问题
In natural languages, we would say "some color is a primary color if the color is red, blue, or yellow."
In every programming language I've seen, that translates into something like:
isPrimaryColor = someColor == "Red" or someColor == "Blue" or someColor == "Yellow"
Why isn't there a syntax that more closely matches the English sentence. After all, you wouldn't say "some color is a primary color if that color is red, or that color is blue, or that color is yellow."
I realize simply isPrimaryColor = someColor == ("Red" or "Blue" or "Yellow") because instead of Red Blue and Yellow they could be boolean statement in which case boolean logic applies, but what about something like:
isPrimaryColor = someColor ( == "Red" or == "Blue" or == "Yellow")
As an added bonus that syntax would allow for more flexibility, say you wanted to see if a number is between 1 and 100 or 1000 and 2000, you could say:
someNumber ((>= 1 and <=100) or (>=1000 and <=2000))
Edit:
Very interesting answers, and point taken that I should learn more languages. After reading through the answers I agree that for strictly equality comparison something similar to set membership is a clear and concise way of expressing the same thing (for languages that have language support for concise inline lists or sets and testing membership)
One issues that came up is that if the value to compare is the result of an expensive calculation a temporary variable would need to be (well, should be) created. The other issue is that there may be different evaluations that need to be checked, such as "the result of some expensive calculation should be prime and between 200 and 300"
These scenarios are also covered by more functional languages (though depending on the language may not be more concise), or really any language that can take a function as a parameter. For instance the previous example could be
MeetsRequirements(GetCalculatedValue(), f(x):x > 200, f(x):x < 300, IsPrime)
回答1:
In Haskell, it is easy to define a function to do this:
matches x ps = foldl (||) False $ map (\ p -> p x) ps
This function takes a value list of predicates (of type a -> Bool) and returns True if any of the the predicates match the value.
This allows you to something like this:
isMammal m = m `matches` [(=="Dog"), (=="Cat"), (=="Human")]
The nice thing is that it doesn't have to just be equality, you can use anything with the correct type:
isAnimal a = a `matches` [isMammal, (=="Fish"), (=="Bird")]
回答2:
I think that most people consider something like
isPrimaryColor = ["Red", "Blue", "Yellow"].contains(someColor)
to be sufficiently clear that they don't need extra syntax for this.
回答3:
In python you can do something like this:
color = "green"
if color in ["red", "green", "blue"]:
print 'Yay'
It is called in operator, which tests for set membership.
回答4:
In perl 6 you could do this with junctions:
if $color eq 'Red'|'Blue'|'Green' {
doit()
}
Alternately you could do it with the smart match operator (~~). The following is roughly equivalent to python's if value in list: syntax, except that ~~ does a lot more in other contexts.
if ($color ~~ qw/Red Blue Green/) {
doit()
}
The parens also make it valid perl 5 (>=5.10); in perl 6 they're optional.
回答5:
Ruby
Contained in list:
irb(main):023:0> %w{red green blue}.include? "red"
=> true
irb(main):024:0> %w{red green blue}.include? "black"
=> false
Numeric Range:
irb(main):008:0> def is_valid_num(x)
irb(main):009:1> case x
irb(main):010:2> when 1..100, 1000..2000 then true
irb(main):011:2> else false
irb(main):012:2> end
irb(main):013:1> end
=> nil
irb(main):014:0> is_valid_num(1)
=> true
irb(main):015:0> is_valid_num(100)
=> true
irb(main):016:0> is_valid_num(101)
=> false
irb(main):017:0> is_valid_num(1050)
=> true
回答6:
So far, nobody has mentioned SQL. It has what you are suggesting:
SELECT
employee_id
FROM
employee
WHERE
hire_date BETWEEN '2009-01-01' AND '2010-01-01' -- range of values
AND employment_type IN ('C', 'S', 'H', 'T') -- list of values
回答7:
COBOL uses 88 levels to implement named values, named groups of values
and named ranges of values.
For example:
01 COLOUR PIC X(10).
88 IS-PRIMARY-COLOUR VALUE 'Red', 'Blue', 'Yellow'.
...
MOVE 'Blue' TO COLOUR
IF IS-PRIMARY-COLOUR
DISPLAY 'This is a primary colour'
END-IF
Range tests are covered as follows:
01 SOME-NUMBER PIC S9(4) BINARY.
88 IS-LESS-THAN-ZERO VALUE -9999 THRU -1.
88 IS-ZERO VALUE ZERO.
88 IS-GREATER-THAN-ZERO VALUE 1 THRU 9999.
...
