For the method tax_deductible_expenses to run it ends up requiring 19
user defined values. That's a whole lot of arguments to pass into a
function. Obviously I could wrap them up in a StructMap but then I get
expressions like (+ 1 (args :B)) which doesn't seem much better than
(+1 (B)).
Pass them in as a map, and destructure at the start of the function:
(defn tax-deductible-expenses [{:keys [management-fee
tenant-finding-fee ...]}]
...)
The issue I'm really trying to put my finger on is - these arguments
really and truly are 'constants' within the context of the calculator.
But they are constants whose values are not known as compile time but
rather are known at run time.
That they're "constant" does not mean that you shouldn't pass them as
arguments to your functions.
Most values obtained from users or config files are such "run-time
constants". Heck, many values in most programs are -- "HTTP listener
port", "log file location", etc.
You still invoke your HTTP server with a :port argument.
Indeed, the more fixed values you have, the less likely it is that you
should define a var for each. Maybe one var containing a map, but I'd
still pass the map around rather than having each function implicitly
refer to it. It makes testing easier.
Does Clojure have a way to express a
'late bound' constant or is the 'right' solution to pass around 19+
arguments to functions or passing around StructMaps or making
everything into thunks?
The reason you pass them around as arguments is so that the behavior
of your functions is precisely determined *only* by its arguments --
they are pure.
That means that you can memoize them, easily write tests for them,
have them work correctly when part of a lazy sequence (which will
often be evaluated outside the scope of your bindings), etc.
For example: how would you compare the tax-deductible-expenses of two
clients? You'd need to invoke the function twice, with a huge nest of
bindings around each call. Much better would be to store the
appropriate values in two maps, then say
(< (tax-deductible-expenses 0 john-data)
(tax-deductible-expenses 0 bill-data))
-R
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