I have pedagogical questions. Why do you prefer "effect" to "side
effect"? I know that "pure" is misleading to programmers new to
Haskell, but I have thought that "side effect" was more likely to be
self-explanatory.
I also reach for longer phrases like "free from side effects" if I'm
talking to my students.
cheers,
bergey
On 2015-12-11 at 16:36, Christopher Allen
Side bar: although using "pure" with respect to effects is the common usage now, I'd urge you to consider finding a different wording since the original (and IMHO more meaningful) denotation of pure functional programming was about semantics and not the presence or absence of effects. The meaning was that you had a programming language whose semantics were lambda-calculus-and-nothing-more. This can be contrasted with ML where the lambda calculus is augmented with an imperative language that isn't functional or a lambda calculus. Part of the problem with making purity about effects rather than semantics is the terrible imprecision confuses new people. They'll often misunderstand it as, "Haskell programs can't perform effects" or they'll think it means stuff in "IO" isn't pure - which is false. We benefit from having a pure functionalal programming language _especially_ in programs that emit effects. Gabriel Gonzalez has a nice article demonstrating some of this: http://www.haskellforall.com/2015/03/algebraic-side-effects.html
When I want to talk about effects, I say "effect". When I want to say something that doesn't emit effects, I say "effect-free" and when it does, "effectful". Sometimes I'll say "in IO" for the latter as well, where "in IO" can be any type that has IO in the outermost position of the final return type.