Re: (declare (pure ...))

[Pietro Cerutti]

On 10 Feb 2024, at 11:10, Al <frm.ml@mailgw.com> wrote:

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.

This is not how I reason about referential transparency. It is a property of functions applied to values, not variables.

The fact that you can define x to different values or even rebind it in the scope of a let binding doesn't make (lambda (x) (+ x 1)) less referentially transparent.

If you change the value of a slot of a vector, then the vector has a different value than before, even if the same name binds to the value.

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Pietro Cerutti

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Sent from a small device - please excuse brevity and typos.