MOVE +358 TO SOME-NUMBER
EVALUATE TRUE
WHEN IS-LESS-THAN-ZERO
DISPLAY 'Negative Number'
WHEN IS-ZERO
DISPLAY 'Zero'
WHEN IS-GREATER-THAN-ZERO
DISPLAY 'Positive Number'
WHEN OTHER
DISPLAY 'How the heck did this happen!'
END-EVALUATE
I guess this all happened because COBOL was supposed to emulate English to some extent.
回答8:
You'll love Perl 6 because it has:
chaining comparison operators:
(1 <= $someNumber <= 100) || (1000 <= $someNumber <= 2000))junctive operators:
$isPrimaryColor = $someColor ~~ "Red" | "Blue" | "Yellow"
And you can combine both with ranges:
$someNumber ~~ (1..100) | (1000..2000)
回答9:
Python actually gives you the ability to do the last thing quite well:
>>> x=5
>>> (1<x<1000 or 2000<x<3000)
True
回答10:
In Python you can say ...
isPrimaryColor = someColor in ('Red', 'Blue', 'Yellow')
... which I find more readable than your (== "Red" or == "Blue") syntax. There's a few reasons to add syntax support for a language feature:
- Efficiency: Not a reason here, since there's no speed improvement.
- Functionality: Also not a concern; there's nothing you can do in the new syntax that you can't do in the old.
- Legibility: Most languages handle the case where you're checking the equality of multiple values just fine. In other cases (e.g.,
someNumber (> 1 and < 10)) it might be more useful, but even then it doesn't buy you much (and Python allows you to say1 < someNumber < 10, which is even clearer).
So it's not clear the proposed change is particularly helpful.
回答11:
My guess would be that languages are designed by force of habit. Early languages only would have had binary comparison operators because they are simpler to implement. Everyone got used to saying (x > 0 and x < y) until language designers didn't ever bother to support the common form in mathematics, (0 < x < y).
In most languages a comparison operator returns a boolean type. In the case of 0 < x < y, if this is interpreted as (0 < x) < y it would be meaningless, since < does not make sense for comparing booleans. Therefore, a new compiler could interpret 0 < x < y as tmp:=x, 0 < tmp && tmp < y without breaking backward compatibility. In the case of x == y == z, however, if the variables are already booleans, it is ambiguous whether this means x == y && y == z or (x == y) == z.
In C# I use the following extension method so that you can write someColor.IsOneOf("Red", "Blue", "Yellow"). It is less efficient than direct comparison (what with the array, loop, Equals() calls and boxing if T is a value type), but it sure is convenient.
public static bool IsOneOf<T>(this T value, params T[] set)
{
object value2 = value;
for (int i = 0; i < set.Length; i++)
if (set[i].Equals(value2))
return true;
return false;
}
回答12:
Icon has the facility you describe.
if y < (x | 5) then write("y=", y)
I rather like that aspect of Icon.
回答13:
In C#:
if ("A".IsIn("A", "B", "C"))
{
}
if (myColor.IsIn(colors))
{
}
Using these extensions:
public static class ObjectExtenstions
{
public static bool IsIn(this object obj, params object [] list)
{
foreach (var item in list)
{
if (obj == item)
{
return true;
}
}
return false;
}
public static bool IsIn<T>(this T obj, ICollection<T> list)
{
return list.Contains(obj);
}
public static bool IsIn<T>(this T obj, IEnumerable<T> list)
{
foreach (var item in list)
{
if (obj == item)
{
return true;
}
}
return false;
}
}
回答14:
You'll have to go a bit down the abstraction layer to find out the reason why. x86's comparison/jump instructions are binary (since they can be easily computed in a few clock cycles), and that's the way things have been.
If you want, many languages offer an abstraction for that. In PHP, for example you could use:
$isPrimaryColor = in_array($someColor, array('Red', 'White', 'Blue'));
回答15:
I don't see an Objective-C answer yet. Here is one:
BOOL isPRimaryColour = [[NSSet setWithObjects: @"red", @"green", @"blue", nil] containsObject: someColour];
回答16:
The question is reasonable, and I wouldn't regard the change as syntactic sugar. If the value being compared is the result of computation, it would be nicer to say:
if (someComplicatedExpression ?== 1 : 2 : 3 : 5)
than to say
int temp; temp = someComplicatedExpression; if (temp == 1 || temp == 2 || temp == 3 || temp == 5)
particularly if there was no other need for the temp variable in question. A modern compiler could probably recognize the short useful lifetime of 'temp' and optimize it to a register, and could probably recognize the "see if variable is one of certain constants" pattern, but there'd be no harm in allowing a programmer to save the compiler the trouble. The indicated syntax wouldn't compile on any existing compiler, but I don't think it would be any more ambiguous than (a+b >> c+d) whose behavior is defined in the language spec.
As to why nobody's done that, I don't know.
