[racket] Code that double checks is good, bad, other?

Hello all,

Sometimes I run into situations where a callee has certain input
constraints that the caller must satisfy. Particularly when the caller is
just below the user interface level, this can lead to redundant checks. For
example, if the the callee takes an integer, and the user input function
pulls from a control that only accepts integers to begin with, there is
really no need for the caller to validate that the thing received from the
control is really an integer before calling the callee with it. It's this
middle function between UI and underlying low level model function that
seems to have redundancy. Actually, the callee wouldn't have to verify
either, but the callee would be used elsewhere where there might be no
guarantee.

An analogy is the water company should put water into the system, and water
should be verified at the tap before blinding drinking it, but does every
piece of underground pipe need to be like "yep it's water"?

The answer is seemingly "no".

But then it's an issue in a larger program of refactoring every bit of code
to take maximum advantage of every statically-guaranteed piece of
information, running as unsafe-yet-still-safe as possible. That seems like
a lot of manual effort that can be avoided by just throwing in what may or
may not be a redundant check depending on which series of pipes the
information flows through.

So as a practical matter, it's an issue of catching errors (bugs) as soon
as possible vs. there aren't supposed to be any bugs to begin with. So one
can argue that either 1. I don't understand my program well enough or 2.
bigger programs are hard to understand, and some measure of practicality is
warranted. Perhaps the question is more philosophical in the end...is it
better to have something quickly and useful that works 99% of the time, or
a perfect guarantee but potentially longer development time? The answer
depends on whether it's for weapon systems guidance or a hello world
example.

This question is probably more fit for stack-X, but I wonder what the
Racket people think of "excessive" checks.
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