Forgive me if this is a kind of silly question, but I've been going through "Programming Elm" and one thing struck me as a little odd: in the text he shows an example of creating a record,
dog = { name = "Tucker", age = 11 }
and then right after that he shows a function that returns a record
haveBirthday d = { name = d.name, age = d.age + 1 }
To me, the syntax for both seems remarkably similar. How does the compiler know which is which? By the + on the right hand side of the function, that implies change, so it has to be a function? By the fact that there's an argument, d? Or is it that the difference between generating a record and a function is quite obvious, and it's just in this case that they seem so alike? Or is it that in some subtle way that I don't yet have the Zen to grasp, they are in fact the same thing? (That is, something like "everything is a function"?)
I've looked at https://elm-lang.org/docs/syntax#functions -- the docs are very user-friendly, but brief. Are there any other resources that give a more didactic view of the syntax (like this book does for Haskell)?
Thanks for any help along the way.
In an imperative language where "side-effects" are the norm, the term "function" is often used to describe what's more appropriately called a procedure or sub-routine. A set of instruction to be executed when called, and where order of execution and re-evaluation is essential because mutation and other side-effects can change anything from anywhere at any time.
In functional programming, however, the notion of a function is closer to the mathematical sense of the term, where its return value is computed entirely based on its arguments. This is especially true for a "pure" functional language like Elm, which normally does not allow "side-effects". That is, effects that interact with the "outside world" without going through the input arguments or return value. In a pure functional language it does not make sense to have a function that does not take any arguments, because it would always do the same thing, and computing the same value again and again is just wasteful. A function with no arguments is effectively just a value. And a function definition and value binding can therefore be distinguished solely based on whether or not it has any arguments.
But there are also many hybrid programming languages. Most functional languages are hybrids in fact, that allow side-effects but still stick close to the mathematical sense of a function. These languages also typically don't have functions without arguments, but use a special type called unit, or (), which has only one value, also called unit or (), which is used to denote a function that takes no significant input, or which returns nothing significant. Since unit has only one value, it carries no significant information.
Many functional languages don't even have functions that take multiple arguments either. In Elm and many other languages, a function takes exactly one argument. No more and no less, ever. You might have seen Elm code which appears to have multiple arguments, but that's all an illusion. Or syntax sugar as it's called in language theoretic lingo.
When you see a function definition like this:
add a b = a + b
that actual translates to this:
add = \a -> \b -> a + b
A function that takes an argument a, then returns another function which takes an argument b, which does the actual computation and returns the result. This is called currying.
Why do this? Because it makes it very convenient to partially apply functions. You can just leave out the last, or last few, arguments, then instead of an error you get a function back which you can fully apply later to get the result. This enables you to do some really handy things.
Let's look at an example. To fully apple add from above we'd just do:
add 2 3
The compiler actually parses this as (add 2) 3, so we've kind of done partial application already, but then immediately applied to another value. But what if we want to add 2 to a whole bunch of things and don't want write add 2 everywhere, because DRY and such? We write a function:
add2ToThings thing =
add 2 thing
(Ok, maybe a little bit contrived, but stay with me)
Partial application allows us to make this even shorter!
add2ToThings =
add 2
You see how that works? add 2 returns a function, and we just give that a name. There have been numerous books written about this marvellous idea in OOP, but they call it "dependency injection" and it's usually slightly more verbose when implemented with OOP techniques.
Anyway, say we have a list of "thing"s, we can get a new list with 2 added to everything by mapping over it like this:
List.map add2ToThings things
But we can do even better! Since add 2 is actually shorter than the name we gave it, we might as well just use it directly:
List.map (add 2) things
Ok, but then say we want to filter out every value that is exactly 5. We can actually partially apply infix operators too, but we have to surround the operator in parentheses to make it behave like an ordinary function:
List.filter ((/=) 5) (List.map (add 2) things)
This is starting to look a bit convoluted though, and reads backwards since we filter after we map. Fortunately we can use Elm's pipe operator |> to clean it up a bit:
things
|> List.map (add 2)
|> List.filter ((/=) 5)
The pipe operator was "discovered" because of partial application. Without that it couldn't have been implemented as an ordinary operator, but would have to be implemented as a special syntax rule in the parser. It's implementation is (essentially) just:
x |> f = f x
It takes an arbitrary argument on its left side and a function on its right side, then applies the function to the argument. And because of partial application we can conveniently get a function to pass in on the right side.
