I want to validate my inputs to function with some conditions and show as a compiler warning/error.
How it is possible?
For example:
func getPoints(start: Int, end: Int) {
}
I want to show compiler warning/error when someone tries to give input high for start than end.
getPoints(start: 3, end: 10) // No warnings
getPoints(start: 6, end: 2) // Compiler warning like: end value can not be less than start value
Actually this is for a framework purpose. I want to ensure that the parameters are not bad inputs.
Such a constraint can't be enforced at compile time. Take Range for example, which enforces that the lowerBound always compares as less or equal to the upperBound. That's just an assertion that runs at run-time, and crashes if it's not met.
I would suggest you just change your API design to use a Range<Int> or ClosedRange<Int> taking pairs of Ints to model ranges is a bad idea, for many reasons:
It doesn't communicate the semantics of a range. Two integers could be anything, but a range is something much more specific.
It doesn't have any of the useful methods, like contains(_:), or support for pattern matching via the ~= operator.
Its error prone, because when passing pairs around, you might make a copy/paste error leading you to accidentally use the same param twice.
It reads better: getPoint(3...10)
You can't generate a warning at compile time, since the arguments are not evaluated beyond checking for type conformance.
In your example, you have used constants, so it would, in theory, be possible to perform the check you want, but what if you passed a variable, or the result of another function? How much of your code would the compiler need to execute in order to perform the check?
You need to enforce your requirements at run time. For example, you could have your function throw if the parameters were incorrect:
enum MyErrors: Error {
case rangeError
}
func getPoints(start: Int, end: Int) throws {
guard start <= end else {
throw MyErrors.rangeError
}
...
}
Or you could have the function simply handle the problem:
func getPoints(start: Int, end: Int) {
let beginning = min(start,end)
let ending = max(start,end)
...
}
Also, I recommend Alexander's suggestion of using Range instead of Int; it is always a good idea to take advantage of Foundation types, but I will leave my answer as it shows some approaches for handling issues at runtime.
What are the benefits and drawbacks of the ?: operator as opposed to the standard if-else statement. The obvious ones being:
Conditional ?: Operator
Shorter and more concise when dealing with direct value comparisons and assignments
Doesn't seem to be as flexible as the if/else construct
Standard If/Else
Can be applied to more situations (such as function calls)
Often are unnecessarily long
Readability seems to vary for each depending on the statement. For a little while after first being exposed to the ?: operator, it took me some time to digest exactly how it worked. Would you recommend using it wherever possible, or sticking to if/else given that I work with many non-programmers?
I would basically recommend using it only when the resulting statement is extremely short and represents a significant increase in conciseness over the if/else equivalent without sacrificing readability.
Good example:
int result = Check() ? 1 : 0;
Bad example:
int result = FirstCheck() ? 1 : SecondCheck() ? 1 : ThirdCheck() ? 1 : 0;
This is pretty much covered by the other answers, but "it's an expression" doesn't really explain why that is so useful...
In languages like C++ and C#, you can define local readonly fields (within a method body) using them. This is not possible with a conventional if/then statement because the value of a readonly field has to be assigned within that single statement:
readonly int speed = (shiftKeyDown) ? 10 : 1;
is not the same as:
readonly int speed;
if (shifKeyDown)
speed = 10; // error - can't assign to a readonly
else
speed = 1; // error
In a similar way you can embed a tertiary expression in other code. As well as making the source code more compact (and in some cases more readable as a result) it can also make the generated machine code more compact and efficient:
MoveCar((shiftKeyDown) ? 10 : 1);
...may generate less code than having to call the same method twice:
if (shiftKeyDown)
MoveCar(10);
else
MoveCar(1);
Of course, it's also a more convenient and concise form (less typing, less repetition, and can reduce the chance of errors if you have to duplicate chunks of code in an if/else). In clean "common pattern" cases like this:
object thing = (reference == null) ? null : reference.Thing;
... it is simply faster to read/parse/understand (once you're used to it) than the long-winded if/else equivalent, so it can help you to 'grok' code faster.
Of course, just because it is useful does not mean it is the best thing to use in every case. I'd advise only using it for short bits of code where the meaning is clear (or made more clear) by using ?: - if you use it in more complex code, or nest ternary operators within each other it can make code horribly difficult to read.
