In a function that permits liberal inputs, I often find that the input
processing I do in a contract is duplicated at the beginning of the body of the
function. Is this avoidable?
Certain functions want to be liberal with input because there are multiple
common ways to represent the data. For instance, I have a function that
operates on CSS RGB colors. This function should be prepared to accept these
forms of input & understand that they're all the same:
"#c00"
"#cc0000"
'("204" "0" "0")
#xcc0000
'(0.8 0 0)
Let's say that my internal representation of an RGB color is described by the
contract rgb-color/c:
(define rgb-color/c (list/c (real-in 0 1) (real-in 0 1) (real-in 0 1)))
But I can't use rgb-color/c as the input contract for the function because it's
too narrow. So I make a second contract that tests for things that can be
converted to an rgb-color:
(define (rgb-colorish? x) (or/c rgb-color/c [tests for the other input formats
...] )
To determine if the input is rgb-colorish?, this contract usually just ends up
trying to convert the input to rgb-color. If it works, then the contract
returns true.
But after the contract returns, I have to convert the input to an rgb-color
anyhow. So I'm doing exactly the same work that the contract just finished. If
the conversion is expensive, I'm doing it twice.
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