I'm pretty new with Kotlin and I'm trying to figure out Kotlin's scope functions.
My code looks like this:
with(something) {
when {
equals("test") -> var1 = "test123"
startsWith("test2") -> var2 = "test456"
contains("test3") -> myNullableVar?.let { it.var3 = "test789" }
}
}
So before I entered the third check with the .let function my with function does not need to be exhaustive (I'm not returning something, I'm only doing assignments). In my third check I'm using .let as a null-check ... but only for an assignment of it.var3 (if it is not null). I don't need to return anything while I know that Kotlin's .let function returns the result of the body by standard.
Nevertheless now my with/when needs to be exhaustive otherwise it won't compile anymore.
This got me thinking and trying out different things. I found these ways to solve this issue:
I can add an else to my with/when so it becomes exhaustive but actually I don't need an else and I don't want to use it in this case.
I can add another .let, so it looks like this: myNullableVar?.let { it.var3 = "test789" }.let{} .... but this looks kinda hacky to me. Is it supposed to work like this?
Use If(xy==null){...}else{...} stuff but I thought I can solve this with Kotlin differently
Because I'm new with Kotlin I'm not really sure how to handle this case properly. I would probably just go with my second idea because "it works". Or should I don't use .let for null-checks? Add another empty .let{}? Or did I not get the null-safety concept at all? I feel a little bit lost here. Thanks for any help.
This seems to be an unfortunate combination of features…
A when can be non-exhaustive only when it doesn't return a value. The problem is that the with() function does return a value. And since the when is at the bottom, its value is what gets returned, so in this case it must be exhaustive.
So why doesn't it insist on an else branch even if you omit the "test3" branch? That's because assignments don't yield a value. (They evaluate to Unit, which is Kotlin's special type for functions that don't return a useful value.) If every branch gives Unit, then Kotlin seems* to be happy to infer a default branch also giving Unit.
But the "test3" branch returns something else — the type of myNullableVar. So what type does the when infer? The nearest common supertype of that type and Unit, which is the top type Any?. And now it needs an explicit else branch!
So what to do?
You've found a few options, none of which is ideal. So here are a few more, ditto!
You could return an explicit Unit from that branch:
contains("test3") -> { myNullableVar?.let { it.var3 = "test789" }; Unit }
You could return an explicit Unit from the with():
contains("test3") -> myNullableVar?.let { it.var3 = "test789" }
}
Unit
}
You could give an explicit type for the with(). (It has two type parameters, so you'd need to give both, starting with the type of its parameter):
with<String, Unit>("abc") {
I haven't found a single obvious best answer, I'm afraid…
And to answer your last question: yes, ?.let{ is perfectly idiomatic and common for null checks. In this particular case, replacing it with an if happens to solve the type problem:
contains("test3") -> { if (myNullableVar != null) myNullableVar.var3 = "test789" }
But as well as being long-winded, if myNullableVar is a property and not a local variable, then it opens up a race condition (what if another thread sets it to null in between the test and the assignment?) so the compiler would complain — which is exactly why people use let instead!
(* I can't find a reference for this behaviour. Is there an official word on it?)
I'm trying to make a call to one service after checking a condition from another service in an iterative way, like so:
if (productService.isProductNotExcluded(product)){
List<Properties> properties = propertiesService.getProductDetailProperties(product)
...
}
But since isProductExcluded is returning Mono<Boolean> I'm using this approach, which seems really odd:
Flux<Properties> properties = productService.isProductNotExcluded(productId)
.filter(notExcluded -> notExcluded)
.map(ok-> propertiesService.getProductDetailProperties(product))
...
Which is the correct way to deal with this kind of situation?
For a predicate which returns a Mono<Boolean>, you can also use filterWhen which takes a publisher as a predicate. Something like this:
Flux<Properties> properties = Mono.just(productId)
.filterWhen(prodId -> productService.isProductNotExcluded(prodId))
.map(validProductId -> propertiesService.getProductDetailProperties(validProductId));
What you are doing is not odd. I personally wouldn't return a boolean in a reactive function Mono<Boolean> if I can avoid it, but it's not wrong and sometimes you don't have a choice.