回答17:
I'm reminded of when I first started to learn programming, in Basic, and at one point I wrote
if X=3 OR 4
I intended this like you are describing, if X is either 3 or 4. The compiler interpreted it as:
if (X=3) OR (4)
That is, if X=3 is true, or if 4 is true. As it defined anything non-zero as true, 4 is true, anything OR TRUE is true, and so the expression was always true. I spent a long time figuring that one out.
I don't claim this adds anything to the discussion. I just thought it might be a mildly amusing anecdote.
回答18:
As a mathematician, I would say that the colour is primary if and only if it is a member of the set {red, green, blue} of primary colours.
And this is exactly how you could say in Delphi:
isPrimary := Colour in [clRed, clGreen, clBlue]
In fact, I employ this technique very often. Last time was three days ago. Implementing my own scripting language's interpreter, I wrote
const
LOOPS = [pntRepeat, pntDoWhile, pntFor];
and then, at a few lines,
if Nodes[x].Type in LOOPS then
The Philosophical Part of the Question
@supercat, etc. ("As to why nobody's done that, I don't know."):
Probably because the designers of programming languages are mathematicians (or, at least, mathematically inclined). If a mathematician needs to state the equality of two objects, she would say
X = Y,
naturally. But if X can be one of a number of things A, B, C, ..., then she would define a set S = {A, B, C, ...} of these things and write
X ∈ S.
Indeed, it is extremely common that you (mathematicians) write X ∈ S, where S is the set
S = {x ∈ D; P(x)}
of objects in some universe D that has the property P, instead of writing P(X). For instance, instead of saying "x is a positive real number", or "PositiveReal(x)", one would say x ∈ ℝ⁺.
回答19:
It's because programming languages are influenced by mathematics, logic and set theory in particular. Boolean algebra defines ∧, ∨ operators in a way that they do not work like spoken natural language. Your example would be written as:
Let p(x) be unary relation which holds if and only if x is a primary color
p(x) ⇔ r(x) ∨ g(x) ∨ b(x)
or
p(x) ⇔ (x=red) ∨ (x=green) ∨ (x=blue)
As you see, it's pretty similar to notation that would be used in programming language. As mathematics provide strong theoretic foundations, programming languages are based on mathematics rather than natural language which always leaves a lot of space for interpretation.
EDIT: Above statement could be simplified by using set notation:
p(x) ⇔ x ∈ {red, green, blue}
and indeed, some programming languages, most notably Pascal, included set, so you could type:
type
color = (red, green, blue, yellow, cyan, magenta, black, white);
function is_primary (x : color) : boolean;
begin
is_primary := x in [red, green, blue]
end
But sets as a language feature didn't catch on.
PS. Sorry for my imperfect English.
回答20:
The latter examples you give are effectively syntactic sugar, they'd have to evaluate to the same code as the longer form as at some point the executed code has to compare your value with each of the conditions in turn.
The array comparison syntax, given in several forms here, closer and I suspect there are other languages which get even closer.
The main problem with making syntax closer to natural language is that the latter is not just ambiguous, it's hideously ambiguous. Even keeping ambiguity to a minimum we still manage to introduce bugs into our apps, can you imagine what it would be like if you programmed in natural english?!
回答21:
Just to add to language examples
Scheme
(define (isPrimaryColor color)
(cond ((member color '(red blue yellow)) #t)
(else #f)))
(define (someNumberTest x)
(cond ((or (and (>= x 1) (<= x 100)) (and (>= x 10000 (<= x 2000))) #t)
(else #f)))
回答22:
Two possibilities
Java
boolean isPrimary = Arrays.asList("red", "blue", "yellow").contains(someColor);
Python
a = 1500
if 1 < a < 10 or 1000 < a < 2000:
print "In range"
回答23:
This can be replicated in Lua with some metatable magic :D
local function operator(func)
return setmetatable({},
{__sub = function(a, _)
return setmetatable({a},
{__sub = function(self, b)
return f(self[1], b)
end}
)
end}
)
end
local smartOr = operator(function(a, b)
for i = 1, #b do
if a == b[i] then
return true
end
end
return false
end)
local isPrimaryColor = someColor -smartOr- {"Red", "Blue", "Either"}
Note: You can change the name of -smartOr- to something like -isEither- to make it even MORE readable.
回答24:
Languages on computers compare as binary because they are all for a machine that uses binary to represent information. They were designed using similar logic and with broadly similar goals. The English language wasn't designed logically, designed to describe algorithms, and human brains (the hardware it runs on) aren't based on binary. They're tools designed for different tasks.
来源：https://stackoverflow.com/questions/3205065/why-do-most-programming-languages-only-have-binary-equality-comparison-operators