CODEWITHSUNDEEP
lisp
Candid Moe
Candid Moe

Reputation: 398

Lisp what GET function returns?

Acording to Accesor GET, get finds a property on the property list2 of symbol whose property indicator is identical to indicator, and returns its corresponding property value. If the property doesn't exists, it returns NIL.

Then, if

(get 'clyde 'species) => nil

the expression

(setf (get 'clyde 'species) 'elephant)

must be the same as

(setf nil 'elephant)

and fail, but it's not the case.

How it comes that the same get produces a value in one case and a place in the other?

Edit

I found the answer here: How does using the SETF function to extend SETF work?

Upvotes: 0

Views: 351

Answers (2)

ignis volens
ignis volens

Reputation: 9252

setf is a macro: the job of a macro is to transform code to other code. So (setf (get ...) ...) does not ever call get: rather setf's macro function looks at the code it is given and turns it into other code, which is (after any other macros get to do their work) called. Here is what (setf (get 'foo 'x) 3) expands into in two implementations:

(setf (get 'foo 'x) 3)
 -> (system::%put 'foo 'x 3)

(setf (get 'foo 'x) 3)
 -> (ccl::set-get 'foo 'x 3)

As you can see, get is not being called, and there is no reason why it would be in this case.

Upvotes: 0

sds
sds

Reputation: 60014

Please take a look at Places and Generalized References.

Basically, setf is a macro, so

(setf (get 'clyde 'species) 'elephant)

is not evaluated sequentially as you seem to think it is, but instead expanded to whatever

(macroexpand '(setf (get 'clyde 'species) 'elephant))

returns in your implementation, and then the result is evaluated.

The bottom line is: when (setf (get 'clyde 'species) 'elephant) is being evaluated, (get 'clyde 'species) is not getting evaluated at all.

Upvotes: 2

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