I personally would have an if/else statement in the map, for clarity. I would also change the name of the function, and rewrite the isNot part.
Flux<Properties> properties = productService.isExcluded(productId)
.flatMap(isExcluded -> {
if(!isExcluded)
return propertiesService.getProductDetailProperties(product);
else
return mono.empty();
});
This is matter of opinion and coding taste, but I find this to be a lot more readable, because you can read the code and understand it straight away. But this is a personal taste.
all() operator can be used.
According to the doc. all() Emits a single boolean true if all values of this sequence match
Mono all(Predicate<? super T> predicate) {}
Is there a special name for doing assignment in a conditional? Here is an example of what I am asking about in C:
// Assume a and b have been previously defined and are compatible types
if( (a = b) ) { // <- What do you call that?
//do something
}
A couple friends of mine and I are convinced that there is a name for it, and some other people have agreed that there is one, but we can't find it anywhere. Has anyone here ever heard a term for it?
Assuming that an assignment is intentional, there is no special name for this. C language specification places a very weak requirement on the controlling expression of an if statement:
6.8.4.1-1: The controlling expression of an if statement shall have scalar type.
An assignment expression satisfies this requirement, as long as a and b are scalar. An implicit comparison to zero is performed on the result of this assignment:
6.8.4.1-2: In both forms, the first substatement is executed if the expression compares unequal to 0. In the else form, the second substatement is executed if the expression compares equal to 0.
Note that compilers would issue a warning when they see an assignment like that, because missing second = is a common source of errors. You can prevent these warnings using parentheses, as described in this Q&A.
It doesn’t really have a name, though people do call it various things. If the code follows your question:
if( a = b )...
then common terms are: bug, error, etc. However if b is not a variable but an expression, e.g. as in the common C patterns:
if( (c = getchar()) != EOF )...
while( *q++ = *p++ )...
then it might be called an idiom, pattern, etc.
I don’t know if it has a name but I’d call it “useful feature that nobody understands”.
It is very useful indeed.
We can consider it a slang.
In C++ for example you can use this by declaring a variable directly, and this is useful for safety checks:
if (Object *a = takeObject()) {
// a is not nullptr
}
Or when I don’t want to repeat a statement in a loop:
while (a = next()) {
}
Instead of:
a = next();
while (a) {
a = next();
}
But commonly these are just mistakes which compilers like gcc and clang give warnings about (and they force you to put an horrible double tuple to silence the warning ew!).
This site tickled my sense of humour - http://www.antiifcampaign.com/ but can polymorphism work in every case where you would use an if statement?
Smalltalk, which is considered as a "truly" object oriented language, has no "if" statement, and it has no "for" statement, no "while" statement. There are other examples (like Haskell) but this is a good one.
Quoting Smalltalk has no “if” statement:
Some of the audience may be thinking
that this is evidence confirming their
suspicions that Smalltalk is weird,
but what I’m going to tell you is
this:
An “if” statement is an abomination in an Object Oriented language.
Why? Well, an OO language is composed
of classes, objects and methods, and
an “if” statement is inescapably none
of those. You can’t write “if” in an
OO way. It shouldn’t exist.
Conditional execution, like everything
else, should be a method. A method of
what? Boolean.
Now, funnily enough, in Smalltalk,
Boolean has a method called
ifTrue:ifFalse: (that name will look
pretty odd now, but pass over it for
now). It’s abstract in Boolean, but
Boolean has two subclasses: True and
False. The method is passed two blocks
of code. In True, the method simply
runs the code for the true case. In
False, it runs the code for the false
case. Here’s an example that hopefully
explains:
(x >= 0) ifTrue: [
'Positive'
] ifFalse: [
'Negative'
]
You should be able to see ifTrue: and
ifFalse: in there. Don’t worry that
they’re not together.