So in three lines of ordinary idiomatic Elm code we've used partial application four times. Without that, and currying, we'd have to write something like:
List.filter (\thing -> thing /= 5) (List.map (\thing -> add 2 thing) things)
Or we might want to write it with some variable bindings to make it more readable:
let
add2ToThings thing =
add 2 thing
thingsWith2Added =
List.map add2ToThings things
thingsWith2AddedAndWithout5 =
List.filter (\thing -> thing /= 5) thingWith2Added
in
thingsWith2AddedAndWithout5
And so that's why functional programming is awesome.
Could someone help me to understand the difference between:
Mono.defer()
Mono.create()
Mono.just()
How to use it properly?
Mono.just(value) is the most primitive - once you have a value you can wrap it into a Mono and subscribers down the line will get it.
Mono.defer(monoSupplier) lets you provide the whole expression that supplies the resulting Mono instance. The evaluation of this expression is deferred until somebody subscribes. Inside of this expression you can additionally use control structures like Mono.error(throwable) to signal an error condition (you cannot do this with Mono.just).
Mono.create(monoSinkConsumer) is the most advanced method that gives you the full control over the emitted values. Instead of the need to return Mono instance from the callback (as in Mono.defer), you get control over the MonoSink<T> that lets you emit values through MonoSink.success(), MonoSink.success(value), MonoSink.error(throwable) methods.
Reactor documentation contains a few good examples of possible Mono.create use cases: link to doc.
The general advice is to use the least powerful abstraction to do the job: Mono.just -> Mono.defer -> Mono.create.
Although in general I agree with (and praise) #IlyaZinkovich's answer, I would be careful with the advice
The general advice is to use the least powerful abstraction to do the job: Mono.just -> Mono.defer -> Mono.create.
In the reactive approach, especially if we are beginners, it's very easy to overlook which the "least powerful abstraction" actually is. I am not saying anything else than #IlyaZinkovich, just depicting one detailed aspect.
Here is one specific use case where the more powerful abstraction Mono.defer() is preferable over Mono.just() but which might not be visible at the first glance.
See also:
https://stackoverflow.com/a/54412779/2886891
https://stackoverflow.com/a/57877616/2886891
We use switchIfEmpty() as a subscription-time branching:
// First ask provider1
provider1.provide1(someData)
// If provider1 did not provide the result, ask the fallback provider provider2
.switchIfEmpty(provider2.provide2(someData))
public Mono<MyResponse> provide2(MyRequest someData) {
// The Mono assembly is needed only in some corner cases
// but in fact it is always happening
return Mono.just(someData)
// expensive data processing which might even fail in the assemble time
.map(...)
.map(...)
...
}
provider2.provide2() accepts someData only when provider1.provide1() does not return any result, and/or the method assembly of the Mono returned by provider2.provide2() is expensive and even fails when called on wrong data.
It this case defer() is preferable, even if it might not be obvious at the first glance:
provider1.provide1(someData)
// ONLY IF provider1 did not provide the result, assemble another Mono with provider2.provide()
.switchIfEmpty(Mono.defer(() -> provider2.provide2(someData)))
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
}
Simple question, from a readability standpoint, which method name do you prefer for a boolean method:
public boolean isUserExist(...)
or:
public boolean doesUserExist(...)
or:
public boolean userExists(...)
public boolean userExists(...)
Would be my prefered. As it makes your conditional checks far more like natural english:
if userExists ...
But I guess there is no hard and fast rule - just be consistent
I would say userExists, because 90% of the time my calling code will look like this:
if userExists(...) {
...
}
and it reads very literally in English.
if isUserExist and if doesUserExist seem redundant.
Beware of sacrificing clarity whilst chasing readability.
