Hi everyone: Suppose I have a function "foo" that should receive two functions as parameters. If I have two lambda functions, I can call "foo" as follows:
foo (-> 1),(-> 2)
In this case, "foo" receives two functions, one that just returns 1 and another that returns 2.
However, usually lambda functions are more complicated, so putting both functions on a single line is impractical. Instead, I would like to write two multiline lambda functions. However, I can't figure out for the life of me how to accomplish this in coffeescript- Ideally, I would want to write it as follows, but it throws an error:
foo
->
1
,
->
2
The best I can come up with that works is super ugly:
foo.apply [
->
1
,
->
2
]
Can any Coffeescript guru show me how I can do this, without getting an error? Thanks!
I believe this is one situation where anonymous functions seem to not be the answer. They are very practical and idiomatic in a lot of situations but even they have limitations and can be less readable if used in extreme situations.
I would define the two functions in variables and then use them as parameters:
func1 = ->
x = 2
y = 3
z = x+y
return z+2*y
func2 = ->
a = "ok"
return a + " if you want this way"
foo func1, func2
But if you decide lambdas would be preferable, just use the parenthesis around the parameters of foo:
foo ((->
x = 2
y = 3
z = x+y
return z+2*y
),(->
a = "ok"
return a + " if you want this way"
)
)
It is not because you are using CoffeScript that you should avoid parenthesis at any cost :)
This should suffice (you could indent the second lamda if you want):
f (->
x = 1
1 + 2 * x),
->
y = 2
2 * y
given the function f:
f = (a,b) -> a() + b()
the result should give 3 + 4 = 7
Functions are implicitly called if a variable or function follows them. That's why
foo
->
2
,
->
3
won't work; the coffeescript compiler only sees a variable followed by an unexpected indent on the next line. Explicitly calling it
foo(
->
2
, ->
3
)
will work.
You can implicitly call a function with multiple paramenters, you just need to line up the comma with the beginning of the function call
foo ->
2
, ->
3
Related
Erlang functions can share the same name but have different arity. For example:
name(X) -> X.
name(X,Y) -> X * Y.
I would like to be able to return a fun from a function that behaves in the same way.
In this example I want a way to return fun/1 and fun/2 as one return, so the returned fun can be called in either way, exactly as the function example above.
function() ->
fun(X) -> X end,
fun(X,Y) -> X * Y end.
I could return a tuple {F1, F2} but that's ugly to use.
I could return a fun that takes a list as it's argument, but that's also rather ugly with calls like F([X]) or F([X, Y]).
Here you have 2 different functions with the same name:
name(X) -> X.
name(X,Y) -> X * Y.
These two functions, as anonymous functions, are: fun name/1 and fun name/2 respectively.
If you put any of them in a variable, say… F1 = fun name/1 or F2 = fun name/2, you will not be able to use those vars later interchangeably, since F1(1) will work, but F1(1,2) will fail (and viceversa with F2).
If you don't know the # of arguments you'll receive in runtime (let's say you receive a list of arguments of variable length), then you'll need to use erlang:apply/2 or erlang:apply/3 to dynamically evaluate the function. In that case, I can offer you 2 ways (depending on the version of erlang:apply that you prefer):
With apply/2:
use_name(Args) ->
Fun = function(length(Args)),
erlang:apply(Fun, Args).
function(1) -> fun(X) -> X end;
function(2) -> fun(X, Y) -> X * Y end.
With apply/3:
use_name(Args) ->
erlang:apply(?MODULE, name, Args).
name(X) -> X.
name(X,Y) -> X * Y.
Hope this helps.
fun({0, M}) -> {M+1, M-2};
fun({N, M}) ->
{A, B} = fun({N-1, M+1}),
{B, A+1}.
so I am kinda unsure of what the A and B would be and how the next recursive call would be. let say 2,2
so it would be
f(2,2) -> {A,B} = fun({1,3}), {B,A+1}
f(1,3) -> {A,B} = fun({0,4}), {B,A+1}
f(0,4) -> {5,2}
but where does A and B go and do they change in each recursive call?
As a very basic explanation of "where is my variable", consider the countdown function in this example module:
-module(example).
-export([countdown/1]).
-spec countdown(non_neg_integer()) -> ok.
countdown(0) ->
io:format("Blastoff!~n");
countdown(N) when N > 0 ->
ok = io:format("Counting down in: ~w~n", [N]),
Next = N - 1,
countdown(Next).
If we hit the base case, which is 0, then we stop. The return value of the function overall is the atom ok (because that is the return value of a successful call to io:format/2).
If the input is greater than 0 then we match on the second clause, which means we assign N the sole input argument for this particular iteration. The next thing we do is make our output call. Then we assign Next to the value N - 1. Then we call the same function again (do a loop) using the value of Next in body of the the current call as the input argument.