The expression (x >= 0) evaluates to
true or false. Say it’s true, then we
have:
true ifTrue: [
'Positive'
] ifFalse: [
'Negative'
]
I hope that it’s fairly obvious that
that will produce ‘Positive’.
If it was false, we’d have:
false ifTrue: [
'Positive'
] ifFalse: [
'Negative'
]
That produces ‘Negative’.
OK, that’s how it’s done. What’s so
great about it? Well, in what other
language can you do this? More
seriously, the answer is that there
aren’t any special cases in this
language. Everything can be done in an
OO way, and everything is done in an
OO way.
I definitely recommend reading the whole post and Code is an object from the same author as well.
That website is against using if statements for checking if an object has a specific type. This is completely different from if (foo == 5). It's bad to use ifs like if (foo instanceof pickle). The alternative, using polymorphism instead, promotes encapsulation, making code infinitely easier to debug, maintain, and extend.
Being against ifs in general (doing a certain thing based on a condition) will gain you nothing. Notice how all the other answers here still make decisions, so what's really the difference?
Explanation of the why behind polymorphism:
Take this situation:
void draw(Shape s) {
if (s instanceof Rectangle)
//treat s as rectangle
if (s instanceof Circle)
//treat s as circle
}
It's much better if you don't have to worry about the specific type of an object, generalizing how objects are processed:
void draw(Shape s) {
s.draw();
}
This moves the logic of how to draw a shape into the shape class itself, so we can now treat all shapes the same. This way if we want to add a new type of shape, all we have to do is write the class and give it a draw method instead of modifying every conditional list in the whole program.
This idea is everywhere in programming today, the whole concept of interfaces is all about polymorphism. (Shape is an interface defining a certain behavior, allowing us to process any type that implements the Shape interface in our method.) Dynamic programming languages take this even further, allowing us to pass any type that supports the necessary actions into a method. Which looks better to you? (Python-style pseudo-code)
def multiply(a,b):
if (a is string and b is int):
//repeat a b times.
if (a is int and b is int):
//multiply a and b
or using polymorphism:
def multiply(a,b):
return a*b
You can now use any 2 types that support the * operator, allowing you to use the method with types that haven't event been created yet.
See polymorphism and what is polymorhism.
Though not OOP-related: In Prolog, the only way to write your whole application is without if statements.
Yes actually, you can have a turing-complete language that has no "if" per se and only allows "while" statements:
http://cseweb.ucsd.edu/classes/fa08/cse200/while.html
As for OO design, it makes sense to use an inheritance pattern rather than switches based on a type field in certain cases... That's not always feasible or necessarily desirable though.
#ennuikiller: conditionals would just be a matter of syntactic sugar:
if (test) body; is equivalent to x=test; while (x) {x=nil; body;}
if-then-else is a little more verbose:
if (test) ifBody; else elseBody;
is equivalent to
x = test; y = true;
while (x) {x = nil; y = nil; ifBody;}
while (y) {y = nil; elseBody;}
the primitive data structure is a list of lists. you could say 2 scalars are equal if they are lists of the same length. you would loop over them simultaneously using the head/tail operators and see if they stop at the same point.
of course that could all be wrapped up in macros.
The simplest turing complete language is probably iota. It contains only 2 symbols ('i' and '*').
Yep. if statements imply branches which can be very costly on a lot of modern processors - particularly PowerPC. Many modern PCs do a lot of pipeline re-ordering and so branch mis-predictions can cost an order of >30 cycles per branch miss.
On console programming it's sometimes faster to just execute the code and ignore it than check if you should execute it!
Simple branch avoidance in C:
if (++i >= 15)
{
i = 0;
)
can be re-written as
i = (i + 1) & 15;
However, if you want to see some real anti-if fu then read this
Oh and on the OOP question - I'll replace a branch mis-prediction with a virtual function call? No thanks....
