[racket] Basic Racket Question

On Wed, Dec 22, 2010 at 1:02 PM, Danny Yoo <dyoo at cs.wpi.edu> wrote:

> >>  (define (netpay gross tax-rate)
> >>    (-(gross)(* gross tax-rate)))
> >>
> >> So I expect the function to calculate as
> >>
> >> = (-(240)(* 240 0.15)
> >> = ( - 240 36)
> >> = 204
>
>
> Just to be more careful: when you're showing the calculation, make
> sure to include the use of the function:
>
>     (netpay 240 0.15)
>      = (-(240)(* 240 0.15)
>     = ( - 240 36)
>     = 204
>
>
> There's a hitch on the first step in the calculation, and it has to do
> with the parens.  Unlike its use in traditional math notation, parens
> are significant in this language: that is, every use of paren has to
> mean something: it's not superfluous: if you have too many or too few,
> it changes the meaning of the program.
>
>
> So, within the larger term here:
>
>    (- (240) (* 240 0.15))
>
> the subterm
>
>    (240)
>
> means "call the function 240".  That may not be what you intend, but
> that what it means in this language.
>
>
> You can see this if you go back to what the error message is saying:
>
>   function call: expected a defined function name or a primitive
> operation name after an open parenthesis, but found a function
> argument name
>
> It's basically trying to point out this problem, that the use of
> "(gross)" within the expression
>
>   (-(gross)(* gross tax-rate))
>
> is trying to use gross as if it were a function, rather than the
> numeric argument to netpay.
>

the subterm

   (240)

means "call the function 240".  That may not be what you intend, but
that what it means in this language.

Awesome pickup, thank you. Can I ask how I could then specify gross a single
item in the formula and not as a function?
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