I usually choose a ternary operator when I'd have a lot of duplicate code otherwise.
if (a > 0)
answer = compute(a, b, c, d, e);
else
answer = compute(-a, b, c, d, e);
With a ternary operator, this could be accomplished with the following.
answer = compute(a > 0 ? a : -a, b, c, d, e);
I find it particularly helpful when doing web development if I want to set a variable to a value sent in the request if it is defined or to some default value if it is not.
A really cool usage is:
x = foo ? 1 :
bar ? 2 :
baz ? 3 :
4;
Sometimes it can make the assignment of a bool value easier to read at first glance:
// With
button.IsEnabled = someControl.HasError ? false : true;
// Without
button.IsEnabled = !someControl.HasError;
I'd recommend limiting the use of the ternary(?:) operator to simple single line assignment if/else logic. Something resembling this pattern:
if(<boolCondition>) {
<variable> = <value>;
}
else {
<variable> = <anotherValue>;
}
Could be easily converted to:
<variable> = <boolCondition> ? <value> : <anotherValue>;
I would avoid using the ternary operator in situations that require if/else if/else, nested if/else, or if/else branch logic that results in the evaluation of multiple lines. Applying the ternary operator in these situations would likely result in unreadable, confusing, and unmanageable code. Hope this helps.
The conditional operator is great for short conditions, like this:
varA = boolB ? valC : valD;
I use it occasionally because it takes less time to write something that way... unfortunately, this branching can sometimes be missed by another developer browsing over your code. Plus, code isn't usually that short, so I usually help readability by putting the ? and : on separate lines, like this:
doSomeStuffToSomething(shouldSomethingBeDone()
? getTheThingThatNeedsStuffDone()
: getTheOtherThingThatNeedsStuffDone());
However, the big advantage to using if/else blocks (and why I prefer them) is that it's easier to come in later and add some additional logic to the branch,
if (shouldSomethingBeDone()) {
doSomeStuffToSomething(getTheThingThatNeedsStuffDone());
doSomeAdditionalStuff();
} else {
doSomeStuffToSomething(getTheOtherThingThatNeedsStuffDone());
}
or add another condition:
if (shouldSomethingBeDone()) {
doSomeStuffToSomething(getTheThingThatNeedsStuffDone());
doSomeAdditionalStuff();
} else if (shouldThisOtherThingBeDone()){
doSomeStuffToSomething(getTheOtherThingThatNeedsStuffDone());
}
So, in the end, it's about convenience for you now (shorter to use :?) vs. convenience for you (and others) later. It's a judgment call... but like all other code-formatting issues, the only real rule is to be consistent, and be visually courteous to those who have to maintain (or grade!) your code.
(all code eye-compiled)
One thing to recognize when using the ternary operator that it is an expression not a statement.
In functional languages like scheme the distinction doesn't exists:
(if (> a b) a b)
Conditional ?: Operator
"Doesn't seem to be as flexible as the if/else construct"
In functional languages it is.
When programming in imperative languages I apply the ternary operator in situations where I typically would use expressions (assignment, conditional statements, etc).
While the above answers are valid, and I agree with readability being important, there are 2 further points to consider:
In C#6, you can have expression-bodied methods.
This makes it particularly concise to use the ternary:
string GetDrink(DayOfWeek day)
=> day == DayOfWeek.Friday
? "Beer" : "Tea";
Behaviour differs when it comes to implicit type conversion.
If you have types T1 and T2 that can both be implicitly converted to T, then the below does not work:
T GetT() => true ? new T1() : new T2();
(because the compiler tries to determine the type of the ternary expression, and there is no conversion between T1 and T2.)
On the other hand, the if/else version below does work:
T GetT()
{
if (true) return new T1();
return new T2();
}
because T1 is converted to T and so is T2
If I'm setting a value and I know it will always be one line of code to do so, I typically use the ternary (conditional) operator. If there's a chance my code and logic will change in the future, I use an if/else as it's more clear to other programmers.
Of further interest to you may be the ?? operator.
The advantage of the conditional operator is that it is an operator. In other words, it returns a value. Since if is a statement, it cannot return a value.
There is some performance benefit of using the the ? operator in eg. MS Visual C++, but this is a really a compiler specific thing. The compiler can actually optimize out the conditional branch in some cases.