The reasoning behind the "anti-if" campaign is similar to what Kent Beck said:
Good code invariably has small methods and
small objects. Only by factoring the system into many small pieces of state
and function can you hope to satisfy the “once and only once” rule. I get lots
of resistance to this idea, especially from experienced developers, but no one
thing I do to systems provides as much help as breaking it into more pieces.
If you don't know how to factor a program with composition and inheritance, then your classes and methods will tend to grow bigger over time. When you need to make a change, the easiest thing will be to add an IF somewhere. Add too many IFs, and your program will become less and less maintainable, and still the easiest thing will be to add more IFs.
You don't have to turn every IF into an object collaboration; but it's a very good thing when you know how to :-)
You can define True and False with objects (in a pseudo-python):
class True:
def if(then,else):
return then
def or(a):
return True()
def and(a):
return a
def not():
return False()
class False:
def if(then,else):
return false
def or(a):
return a
def and(a):
return False()
def not():
return True()
I think it is an elegant way to construct booleans, and it proves that you can replace every if by polymorphism, but that's not the point of the anti-if campaign. The goal is to avoid writing things such as (in a pathfinding algorithm) :
if type == Block or type == Player:
# You can't pass through this
else:
# You can
But rather call a is_traversable method on each object. In a sense, that's exactly the inverse of pattern matching. "if" is useful, but in some cases, it is not the best solution.
I assume you are actually asking about replacing if statements that check types, as opposed to replacing all if statements.
To replace an if with polymorphism requires a method in a common supertype you can use for dispatching, either by overriding it directly, or by reusing overridden methods as in the visitor pattern.
But what if there is no such method, and you can't add one to a common supertype because the super types are not maintained by you? Would you really go to the lengths of introducing a new supertype along with subtypes just to get rid of a single if? That would be taking purity a bit far in my opinion.
Also, both approaches (direct overriding and the visitor pattern) have their disadvantages: Overriding the method directly requires that you implement your method in the classes you want to switch on, which might not help cohesion. On the other hand, the visitor pattern is awkward if several cases share the same code. With an if you can do:
if (o instanceof OneType || o instanceof AnotherType) {
// complicated logic goes here
}
How would you share the code with the visitor pattern? Call a common method? Where would you put that method?
So no, I don't think replacing such if statements is always an improvement. It often is, but not always.
I used to write code a lot as the recommend in the anti-if campaign, using either callbacks in a delegate dictionary or polymorphism.
It's quite a beguiling argument, especially if you are dealing with messy code bases but to be honest, although it's great for a plugin model or simplifying large nested if statements, it does make navigating and readability a bit of a pain.
For example F12 (Go To Definition) in visual studio will take you to an abstract class (or, in my case an interface definition).
It also makes quick visual scanning of a class very cumbersome, and adds an overhead in setting up the delegates and lookup hashes.
Using the recommendations put forward in the anti-if campaign as much as they appear to be recommending looks like 'ooh, new shiny thing' programming to me.
As for the other constructs put forward in this thread, albeit it has been done in the spirit of a fun challenge, are just substitutes for an if statement, and don't really address what the underlying beliefs of the anti-if campaign.
You can avoid ifs in your business logic code if you keep them in your construction code (Factories, builders, Providers etc.). Your business logic code would be much more readable, easier to understand or easier to maintain or extend. See: http://www.youtube.com/watch?v=4F72VULWFvc
Haskell doesn't even have if statements, being pure functional. ;D
You can do it without if per se, but you can't do it without a mechanism that allows you to make a decision based on some condition.
In assembly, there's no if statement. There are conditional jumps.
In Haskell for instance, there's no explicit if, instead, you define a function multiple times, I forgot the exact syntax, but it's something like this:
pseudo-haskell:
def posNeg(x < 0):
return "negative"
def posNeg(x == 0):
return "zero"
def posNeg(x):
return "positive"
When you call posNeg(a), the interpreter will look at the value of a, if it's < 0 then it will choose the first definition, if it's == 0 then it will choose the second definition, otherwise it will default to the third definition.
So while languages like Haskell and SmallTalk don't have the usual C-style if statement, they have other means of allowing you to make decisions.
