Learn X in Y minutes

/*
   Learn Chapel in Y Minutes
   
   This primer will go over basic syntax and concepts in Chapel.
   Last sync with official page: Sun, 08 Mar 2020 08:05:53 +0000
*/

// Comments are C-family style

// one line comment
/*
    multi-line comment
*/

/*
Basic printing
*/

write("Hello, ");
writeln("World!");

// ``write`` and ``writeln`` can take a list of things to print.
// Each thing is printed right next to the others, so include your spacing!
writeln("There are ", 3, " commas (\",\") in this line of code");

// Different output channels:
use IO; // Required for accessing the alternative output channels

stdout.writeln("This goes to standard output, just like plain writeln() does");
stderr.writeln("This goes to standard error");

/*
Variables
*/

// Variables don't have to be explicitly typed as long as
// the compiler can figure out the type that it will hold.
// 10 is an ``int``, so ``myVar`` is implicitly an ``int``
var myVar = 10;
myVar = -10;
var mySecondVar = myVar;
// ``var anError;`` would be a compile-time error.

// We can (and should) explicitly type things.
var myThirdVar: real;
var myFourthVar: real = -1.234;
myThirdVar = myFourthVar;

/*
Types
*/

// There are a number of basic types.
var myInt: int = -1000; // Signed ints
var myUint: uint = 1234; // Unsigned ints
var myReal: real = 9.876; // Floating point numbers
var myImag: imag = 5.0i; // Imaginary numbers
var myCplx: complex = 10 + 9i; // Complex numbers
myCplx = myInt + myImag; // Another way to form complex numbers
var myBool: bool = false; // Booleans
var myStr: string = "Some string..."; // Strings
var singleQuoteStr = 'Another string...'; // String literal with single quotes

// Some types can have sizes.
var my8Int: int(8) = 10; // 8 bit (one byte) sized int;
var my64Real: real(64) = 1.516; // 64 bit (8 bytes) sized real

// Typecasting.
var intFromReal = myReal : int;
var intFromReal2: int = myReal : int;

// Type aliasing.
type chroma = int;        // Type of a single hue
type RGBColor = 3*chroma; // Type representing a full color
var black: RGBColor = (0,0,0);
var white: RGBColor = (255, 255, 255);

/*
Constants and Parameters
*/

// A ``const`` is a constant, and cannot be changed after set in runtime.
const almostPi: real = 22.0/7.0;

// A ``param`` is a constant whose value must be known statically at
// compile-time.
param compileTimeConst: int = 16;

// The ``config`` modifier allows values to be set at the command line.
// Set with ``--varCmdLineArg=Value`` or ``--varCmdLineArg Value`` at runtime.
config var varCmdLineArg: int = -123;
config const constCmdLineArg: int = 777;

// ``config param`` can be set at compile-time.
// Set with ``--set paramCmdLineArg=value`` at compile-time.
config param paramCmdLineArg: bool = false;
writeln(varCmdLineArg, ", ", constCmdLineArg, ", ", paramCmdLineArg);

/*
References
*/

// ``ref`` operates much like a reference in C++. In Chapel, a ``ref`` cannot
// be made to alias a variable other than the variable it is initialized with.
// Here, ``refToActual`` refers to ``actual``.
var actual = 10;
ref refToActual = actual; 
writeln(actual, " == ", refToActual); // prints the same value
actual = -123; // modify actual (which refToActual refers to)
writeln(actual, " == ", refToActual); // prints the same value
refToActual = 99999999; // modify what refToActual refers to (which is actual)
writeln(actual, " == ", refToActual); // prints the same value

/*
Operators
*/

// Math operators:
var a: int, thisInt = 1234, thatInt = 5678;
a = thisInt + thatInt;  // Addition
a = thisInt * thatInt;  // Multiplication
a = thisInt - thatInt;  // Subtraction
a = thisInt / thatInt;  // Division
a = thisInt ** thatInt; // Exponentiation
a = thisInt % thatInt;  // Remainder (modulo)

// Logical operators:
var b: bool, thisBool = false, thatBool = true;
b = thisBool && thatBool; // Logical and
b = thisBool || thatBool; // Logical or
b = !thisBool;            // Logical negation

// Relational operators:
b = thisInt > thatInt;           // Greater-than
b = thisInt >= thatInt;          // Greater-than-or-equal-to
b = thisInt < a && a <= thatInt; // Less-than, and, less-than-or-equal-to
b = thisInt != thatInt;          // Not-equal-to
b = thisInt == thatInt;          // Equal-to

// Bitwise operators:
a = thisInt << 10;     // Left-bit-shift by 10 bits;
a = thatInt >> 5;      // Right-bit-shift by 5 bits;
a = ~thisInt;          // Bitwise-negation
a = thisInt ^ thatInt; // Bitwise exclusive-or

// Compound assignment operators:
a += thisInt;          // Addition-equals (a = a + thisInt;)
a *= thatInt;          // Times-equals (a = a * thatInt;)
b &&= thatBool;        // Logical-and-equals (b = b && thatBool;)
a <<= 3;               // Left-bit-shift-equals (a = a << 10;)

// Unlike other C family languages, there are no
// pre/post-increment/decrement operators, such as:
//
// ``++j``, ``--j``, ``j++``, ``j--``

// Swap operator:
var old_this = thisInt;
var old_that = thatInt;
thisInt <=> thatInt; // Swap the values of thisInt and thatInt
writeln((old_this == thatInt) && (old_that == thisInt));

// Operator overloads can also be defined, as we'll see with procedures.