The scenario I most find myself using it is for defaulting values and especially in returns
return someIndex < maxIndex ? someIndex : maxIndex;
Those are really the only places I find it nice, but for them I do.
Though if you're looking for a boolean this might sometimes look like an appropriate thing to do:
bool hey = whatever < whatever_else ? true : false;
Because it's so easy to read and understand, but that idea should always be tossed for the more obvious:
bool hey = (whatever < whatever_else);
If you need multiple branches on the same condition, use an if:
if (A == 6)
f(1, 2, 3);
else
f(4, 5, 6);
If you need multiple branches with different conditions, then if statement count would snowball, you'll want to use the ternary:
f( (A == 6)? 1: 4, (B == 6)? 2: 5, (C == 6)? 3: 6 );
Also, you can use the ternary operator in initialization.
const int i = (A == 6)? 1 : 4;
Doing that with if is very messy:
int i_temp;
if (A == 6)
i_temp = 1;
else
i_temp = 4;
const int i = i_temp;
You can't put the initialization inside the if/else, because it changes the scope. But references and const variables can only be bound at initialization.
The ternary operator can be included within an rvalue, whereas an if-then-else cannot; on the other hand, an if-then-else can execute loops and other statements, whereas the ternary operator can only execute (possibly void) rvalues.
On a related note, the && and || operators allow some execution patterns which are harder to implement with if-then-else. For example, if one has several functions to call and wishes to execute a piece of code if any of them fail, it can be done nicely using the && operator. Doing it without that operator will either require redundant code, a goto, or an extra flag variable.
With C# 7, you can use the new ref locals feature to simplify the conditional assignment of ref-compatible variables. So now, not only can you do:
int i = 0;
T b = default(T), c = default(T);
// initialization of C#7 'ref-local' variable using a conditional r-value⁽¹⁾
ref T a = ref (i == 0 ? ref b : ref c);
...but also the extremely wonderful:
// assignment of l-value⁽²⁾ conditioned by C#7 'ref-locals'
(i == 0 ? ref b : ref c) = a;
That line of code assigns the value of a to either b or c, depending on the value of i.
Notes
1. r-value is the right-hand side of an assignment, the value that gets assigned.
2. l-value is the left-hand side of an assignment, the variable that receives the assigned value.
So I've got 3 conditions for an if-statements, and I want to ensure the first two are true before checking the third one. Here's some pseudo code demonstrating what I mean:
if (A && B && C) {
doSomething
}
Is that equivalent to:
if (A && B) {
if (C) {
doSomething
}
}
Or can we not rely on the ordering of the conditions in a short circuit evaluation?
Thanks in advance!
Yes, short circuiting respects the order and will only evaluate until it finds a failure (hence taking a short cut when it knows it can't possibly succeed any more no matter how any future check might evaluate).
If you need all 3 to be true to execute the code, then both will fit your purpose.
Literally that statement reads if A and B and C are ALL true, execute that code. If however, you want to do something where A,B are true and C is false, you'll want to use your second piece of code and add an else statement
Suppose I want to print all locations with a hard coded value, in a context were d is an M3 Declaration:
top-down visit(d) {
case \number(str numberValue) :
println("hardcode value <numberValue> encountered at <d#\src>");
}
The problem is that the location <d#\src> is too generic, it yields the entire declaration (like the whole procedure). The expression <numberValue#\src> seems more appropriate, but it is not allowed, probably because we are too low in the parse tree.
So, the question is how to get a parent E (the closest parent, like the expression itself) of <numberValue> such that <E#\src> is defined?
One option is to add an extra level in the top-down traversal:
top-down visit(d) {
case \expressionStatement(Expression stmt): {
case \number(str numberValue) :
println("hardcode value <numberValue> encountered at <stmt#\src>");
}
}
This works, but has some drawbacks:
It is ugly, in order to cover all cases we have to add many more variants;
It is very inefficient.
So, what is the proper way to get the location of a numberValue (and similar low-level constructs like stringValue's, etc)?
You should be able to do the following:
top-down visit(d) {
case n:\number(str numberValue) :
println("hardcode value <numberValue> encountered at <n#\src>");
}
Saying n:\number(str numberValue) will bind the name n to the \number node that the case matched. You can then use this in your message to get the location for n. Just make sure that n isn't already in scope. You should be able to create similar patterns for the other scenarios you mentioned as well.