Although if (user.ExistsInDatabase(db)) reads nicer than if (user.CheckExistsInDatabase(db)), consider the case of a class with a builder pattern, (or any class which you can set state on):
user.WithName("Mike").ExistsInDatabase(db).ExistsInDatabase(db2).Build();
It's not clear if ExistsInDatabase is checking whether it does exist, or setting the fact that it does exist. You wouldn't write if (user.Age()) or if (user.Name()) without any comparison value, so why is if (user.Exists()) a good idea purely because that property/function is of boolean type and you can rename the function/property to read more like natural english? Is it so bad to follow the same pattern we use for other types other than booleans?
With other types, an if statement compares the return value of a function to a value in code, so the code looks something like:
if (user.GetAge() >= 18) ...
Which reads as "if user dot get age is greater than or equal to 18..." true - it's not "natural english", but I would argue that object.verb never resembled natural english and this is simply a basic facet of modern programming (for many mainstream languages). Programmers generally don't have a problem understanding the above statement, so is the following any worse?
if (user.CheckExists() == true)
Which is normally shortened to
if (user.CheckExists())
Followed by the fatal step
if (user.Exists())
Whilst it has been said that "code is read 10x more often than written", it is also very important that bugs are easy to spot. Suppose you had a function called Exists() which causes the object to exist, and returns true/false based on success. You could easily see the code if (user.Exists()) and not spot the bug - the bug would be very much more obvious if the code read if (user.SetExists()) for example.
Additionally, user.Exists() could easily contain complex or inefficient code, round tripping to a database to check something. user.CheckExists() makes it clear that the function does something.
See also all the responses here: Naming Conventions: What to name a method that returns a boolean?
As a final note - following "Tell Don't Ask", a lot of the functions that return true/false disappear anyway, and instead of asking an object for its state, you tell it to do something, which it can do in different ways based on its state.
The goal for readability should always be to write code the closest possible to natural language. So in this case, userExists seems the best choice. Using the prefix "is" may nonetheless be right in another situations, for example isProcessingComplete.
My simple rule to this question is this:
If the boolean method already HAS a verb, don't add one. Otherwise, consider it. Some examples:
$user->exists()
$user->loggedIn()
$user->isGuest() // "is" added
I would go with userExists() because 1) it makes sense in natural language, and 2) it follows the conventions of the APIs I have seen.
To see if it make sense in natural language, read it out loud. "If user exists" sounds more like a valid English phrase than "if is user exists" or "if does user exist". "If the user exists" would be better, but "the" is probably superfluous in a method name.
To see whether a file exists in Java SE 6, you would use File.exists(). This looks like it will be the same in version 7. C# uses the same convention, as do Python and Ruby. Hopefully, this is a diverse enough collection to call this a language-agnostic answer. Generally, I would side with naming methods in keeping with your language's API.
There are things to consider that I think were missed by several other answers here
It depends if this is a C++ class method or a C function. If this is a method then it will likely be called if (user.exists()) { ... } or if (user.isExisting()) { ... }
not if (user_exists(&user)) .
This is the reason behind coding standards that state bool methods should begin with a verb since they will read like a sentence when the object is in front of them.
Unfortunately lots of old C functions return 0 for success and non-zero for failure so it can be difficult to determine the style being used unless you follow the all bool functions begin with verbs or always compare to true like so if (true == user_exists(&user))
Why not rename the property then?
if (user.isPresent()) {
Purely subjective.
I prefer userExists(...) because then statements like this read better:
if ( userExists( ... ) )
or
while ( userExists( ... ) )
In this particular case, the first example is such horrible English that it makes me wince.
I'd probably go for number three because of how it sounds when reading it in if statements. "If user exists" sounds better than "If does user exists".
This is assuming it's going to be to used in if statement tests of course...
I like any of these:
userExists(...)
isUserNameTaken(...)
User.exists(...)
User.lookup(...) != null
Method names serves for readability, only the ones fit into your whole code would be the best which most of the case it begins with conditions thus subjectPredicate follows natural sentence structure.
Since I follow the convention to put verb before function name, I would do the same here too:
//method name
public boolean doesExists(...)
//this way you can also keep a variable to store the result
bool userExists = user.doesExists()
//and use it like a english phrase
if (userExists) {...}
//or you can use the method name directly also and it will make sense here too
if (user.doesExists()) {...}