The next iteration all the variables are totally new because this is a fresh execution context. The old N and Next no longer exist. In fact, they don't event exist on a stack anywhere because Erlang uses "tail call optimization" to maintain recursive tail calls in constant space, the same way most other languages would do an explicit for or while or do while or [insert form].
As Alexy points out, be careful about the token fun -- it is a keyword in Erlang, not a legal function name. It is the non-name of an anonymous function (also known as a lambda). In other words, unless you provide a label, the name of every anonymous function is just fun.
fun is also the keyword that is used to reference a function by label (to use as a value itself) instead of calling it. For example, countdown(10) calls the function above with an argument of 10. Referencing the function as fun countdown/1 returns the function itself as a value. That is, incidentally, why the function export declaration at the top of the module is written as -module([countdown/1]), because that is the explicit name of this function. Consider this:
1> c(example).
{ok,example}
2> example:countdown(2).
Counting down in: 2
Counting down in: 1
Blastoff!
ok
3> Countdown = fun example:countdown/1.
#Fun<example.countdown.1>
4> Countdown(2).
Counting down in: 2
Counting down in: 1
Blastoff!
ok
While I'm on the subject...
Erlang has very few keywords compared to most languages (and very little syntax, actually). Here is the list of reserved words:
after and andalso band begin bnot bor bsl bsr bxor case catch cond div end fun if let not of or orelse receive rem try when xor
You just need to go back up:
f({1, 3}) -> {A, B} = {5, 2}, {B, A+1} -> {2, 6}
f({2, 2}) -> {A, B} = {2, 6}, {B, A+1} -> {6, 3}
(note that fun is a keyword in Erlang and that f(N,M) is not the same as f({N,M}))
and do they change in each recursive call
Yes, as you can see.
I'm learning F# and I cannot figure out what the difference between let, fun and function is, and my text book doesn't really explain that either. As an example:
let s sym = function
| V x -> Map.containsKey x sym
| A(f, es) -> Map.containsKey f sym && List.forall (s sym) es;;
Couldn't I have written this without the function keyword? Or could I have written that with fun instead of function? And why do I have to write let when I've seen some examples where you write
fun s x =
...
What's the difference really?
I guess you should really ask MSDN, but in a nutshell:
let binds a value with a symbol. The value can be a plain type like an int or a string, but it can also be a function. In FP functions are values and can be treated in the same way as those types.
fun is a keyword that introduces an anonymous function - think lambda expression if you're familiar with C#.
Those are the two important ones, in the sense that all the others usages you've seen can be thought as syntax sugar for those two. So to define a function, you can say something like this:
let myFunction =
fun firstArg secondArg ->
someOperation firstArg secondArg
And that's very clear way of saying it. You declare that you have a function and then bind it to the myFunction symbol.
But you can save yourself some typing by just conflating anonymous function declaration and binding it to a symbol with let:
let myFunction firstArg secondArg =
someOperation firstArg secondArg
What function does is a bit trickier - you combine an anonymous single-argument function declaration with a match expression, by matching on an implicit argument. So these two are equivalent:
let myFunction firstArg secondArg =
match secondArg with
| "foo" -> firstArg
| x -> x
let myFunction firstArg = function
| "foo" -> firstArg
| x -> x
If you're just starting on F#, I'd steer clear of that one. It has its uses (mainly for providing succinct higher order functions for maps/filters etc.), but results in code less readable at a glance.
These things are sort of shortcuts to each other.
The most fundamental thing is let. This keyword gives names to stuff:
let name = "stuff"
Speaking more technically, the let keyword defines an identifier and binds it to a value:
let identifier = "value"
After this, you can use words name and identifier in your program, and the compiler will know what they mean. Without the let, there wouldn't be a way to name stuff, and you'd have to always write all your stuff inline, instead of referring to chunks of it by name.
Now, values come in different flavors. There are strings "some string", there are integer numbers 42, floating point numbers 5.3, Boolean values true, and so on. One special kind of value is function. Functions are also values, in most respects similar to strings and numbers. But how do you write a function? To write a string, you use double quotes, but what about function?
Well, to write a function, you use the special word fun:
let squareFn = fun x -> x*x
Here, I used the let keyword to define an identifier squareFn, and bind that identifier to a value of the function kind. Now I can use the word squareFn in my program, and the compiler will know that whenever I use it I mean a function fun x -> x*x.
This syntax is technically sufficient, but not always convenient to write. So in order to make it shorter, the let binding takes an extra responsibility upon itself and provides a shorter way to write the above:
let squareFn x = x*x
That should do it for let vs fun.
Now, the function keyword is just a short form for fun + match. Writing function is equivalent to writing fun x -> match x with, period.
