Re: (declare (pure ...))

[Al]

On 2024-02-10 11:53, Pietro Cerutti wrote:

> I don't see why vector-ref would be any less pure than, say, a let 
> binding. Or do you mean vector-set! ?
vector-ref, applied to a global, could return different values even when 
called with the same arguments. Between calls, some other code could 
modify the contents of the vector. So according to referential 
transparency, vector-ref could not be pure. Likewise a function that 
calls vector-ref could not be pure.


Further, according to referential transparency no procedure that *reads* 
a global (not just vectors -- even imediate values like numbers, 
booleans) can be pure. It might return different values if impure code 
modifies the global between calls to the procedure in question.


In contrast, a let binding can only be modified by the code in the 
current scope.


There should still be a way to communicate to the optimizer that 
vector-ref, or some procedure that uses vector-ref on a global 
identifier (but does not call set! / vector-set! on globals) does not, 
uh, modify any globals.


-- Al