Why Functional Programming?
Functional programs contain no assignment statements, so variables, once given a value,
never change. More generally, functional programs contain no side-effects at all. A function
call can have no effect other than to compute its result. This eliminates a major source of bugs,
and also makes the order of execution irrelevant—since no side-effect can change an expres-
sion’s value, it can be evaluated at any time. This relieves the programmer of the burden of pre-
scribing the flow of control. Since expressions can be evaluated at any time, one can freely
replace variables by their values and vice versa—that is, programs are “referentially transpar-
ent.” This freedom helps make functional programs more tractable mathematically than their
conventional counterparts.
—John Hughes
“Why Functional Programming Matters”
I call it my billion-dollar mistake … My goal was to ensure that all use of references should be
absolutely safe, with checking performed automatically by the compiler. But I couldn’t resist the
temptation to put in a null reference, simply because it was so easy to implement. This has led
to innumerable errors, vulnerabilities, and system crashes, which have probably caused a bil-
lion dollars of pain and damage in the last forty years.
—Tony Hoare
Program testing can be a very effective way to show the presence of bugs, but is hopelessly inad-
equate for showing their absence.
—Edsger W. Dijkstra
Testing by itself does not improve software quality. Test results are an indicator of quality, but
in and of themselves, they don’t improve it. Trying to improve software quality by increasing
the amount of testing is like trying to lose weight by weighing yourself more often.
—Steve McConnell
The proper use of comments is to compensate for our failure to express ourselves in code.
—Robert C. Martin
In programming the hard part isn’t solving problems, but deciding what problems to solve.
—Paul Graham
Object oriented programming makes code understandable by encapsulating moving parts.
Functional programming makes code understandable by minimizing moving parts.
—Michael Feathers
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