For example, the following three definitions are equivalent:
let f = fun x ->
match x with
| 0 -> "Zero"
| _ -> "Not zero"
let f x = // Using the extra convenient form of "let", as discussed above
match x with
| 0 -> "Zero"
| _ -> "Not zero"
let f = function // Using "function" instead of "fun" + "match"
| 0 -> "Zero"
| _ -> "Not zero"
cube (x,y,z) =
filter (pcubes x) cubes
cubes = [(a,b,c) | a <- [1..30],b <- [1..30],c <- [1..30]]
pcubes x (b,n,m) = (floor(sqrt(b*n)) == x)
so this code works, cubes makes a list of tuples,pcubes is used with filter to filter all the cubes in which floor(sqrt(b*n)) == x is satisfied,but the person who has modified my code wrote pcubes x in filter (pcubes x) cubes,how does this work.pcubes x makes a function that will initial the cubes x (b,n,m) that will take in a tuple and output a bool.the bool will be used in the filter function. How does this sort of manipulation happen? how does pcubes x access the (b,n,m) part of the function?
In Haskell, we don't usually use tuples (ie: (a,b,c)) to pass arguments to functions. We use currying.
Here's an example:
add a b = a + b
Here add is a function that takes a number, the returns another function that takes a number, then returns a number. We represent it's type as so:
add :: Int -> (Int -> Int)
Because of the way -> behaves, we can remove the parentheses in this case:
add :: Int -> Int -> Int
It is called like this:
(add 1) 2
but because of the way application works, we can just write:
add 1 2
Doesn't that look like our definition above, of the form add a b...?
Your function pcubes is similar. Here's how I'd write it:
pcubes x (b,n,m) = floor (sqrt (b*n)) == x
And as someone else said, it's type could be represented as:
pcubes :: Float -> (Float, Float, Float) -> Bool
When we write pcubes 1 the type becomes:
pcubes 1 :: (Float, Float, Float) -> Bool
Which, through currying, is legal, and can quite happily be used elsewhere.
I know, this is crazy black functional magic, as it was for me, but before long I guarantee you'll never want to go back: curried functions are useful.
A note on tuples: Expressions like (a,b,c) are data . They are not purely function-argument expressions. The fact that we can pull it into a function is called pattern matching, though it's not my turn to go into that.
Update 2: examples removed, because they were misleading. The ones below are more relevant.
My question:
Is there a programming language with such a construct?
Update:
Now when I think about it, Prolog has something similar.
I even allows defining operations at definition line.
(forget about backtracking and relations - think about syntax)
I asked this question because I believe, it's a nice thing to have symmetry in a language.
Symmetry between "in" parameters and "out" parameters.
If returning values like that would be easy, we could drop explicit returning in designed language.
retruning pairs ... I think this is a hack. we do not need a data structure to pass multiple parameters to a function.
Update 2:
To give an example of syntax I'm looking for:
f (s, d&) = // & indicates 'out' variable
d = s+s.
main =
f("say twice", &twice) // & indicates 'out' variable declaration
print(twice)
main2 =
print (f("say twice", _))
Or in functional + prolog style
f $s (s+s). // use $ to mark that s will get it's value in other part of the code
main =
f "say twice" $twice // on call site the second parameter will get it's value from
print twice
main2 =
print (f "Say twice" $_) // anonymous variable
In a proposed language, there are no expressions, because all returns are through parameters. This would be cumbersome in situations where deep hierarchical function calls are natural. Lisp'ish example:
(let x (* (+ 1 2) (+ 3 4))) // equivalent to C x = ((1 + 2) * (3 + 4))
would need in the language names for all temporary variables:
+ 1 2 res1
+ 3 4 res2
* res1 res2 x
So I propose anonymous variables that turn a whole function call into value of this variable:
* (+ 1 2 _) (+ 3 4 _)
This is not very natural, because all the cultural baggage we have, but I want to throw away all preconceptions about syntax we currently have.
<?php
function f($param, &$ret) {
$ret = $param . $param;
}
f("say twice", $twice);
echo $twice;
?>
$twice is seen after the call to f(), and it has the expected value. If you remove the ampersand, there are errors. So it looks like PHP will declare the variable at the point of calling. I'm not convinced that buys you much, though, especially in PHP.
"Is there a programming language with such a construct?"
Your question is in fact a little unclear.
In a sense, any language that supports assignment to [the variable state associated with] a function argument, supports "such a construct".
C supports it because "void f (type *address)" allows modification of anything address points to. Java supports it because "void f (Object x)" allows any (state-modifying) invocation of some method of x. COBOL supports it because "PROCEDURE DIVISION USING X" can involve an X that holds a pointer/memory address, ultimately allowing to go change the state of the thing pointed to by that address.
