaluating it has side effects. In the context of ML, I think it's a more useful baseline. So declarations are still declarations, but e.g. ; is just a sequential evaluation operator. > let being a statement is nice because it means it can only happen at the "top level" of a block, and not say, inside a loop condition. I would argue that it's actually a downside - it means that a loop condition cannot have common subexpressions (that nevertheless need to be evaluated on every iteration) factored out.
- steveklabnik 2y ago
  I guess if you required ; after all of them, sure. Rust doesn’t, so I didn’t think of that. I’ve always found code that does this to be more complicated to read and understand than rewriting it in a different way. YMMV, of course.
dragonwriter 2y ago
> Like, in Ruby, where everything truly is an expression, importing a file and then evaluating it has side effects. That's not so much “everything is an expression” as “everything is at runtime”.
- steveklabnik 2y ago
  Eh that’s fair.

int_19h 2y ago

> Like, in Ruby, where everything truly is an expression, importing a file and then evaluating it has side effects. In the context of ML, I think it's a more useful baseline. So declarations are still declarations, but e.g. ; is just a sequential evaluation operator. > let being a statement is nice because it means it can only happen at the "top level" of a block, and not say, inside a loop condition. I would argue that it's actually a downside - it means that a loop condition cannot have common subexpressions (that nevertheless need to be evaluated on every iteration) factored out.

steveklabnik 2y ago
I guess if you required ; after all of them, sure. Rust doesn’t, so I didn’t think of that. I’ve always found code that does this to be more complicated to read and understand than rewriting it in a different way. YMMV, of course.

dragonwriter 2y ago

> Like, in Ruby, where everything truly is an expression, importing a file and then evaluating it has side effects. That's not so much “everything is an expression” as “everything is at runtime”.

steveklabnik 2y ago
Eh that’s fair.