We (two people) are doing a project using Lua as an embedded language.
My teammate uses argument type checks almost everywhere:
function someFunction( a, b,c )
if a == nil then return end
--Some stuff here
if type( b ) ~= "function" then
Error( "blah" )
end
--More here
if someTable[ c ] == nil then someTable[ c ] = {}
end
I don't really like that as I think that most of those checks are unneccessary.. it kind of takes the "spirit" of using Lua. It also makes the code longer, slower and less readable in my opinion.
In general I would do it this way:
function someFunction( a, b,c )
--More here
if someTable[ c ] == nil then someTable[ c ] = {}
end
I leave out almost all type/argument checks and only do those who have a high chance of actually happening.
Now we are unsure of what solution is better and decided to ask you: Security checks in Lua - yes or no?
I'm not familiar with Lua, but Wikipedia seems to think it is duck-typed. I'm going to draw an analogy with Python, so forgive me if it's not appropriate.
In Python, functions are designed with the principle that they need an object that meets certain criteria. If you pass in a different object than what the original author intended, as long as it meets the criteria of the function, it should work. The idea being, "if it looks like a duck, walks like a duck, and quacks like a duck, it is a duck." (Hence the name.) That said, there are a few rare instances where you need an object of a specific type, but this is not the general case.
At any rate, you appear to be "fighting the language", which is a code smell in my book. Most languages are designed and intended to be used in certain ways - figure out what principles and design/coding guidelines the Lua community uses, and follow those.
I type check public functions in my api. But do not for only internally used functions.
Good type checking:
function ( a , b )
assert ( type ( a ) == "number" , "Invalid a" )
b = b or {}
assert ( type ( b ) == "table" , "B must be a table" )
c = assert ( type ( c ) == "string" ) and c or "default"
end
Keep in mind though, lua also has a bit of "duck" typing: if all that is required in an object is callable, then a table with a __call method should be allowable. Same for an indexable object: a table and a userdata can both be indexed (not to mention the other types).
I don't know Lua either, and it's a little unclear whether you're asking only about checking the argument types [type(b)~="function"] or do you want to check their values too [a==nil], but here's what I do:
If the function can only ever be called by some other functions of your own, and those other functions have already checked the argument, then there's no need to check it again. On the other hand, if your design doesn't guarantee the safety of your arguments then you do need to check it yourself.
Basically, what can go wrong will go wrong, but it will wait until after you've finished testing and have shipped. You can't take chances - you do need a cast-iron guarantee. The key to choosing your guarantee is to inspect your design and see what you really need.
(Even if the function is only called by your own other functions, you might still want to include checks if you think you might later forget about all this and call it from somewhere else without checking.)
I guess it depends on what you plan to do next: if other people should actually write Lua code to work with your code, it would be useful to check arguments (or make it possible by having something like enableDebug). Something useful I came along yesterday is DecoratorsAndDocstrings which makes it possible to document/typecheck functions without altering the implementation.
Another idiom used for this purpose is :
argument = argument or default -- or
argument = type(argument)=="number" and argument or error'I should get a number'
Now we are unsure of what solution is better and decided to ask you: Security checks in Lua - yes or no?
It depends on the type of the project. If your project is small - i.e. only you and your teammate are going to manage it - it is OK to skip the checks, because you should know that you are passing to functions, and it will make the code small and readable. The downside is that when error occurs - and it may happen somewhere totally unexpected - you'll have to spend time debugging and tracing your functions.
On the other hand, if you are creating an library/API to be used by others, it is very appropriate to do type checking. For the user of your library who doesn't know of the internals, it is very helpful to know when he is passing the wrong arguments.
You need to check types where it is important (and should not check where it is not important). Usually I type-check most of the public function arguments plus I add a check anywhere I encounter a type error when debugging.
To shorten the type checks, I use a small arguments() helper function:
https://github.com/lua-nucleo/lua-nucleo/blob/master/lua-nucleo/args.lua
Example:
https://github.com/lua-aplicado/lua-aplicado/blob/master/lua-aplicado/filesystem.lua#L63-66
local write_file = function(filename, new_data)
arguments(
"string", filename,
"string", new_data
)
local file, err = io.open(filename, "w")
if not file then
return nil, err
end
file:write(new_data)
file:close()
file = nil
return true
end