This is actually a coding game I like to play with programming languages. It's called "if we had no if" which has its origins at: http://wiki.tcl.tk/4821
Basically, if we disallow the use of conditional constructs in the language: no if, no while, no for, no unless, no switch etc.. can we recreate our own IF function. The answer depends on the language and what language features we can exploit (remember using regular conditional constructs is cheating co no ternary operators!)
For example, in tcl, a function name is just a string and any string (including the empty string) is allowed for anything (function names, variable names etc.). So, exploiting this we can do:
proc 0 {true false} {uplevel 1 $false; # execute false code block, ignore true}
proc 1 {true false} {uplevel 1 $true; # execute true code block, ignore flase}
proc _IF {boolean true false} {
$boolean $true $false
}
#usage:
_IF [expr {1<2}] {
puts "this is true"
} {
#else:
puts "this is false"
}
or in javascript we can abuse the loose typing and the fact that almost anything can be cast into a string and combine that with its functional nature:
function fail (discard,execute) {execute()}
function pass (execute,discard) {execute()}
var truth_table = {
'false' : fail,
'true' : pass
}
function _IF (expr) {
return truth_table[!!expr];
}
//usage:
_IF(3==2)(
function(){alert('this is true')},
//else
function(){alert('this is false')}
);
Not all languages can do this sort of thing. But languages I like tend to be able to.
The idea of polymorphism is to call an object without to first verify the class of that object.
That doesn't mean the if statement should not be used at all; you should avoid to write
if (object.isArray()) {
// Code to execute when the object is an array.
} else if (object.inString()) {
// Code to execute if the object is a string.
}
It depends on the language.
Statically typed languages should be able to handle all of the type checking by sharing common interfaces and overloading functions/methods.
Dynamically typed languages might need to approach the problem differently since type is not checked when a message is passed, only when an object is being accessed (more or less). Using common interfaces is still good practice and can eliminate many of the type checking if statements.
While some constructs are usually a sign of code smell, I am hesitant to eliminate any approach to a problem apriori. There may be times when type checking via if is the expedient solution.
Note: Others have suggested using switch instead, but that is just a clever way of writing more legible if statements.
Well, if you're writing in Perl, it's easy!
Instead of
if (x) {
# ...
}
you can use
unless (!x){
# ...
}
;-)
In answer to the question, and as suggested by the last respondent, you need some if statements to detect state in a factory. At that point you then instantiate a set of collaborating classes that solve the state specific problem. Of course, other conditionals would be required as needed, but they would be minimized.
What would be removed of course would be the endless procedural state checking rife in so much service based code.
Interesting smalltalk is mentioned, as that's the language I used before being dragged across into Java. I don't get home as early as I used to.
I thought about adding my two cents: you can optimize away ifs in many languages where the second part of a boolean expression is not evaluated when it won't affect the result.
With the and operator, if the first operand evaluates to false, then there is no need to evaluate the second one. With the or operator, it's the opposite - there's no need to evaluate the second operand if the first one is true. Some languages always behave like that, others offer an alternative syntax.
Here's an if - elseif - else code made in JavaScript by only using operators and anonymous functions.
document.getElementById("myinput").addEventListener("change", function(e) {
(e.target.value == 1 && !function() {
alert('if 1');
}()) || (e.target.value == 2 && !function() {
alert('else if 2');
}()) || (e.target.value == 3 && !function() {
alert('else if 3');
}()) || (function() {
alert('else');
}());
});
<input type="text" id="myinput" />
This makes me want to try defining an esoteric language where blocks implicitly behave like self-executing anonymous functions and return true, so that you would write it like this:
(condition && {
action
}) || (condition && {
action
}) || {
action
}
Which one of the following do you do:
var = true;
if (...) var = false;
Or
if (...) var = false;
else var = true;
Is there a reason you pick on or the other?
I'm working on the premise that nothing else is happening to var. The next line of code might be something like:
if (var) { ... }
How about var = { ... } directly since it's a boolean?