From that perspective, I'd say almost every language known to mankind supports "such a construct", with the exception perhaps of languages such as Tutorial D, which claim to be "absolutely pointer-free".
I'm having a hard time understanding what you want. You want to put the return type on call signature? I'm sure someone could hack that together but is it really useful?
// fakelang example - use a ; to separate ins and outs
function f(int in1, int in2; int out1, int out2, int out3) {...}
// C++0x-ish
auto f(int in1, int in2) -> int o1, int o2, int o3 {...}
int a, b, c;
a, b, c = f(1, 2);
I get the feeling this would be implemented internally this way:
LEA EAX, c // push output parameter pointers first, in reverse order
PUSH EAX
LEA EAX, b
PUSH EAX
LEA EAX, a
PUSH EAX
PUSH 1 // push input parameters
PUSH 2
CALL f // Caller treat the outputs as references
ADD ESP,20 // clean the stack
For your first code snippet, I'm not aware of any such languages, and frankly I'm glad it is the case. Declaring a variable in the middle of expression like that, and then using it outside said expression, looks very wrong to me. If anything, I'd expect the scope of such variable to be restricted to the function call, but then of course it's quite pointless in the first place.
For the second one - multiple return values - pretty much any language with first-class tuple support has something close to that. E.g. Python:
def foo(x, y):
return (x + 1), (y + 1)
x, y = foo(1, 2)
Lua doesn't have first-class tuples (i.e. you can't bind a tuple value to a single variable - you always have to expand it, possibly discarding part of it), but it does have multiple return values, with essentially the same syntax:
function foo(x, y)
return (x + 1), (y + 1)
end
local x, y = foo(x, y)
F# has first-class tuples, and so everything said earlier about Python applies to it as well. But it can also simulate tuple returns for methods that were declared in C# or VB with out or ref arguments, which is probably the closest to what you describe - though it is still implicit (i.e. you don't specify the out-argument at all, even as _). Example:
// C# definition
int Foo(int x, int y, out int z)
{
z = y + 1;
return x + 1;
}
// explicit F# call
let mutable y = 0
let x = Foo(1, 2, byref y);
// tupled F# call
let x, y = Foo(1, 2)
Here is how you would do it in Perl:
sub f { $_[1] = $_[0] . $_[0] } #in perl all variables are passed by reference
f("say twice", my $twice);
# or f("...", our $twice) or f("...", $twice)
# the last case is only possible if you are not running with "use strict;"
print $twice;
[edit] Also, since you seem interested in minimal syntax:
sub f { $_[1] = $_[0] x 2 } # x is the repetition operator
f "say twice" => $twice; # => is a quoting comma, used here just for clarity
print $twice;
is perfectly valid perl. Here's an example of normal quoting comma usage:
("abc", 1, "d e f", 2) # is the same as
(abc => 1, "d e f" => 2) # the => only quotes perl /\w+/ strings
Also, on return values, unless exited with a "return" early, all perl subroutines automatically return the last line they execute, be it a single value, or a list. Lastly, take a look at perl6's feed operators, which you might find interesting.
[/edit]
I am not sure exactly what you are trying to achieve with the second example, but the concept of implicit variables exists in a few languages, in Perl, it is $_.
an example would be some of perl's builtins which look at $_ when they dont have an argument.
$string = "my string\n";
for ($string) { # loads "my string" into $_
chomp; # strips the last newline from $_
s/my/our/; # substitutes my for our in $_
print; # prints $_
}
without using $_, the above code would be:
chomp $string;
$string =~ s/my/our/;
print $string;
$_ is used in many cases in perl to avoid repeatedly passing temporary variables to functions
Not programming languages, but various process calculi have syntax for binding names at the receiver call sites in the scope of process expressions dependent on them. While Pict has such syntax, it doesn't actually make sense in the derived functional syntax that you're asking about.
You might have a look at Oz. In Oz you only have procedures and you assign values to variables instead of returning them.
It looks like this:
proc {Max X Y Z}
if X >= Y then Z = X else Z = Y end
end
There are functions (that return values) but this is only syntactic sugar.
Also, Concepts, Techniques, and Models of Computer Programming is a great SICP-like book that teaches programming by using Oz and the Mozart Programming System.
I don't think so. Most languages that do something like that use Tuples so that there can be multiple return values. Come to think of it, the C-style reference and output parameters are mostly hacks around not being about to return Tuples...
Somewhat confusing, but C++ is quite happy with declaring variables and passing them as out parameters in the same statement:
void foo ( int &x, int &y, int &z ) ;
int a,b,c = (foo(a,b,c),c);
But don't do that outside of obfuscation contests.
You might also want to look at pass by name semantics in Algol, which your fuller description smells vaguely similar to.