Rebinding parameters to a function call in scheme>
I'm a bit of a scheme newbie and I'm trying to do write a function that 开发者_如何学编程takes a function name, and and an expression and binds the function name as the first argument of the expression.
IE:
Something like:
(bind-function-name '+ '(param 'a "First"))
Would produce a lambda that evaluates
(param '+ 'a "First")
Best I've come up with so far:
(define (bind-function-name name cmd)
(let ((func (car cmd)) (args (cdr cmd)))
(lambda ()
(apply func (cons name args)))))
Which doesn't seem to work, as it complains about the function name being a symbol.
I'm sure this is pretty simple, but I'm just missing something, thought someone could help me out.
First: you are mixing code and data. It looks alike in Scheme but it is not the same. Second: an expression does not have a first argument. Only a function call has a first argument.
So you do not talk about an expression but a function with an argument list. And if you have a function and a list of arguments, it is quite easy to write a bind function, which calls the original function with a modified argument list. It is done this way:
This is an old function, which takes some arguments and does anything with them:
(define (old-func . args)
(display "old-func: ")
(write args)
(newline))
(old-func 1 2 3) -> old-func: (1 2 3)
This is the bind function wich takes the old function and a new first-argument and returns a new function which calls the old function with the modified argument list.
(define (bind-func first-arg old-func)
(lambda args
(apply old-func (cons first-arg args))))
And this is the way it works:
(define new-func (bind-func "new" old-func))
(new-func 1 2 3) -> old-func: ("new" 1 2 3)
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