Why is the 'if' statement considered evil?

Question

I just came from Simple Design and Testing Conference. In one of the session we were talking about evil keywords in programming languages. Corey Haines, who proposed the subject, was convinced that if statement is absolute evil. His alternative was to create functions with predicates. Can you please explain to me why if is evil.

I understand that you can write very ugly code abusing if. But I don't believe that it's that bad.

Answer 1

I'll agree with you; he was wrong. You can go too far with things like that, too clever for your own good.

Code created with predicates instead of ifs would be horrendous to maintain and test.

Answer 2

There is another sense in which if can be evil: when it comes instead of polymorphism.

E.g.

 if (animal.isFrog()) croak(animal)
 else if (animal.isDog()) bark(animal)
 else if (animal.isLion()) roar(animal)

instead of

 animal.emitSound()

But basically if is a perfectly acceptable tool for what it does. It can be abused and misused of course, but it is nowhere near the status of goto.

Answer 3

The only problem with a predicates (in terms of replacing if statements) is that you still need to test them:

function void Test(Predicate<int> pr, int num) 
{
    if (pr(num))
    { /* do something */ }
    else
    { /* do something else */ }
}

You could of course use the terniary operator (?:), but that's just an if statement in disguise...

Answer 4

I think If statements are evil, but If expressions are not. What I mean by an if expression in this case can be something like the C# ternary operator (condition ? trueExpression : falseExpression). This is not evil because it is a pure function (in a mathematical sense). It evaluates to a new value, but it has no effects on anything else. Because of this, it works in a substitution model.

Imperative If statements are evil because they force you to create side-effects when you don't need to. For an If statement to be meaningful, you have to produce different "effects" depending on the condition expression. These effects can be things like IO, graphic rendering or database transactions, which change things outside of the program. Or, it could be assignment statements that mutate the state of the existing variables. It is usually better to minimize these effects and separate them from the actual logic. But, because of the If statements, we can freely add these "conditionally executed effects" everywhere in the code. I think that's bad.

Answer 5

Predicates come from logical/declarative programming languages, like PROLOG. For certain classes of problems, like constraint solving, they are arguably superior to a lot of drawn out step-by-step if-this-do-that-then-do-this crap. Problems that would be long and complex to solve in imperative languages can be done in just a few lines in PROLOG.

There's also the issue of scalable programming (due to the move towards multicore, the web, etc.). If statements and imperative programming in general tend to be in step-by-step order, and not scaleable. Logical declarations and lambda calculus though, describe how a problem can be solved, and what pieces it can be broken down into. As a result, the interpreter/processor executing that code can efficiently break the code into pieces, and distribute it across multiple CPUs/cores/threads/servers.

Definitely not useful everywhere; I'd hate to try writing a device driver with predicates instead of if statements. But yes, I think the main point is probably sound, and worth at least getting familiar with, if not using all the time.

Answer 6

Good that in ruby we have unless ;)

But seriously probably if is the next goto, that even if most of the people think it is evil in some cases is simplifying/speeding up the things (and in some cases like low level highly optimized code it's a must).