Need help understanding bindings in Scheme code
For the code below, I am unable to comprehend how the bindings (x, y, z) occur. Please check out the code, I'll explain my problem in greater detail below:
(define (w x)
(lambda (y z)
开发者_Python百科 (begin
(set! x (+ (* y x) z)) x)))
(define f1 (w 3))
(f1 4 2)
(f1 2 1)
The output is 14, 29. These are values for x.
This means initially, x=3, y=4, z=2. In the 2nd call, i.e. (f1 2 1), x=14, y=2,z=1.
My doubts:
How does the binding occur at first, why is x=3, y=4, and z=2? If it has got to do with lambda expression in the function, please elaborate on how it works..I have a feeling this is where my understanding breaks down..
Next, why is the initial answer of x=14 retained in the second call, i.e. (f1 2 1)?
Thank you for looking into this :)
When w
is run, it creates a closure out of the inner lambda. Because x
came from outside of that inner lambda, x
is stored inside that closure (in this case, f1
).
So, f1
has a variable inside that represents x
, and it starts out as 3. When you run f1
, it evaluates the math and then sets its own x
to be 14.
The moral is, a lambda
is more than just code; it's code combined with the variables it closes over, such as this x
.
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