Does anyone know of a truly declarative language? The behavior I'm looking for is kind of what Excel does, where I can define variables and formulas, and have the formula's result change when the input changes (without having set the answer again myself)
The behavior I'm looking for is best shown with this pseudo code:
X = 10 // define and assign two variables
Y = 20;
Z = X + Y // declare a formula that uses these two variables
X = 50 // change one of the input variables
?Z // asking for Z should now give 70 (50 + 20)
I've tried this in a lot of languages like F#, python, matlab etc, but every time I tried this they come up with 30 instead of 70. Which is correct from an imperative point of view, but I'm looking for a more declarative behavior if you know what I mean.
And this is just a very simple calculation. When things get more difficult it should handle stuff like recursion and memoization automagically.
The code below would obviously work in C# but it's just so much code for the job, I'm looking for something a bit more to the point without all that 'technical noise'
class BlaBla{
public int X {get;set;} // this used to be even worse before 3.0
public int Y {get;set;}
public int Z {get{return X + Y;}}
}
static void main(){
BlaBla bla = new BlaBla();
bla.X = 10;
bla.Y = 20;
// can't define anything here
bla.X = 50; // bit pointless here but I'll do it anyway.
Console.Writeline(bla.Z);// 70, hurray!
}
This just seems like so much code, curly braces and semicolons that add nothing.
Is there a language/ application (apart from Excel) that does this? Maybe I'm no doing it right in the mentioned languages, or I've completely missed an app that does just this.
I prototyped a language/ application that does this (along with some other stuff) and am thinking of productizing it. I just can't believe it's not there yet. Don't want to waste my time.
Any Constraint Programming system will do that for you.
Examples of CP systems that have an associated language are ECLiPSe, SICSTUS Prolog / CP package, Comet, MiniZinc, ...
It looks like you just want to make Z store a function instead of a value. In C#:
var X = 10; // define and assign two variables
var Y = 20;
Func<int> Z = () => X + Y; // declare a formula that uses these two variables
Console.WriteLine(Z());
X = 50; // change one of the input variables
Console.WriteLine(Z());
So the equivalent of your ?-prefix syntax is a ()-suffix, but otherwise it's identical. A lambda is a "formula" in your terminology.
Behind the scenes, the C# compiler builds almost exactly what you presented in your C# conceptual example: it makes X into a field in a compiler-generated class, and allocates an instance of that class when the code block is entered. So congratulations, you have re-discovered lambdas! :)
In Mathematica, you can do this:
x = 10; (* # assign 30 to the variable x *)
y = 20; (* # assign 20 to the variable y *)
z := x + y; (* # assign the expression x+y to the variable z *)
Print[z];
(* # prints 30 *)
x = 50;
Print[z];
(* # prints 70 *)
The operator := (SetDelayed) is different from = (Set). The former binds an unevaluated expression to a variable, the latter binds an evaluated expression.
Wanting to have two definitions of X is inherently imperative. In a truly declarative language you have a single definition of a variable in a single scope. The behavior you want from Excel corresponds to editing the program.
Have you seen Resolver One? It's like Excel with a real programming language behind it.
Here is Daniel's example in Python, since I noticed you said you tried it in Python.
x = 10
y = 10
z = lambda: x + y
# Output: 20
print z()
x = 20
# Output: 30
print z()
Two things you can look at are the cells lisp library, and the Modelica dynamic modelling language, both of which have relation/equation capabilities.
There is a Lisp library with this sort of behaviour:
http://common-lisp.net/project/cells/
JavaFX will do that for you if you use bind instead of = for Z
react is an OCaml frp library. Contrary to naive emulations with closures it will recalculate values only when needed
Objective Caml version 3.11.2
# #use "topfind";;
# #require "react";;
# open React;;
# let (x,setx) = S.create 10;;
val x : int React.signal = <abstr>
val setx : int -> unit = <fun>
# let (y,sety) = S.create 20;;
val y : int React.signal = <abstr>
val sety : int -> unit = <fun>
# let z = S.Int.(+) x y;;
val z : int React.signal = <abstr>
# S.value z;;
- : int = 30
# setx 50;;
- : unit = ()
# S.value z;;
- : int = 70
You can do this in Tcl, somewhat. In tcl you can set a trace on a variable such that whenever it is accessed a procedure can be invoked. That procedure can recalculate the value on the fly.