I prefer the second in Java, doing something like this:
int x;
if (cond) {
x = 1;
} else {
x = 5;
}
because if something is changed later (for example, I turn the else block into an else if), the compiler will tell me that the variable has failed to be initialized, which I might miss if I used your first strategy.
You could also use a ternary operator if your language supports it :)
I would generally only do the first one if there was a chance the IF could fail and the variable must have a default value if it does.
If you set the default, then you reset it again later to something else, although it's a very small amount, its still a waste of resources. So, most of the time, for most of the code, a balanced if/else or even a (?:) syntax, are clearer and more appropriate, except:
Sometimes, if what you doing is building fall-through code (or a decision function), where you start with a specific condition, and then test a whole bunch of other conditions to see if that changes, then you want to definitely set the default first:
int final = 27;
if ( some condition ) final = 86;
if ( another condition ) {
final = 98;
return final;
}
if ( some state ) {
final += 2;
}
return final;
Or something similar to that.
BTW: in your example, if you set 'var', then the next line just tests 'var', you don't really need 'var' do you? If the condition is so ugly that using 'var' helps make it readable, then your probably best to move the condition into it's own function, accepting that the extra function call is there to help readability. In general, you can waste resources, if and only if you get something significant, such as readability, in return.
Paul.
Depends on the context. I would use the second option when it is clear that 'var' needs to be true when IF fails.
I use the first type unless the value to set requires significant computation.
Always the first as many people have said. However it's worth emphasising why, and that's because it makes the program more resistant to future bugs caused by incorrect maintenance.
For example, it's quite common for some additional business condition to arise and a maintenance coder add some extra condition or two inside the if to include more business logic and incorrectly amend the code - for example
if (a==b) {
if (a==c) {
[new logic]
var=false
}
}
else {
var = false
}
On the face of it it looks unlikely, but it happens alarmingly often (in fairness often the situation arises after the original if has got a lot more complex). Putting the initialisation first prevents this.
Do you prefer code that is short and compact, or code that is easier to read?
If you prefer code that is short and compact use
var x = true;
if (...) x = false;
But this can even be "improved". Most languages give initial values, and usually for the boolean type the default is false. So, you could write
var x;
if (...) x = true;
If you prefer code that is easy to read use
if (...) var x = false;
else var x = true;
because it makes your intentions clear.
The performance of both is the same.
Depends on the language. In C++, I would highly recommend setting it to a default as quickly as possible otherwise you risk getting garbage results.
In most other languages, you can be a bit more flexible. In fact, I would argue that it's more readable to explicitly define the conditions than to set a default.
Since the variable is not written to later, for general values I would write the following in Java:
final Type var;
if (cond)
{
var = value1;
}
else
{
var = value2;
}
The Java compiler will catch the error that var is not assigned a value before it is used.
The final keyword expresses the fact that the variable is constant after the conditional.
In your exact case with booleans I would use
final boolean var = !cond;
Using a conditional in this case indicates you are afflicted by "booleanophobia".
In C I would initialize the variable at its declaration.
I generally set the "default" value and use if statements to modify it.
If no default exists then only the if statements.
int timeout = 100;
if (moreTime) timeout = 1000;
int searchOption = null;
if (sometest1) searchOption = 1;
if (sometest2) searchOption = 2;
// then later..
if (searchOption != null)
.....
If the initialization is complex enough that a direct expression can't be cleanly written, I sometimes find it useful to handle this case as
boolean isFoo = determineWhetherFoo(...);
where determineWhetherFoo takes whatever arguments are necessary to make the determination and returns the appropriate boolean result. This makes it very clear what the variable means and what it depends on. Initializing a variable to a possibly-wrong value, followed by a wad of code that may change its value, can sometimes obscure what's being expressed.
Wherever you write an if() also write the else - even if it's empty.
The compiler will optimise it away but it forces you (and any programmers after you) to think about when the if () isn't triggered, what are the consequences?