Following is a working example that does more or less what you ask:
proc main {} {
set x 10
set y 20
define z {$x + $y}
puts "z (x=$x): $z"
set x 50
puts "z (x=$x): $z"
}
proc define {name formula} {
global cache
set cache($name) $formula
uplevel trace add variable $name read compute
}
proc compute {name _ op} {
global cache
upvar $name var
if {[info exists cache($name)]} {
set expr $cache($name)
} else {
set expr $var
}
set var [uplevel expr $expr]
}
main
Groovy and the magic of closures.
def (x, y) = [ 10, 20 ]
def z = { x + y }
assert 30 == z()
x = 50
assert 70 == z()
def f = { n -> n + 1 } // define another closure
def g = { x + f(x) } // ref that closure in another
assert 101 == g() // x=50, x + (x + 1)
f = { n -> n + 5 } // redefine f()
assert 105 == g() // x=50, x + (x + 5)
It's possible to add automagic memoization to functions too but it's a lot more complex than just one or two lines. http://blog.dinkla.net/?p=10
In F#, a little verbosily:
let x = ref 10
let y = ref 20
let z () = !x + !y
z();;
y <- 40
z();;
You can mimic it in Ruby:
x = 10
y = 20
z = lambda { x + y }
z.call # => 30
z = 50
z.call # => 70
Not quite the same as what you want, but pretty close.
not sure how well metapost (1) would work for your application, but it is declarative.
Lua 5.1.4 Copyright (C) 1994-2008 Lua.org, PUC-Rio
x = 10
y = 20
z = function() return x + y; end
x = 50
= z()
70
It's not what you're looking for, but Hardware Description Languages are, by definition, "declarative".
This F# code should do the trick. You can use lazy evaluation (System.Lazy object) to ensure your expression will be evaluated when actually needed, not sooner.
let mutable x = 10;
let y = 20;
let z = lazy (x + y);
x <- 30;
printf "%d" z.Value
Related
let prog =
"""//Kernel code:
extern "C" {
#pragma pack(1)
typedef struct {
int length;
float *pointer;
} global_array_float;
__global__ void kernel_main(global_array_float x){
printf("(on device) x.length=%d\n",x.length); // prints: (on device) x.length=10
printf("(on device) x.pointer=%lld\n",x.pointer); // prints: (on device) x.pointer=0
printf("sizeof(global_array_float)=%d", sizeof(global_array_float)); // 12 bytes just as expected
}
;}"""
printfn "%s" prog
let cuda_kernel = compile_kernel prog "kernel_main"
let test_launcher(str: CudaStream, kernel: CudaKernel, x: CudaGlobalArray<float32>, o: CudaGlobalArray<float32>) =
let block_size = 1
kernel.GridDimensions <- dim3(1)
kernel.BlockDimensions <- dim3(block_size)
printfn "(on host) x.length=%i" x.length // prints: (on host) x.length=10
printfn "(on host) x.pointer=%i" x.pointer // prints: (on host) x.pointer=21535919104
let args: obj [] = [|x.length;x.pointer|]
kernel.RunAsync(str.Stream, args)
let cols, rows = 10, 1
let a = d2M.create((rows,cols))
|> fun x -> fillRandomUniformMatrix ctx.Str x 1.0f 0.0f; x
let a' = d2MtoCudaArray a
//printfn "%A" (getd2M a)
let o = d2M.create((rows,cols)) // o does nothing here as this is a minimalist example.
let o' = d2MtoCudaArray o
test_launcher(ctx.Str,cuda_kernel,a',o')
cuda_context.Synchronize()
//printfn "%A" (getd2M o)
Here is an excerpt from the main repo that I am working on currently. I am very close to having a working F# quotations to Cuda C compiler, but I can't figure out how to pass the arguments into the function properly from the host side.
Despite the pack pragma, the NVRTC 7.5 Cuda compiler is doing some other optimization and I have no idea what it is.
Because I am working off F# quotations, I need to pass the arguments as a single struct for this to work. If I change the function from kernel_main(global_array_float x) to something like kernel_main(int x_length, float *x_pointer) then it works, but I that is not the form which the quotations system gives me upfront and I would like to avoid doing extra work to make F# more like C.
Any idea what I could try?
I've made two mistaken assumptions.
First error is assuming that let args: obj [] = [|x.length;x.pointer|] would get neatly placed on stack next to each other. In actuality these are two different arguments and the second one gets lost somewhere when passed along like in the above.
It can be fixed by making a custom struct type and rewriting the expression like so: let args: obj [] = [|CudaLocalArray(x.length,x.pointer)|].
The other mistaken assumption that I found when I rewrote it like the above is that using [<StructLayout(LayoutKind.Sequential>] does not mean the fields will be packed together. Instead, like for C, pack is a argument, so it needs to be used like so: [<StructLayout(LayoutKind.Sequential,Pack=1)>].
Will somebody please explain why the initial conditions are properly taken care of in the following openmodelica model compiled and simulated in OMEdit v1.9.1 beta2 in Windows, but if line 5 is commentd and 6 uncommented (x,y) is initialized to (0.5,0)?
Thank you.
class Pendulum "Planar Pendulum"
constant Real PI = 3.141592653589793;
parameter Real m = 1,g = 9.81,L = 0.5;
Real F "Force of the Rod";
output Real x(start=L*sin(PI/4)) ,y(start=-0.35355);
//output Real x(start = L * sin(PI / 4)), y(start=-L*sin(PI/4));
output Real vx,vy;
equation
m * der(vx) = -x / L * F;
m * der(vy) = (-y / L * F) - m * g;
der(x) = vx;
der(y) = vy;
x ^ 2 + y ^ 2 = L ^ 2;
end Pendulum;
The short answer is that initial values are treated merely as hints, you have to add the fixed=true attribute to force them as in:
output Real x(start=L*cos(PI/4),fixed=true);
If initialized variables are constrained, the fixed attribute should not be used on all initialized variables but on a 'proper' subset, in this case on just one.
The long answer can be found here
I have two list like this :
def a = [100,200,300]
def b = [30,60,90]
I want the Groovier way of manipulating the a like this :
1) First element of a should be changed to a[0]-2*b[0]
2)Second element of a should be changed to a[1]-4*b[1]
3)Third element of a should be changed to a[2]-8*b[2]
(provided that both a and b will be of same length of 3)
If the list changed to map like this, lets say:
def a1 = [100:30, 200:60, 300:90]
how one could do the same above operation in this case.
Thanks in advance.
For List, I'd go with:
def result = []
a.eachWithIndex{ item, index ->
result << item - ((2**index) * b[index])
}
For Map it's a bit easier, but still requires an external state:
int i = 1
def result = a.collect { k, v -> k - ((2**i++) * v) }
A pity, Groovy doesn't have an analog for zip, in this case - something like zipWithIndex or collectWithIndex.
Using collect
In response to Victor in the comments, you can do this using a collect
def a = [100,200,300]
def b = [30,60,90]
// Introduce a list `c` of the multiplier
def c = (1..a.size()).collect { 2**it }
// Transpose these lists together, and calculate
[a,b,c].transpose().collect { x, y, z ->
x - y * z
}
Using inject
You can also use inject, passing in a map of multiplier and result, then fetching the result out at the end:
def result = [a,b].transpose().inject( [ mult:2, result:[] ] ) { acc, vals ->
acc.result << vals.with { av, bv -> av - ( acc.mult * bv ) }
acc.mult *= 2
acc
}.result
And similarly, you can use inject for the map:
def result = a1.inject( [ mult:2, result:[] ] ) { acc, key, val ->
acc.result << key - ( acc.mult * val )
acc.mult *= 2
acc
}.result
Using inject has the advantage that you don't need external variables declared, but has the disadvantage of being harder to read the code (and as Victor points out in the comments, this makes static analysis of the code hard to impossible for IDEs and groovypp)
def a1 = [100:30, 200:60, 300:90]
a1.eachWithIndex{item,index ->
println item.key-((2**(index+1))*item.value)
i++
}
I try to understand lexical-scoping. In lexical-scoping, I have this code, C like syntax:
main{
f1(){
int y = 8;
}
int y = 1; //*
f1();
}
After the execution of f1() line, will the value of y variable in main (I put * next of it) remain 1 or change to 8?
It will remain 1. You have two completely distinct variables. Changes to one do not affect the other.
Does anyone know how to make Resharper add blank lines after brackets when performing code cleanup?
I want to make the following code
if (x == y)
{
int a = x;
int b = y;
int z = a + b;
}
Look like this
if (x == y)
{
int a = x;
int b = y;
int z = a + b;
}
I have had a look through the Resharper options but can't see anything to add the extra blank lines around your block of code, although I am not sure that format makes it any more readable than without extra spaces. I would suggest asking this question on the Resharper News Group: news://news.jetbrains.com/jetbrains.resharper.community