/*
Tuples
*/

// Tuples can be of the same type or different types.
var sameTup: 2*int = (10, -1);
var sameTup2 = (11, -6);
var diffTup: (int,real,complex) = (5, 1.928, myCplx);
var diffTupe2 = (7, 5.64, 6.0+1.5i);

// Tuples can be accessed using square brackets or parentheses, and are
// 1-indexed.
writeln("(", sameTup[1], ",", sameTup(2), ")");
writeln(diffTup);

// Tuples can also be written into.
diffTup(1) = -1;

// Tuple values can be expanded into their own variables.
var (tupInt, tupReal, tupCplx) = diffTup;
writeln(diffTup == (tupInt, tupReal, tupCplx));

// They are also useful for writing a list of variables, as is common in debugging.
writeln((a,b,thisInt,thatInt,thisBool,thatBool));

/*
Control Flow
*/

// ``if`` - ``then`` - ``else`` works just like any other C-family language.
if 10 < 100 then
  writeln("All is well");

if -1 < 1 then
  writeln("Continuing to believe reality");
else
  writeln("Send mathematician, something's wrong");

// You can use parentheses if you prefer.
if (10 > 100) {
  writeln("Universe broken. Please reboot universe.");
}

if a % 2 == 0 {
  writeln(a, " is even.");
} else {
  writeln(a, " is odd.");
}

if a % 3 == 0 {
  writeln(a, " is even divisible by 3.");
} else if a % 3 == 1 {
  writeln(a, " is divided by 3 with a remainder of 1.");
} else {
  writeln(b, " is divided by 3 with a remainder of 2.");
}

// Ternary: ``if`` - ``then`` - ``else`` in a statement.
var maximum = if thisInt < thatInt then thatInt else thisInt;

// ``select`` statements are much like switch statements in other languages.
// However, ``select`` statements don't cascade like in C or Java.
var inputOption = "anOption";
select inputOption {
  when "anOption" do writeln("Chose 'anOption'");
  when "otherOption" {
    writeln("Chose 'otherOption'");
    writeln("Which has a body");
  }
  otherwise {
    writeln("Any other Input");
    writeln("the otherwise case doesn't need a do if the body is one line");
  }
}

// ``while`` and ``do``-``while`` loops also behave like their C counterparts.
var j: int = 1;
var jSum: int = 0;
while (j <= 1000) {
  jSum += j;
  j += 1;
}
writeln(jSum);

do {
  jSum += j;
  j += 1;
} while (j <= 10000);
writeln(jSum);

// ``for`` loops are much like those in python in that they iterate over a
// range. Ranges (like the ``1..10`` expression below) are a first-class object
// in Chapel, and as such can be stored in variables.
for i in 1..10 do write(i, ", ");
writeln();

var iSum: int = 0;
for i in 1..1000 {
  iSum += i;
}
writeln(iSum);

for x in 1..10 {
  for y in 1..10 {
    write((x,y), "\t");
  }
  writeln();
}

/*
Ranges and Domains
*/

// For-loops and arrays both use ranges and domains to define an index set that
// can be iterated over. Ranges are single dimensional integer indices, while
// domains can be multi-dimensional and represent indices of different types.

// They are first-class citizen types, and can be assigned into variables.
var range1to10: range = 1..10;  // 1, 2, 3, ..., 10
var range2to11 = 2..11; // 2, 3, 4, ..., 11
var rangeThisToThat: range = thisInt..thatInt; // using variables
var rangeEmpty: range = 100..-100; // this is valid but contains no indices

// Ranges can be unbounded.
var range1toInf: range(boundedType=BoundedRangeType.boundedLow) = 1.. ; // 1, 2, 3, 4, 5, ...
var rangeNegInfTo1 = ..1; // ..., -4, -3, -2, -1, 0, 1

// Ranges can be strided (and reversed) using the ``by`` operator.
var range2to10by2: range(stridable=true) = 2..10 by 2; // 2, 4, 6, 8, 10
var reverse2to10by2 = 2..10 by -2; // 10, 8, 6, 4, 2

var trapRange = 10..1 by -1; // Do not be fooled, this is still an empty range
writeln("Size of range '", trapRange, "' = ", trapRange.size);

// Note: ``range(boundedType= ...)`` and ``range(stridable= ...)`` are only
// necessary if we explicitly type the variable.

// The end point of a range can be computed by specifying the total size
// of the range using the count (``#``) operator.
var rangeCount: range = -5..#12; // range from -5 to 6

// Operators can be mixed.
var rangeCountBy: range(stridable=true) = -5..#12 by 2; // -5, -3, -1, 1, 3, 5
writeln(rangeCountBy);

// Properties of the range can be queried.
// In this example, printing the first index, last index, number of indices,
// stride, and if 2 is include in the range.
writeln((rangeCountBy.first, rangeCountBy.last, rangeCountBy.size,
           rangeCountBy.stride, rangeCountBy.contains(2)));

for i in rangeCountBy {
  write(i, if i == rangeCountBy.last then "\n" else ", ");
}

// Rectangular domains are defined using the same range syntax,
// but they are required to be bounded (unlike ranges).
var domain1to10: domain(1) = {1..10};        // 1D domain from 1..10;
var twoDimensions: domain(2) = {-2..2,0..2}; // 2D domain over product of ranges
var thirdDim: range = 1..16;
var threeDims: domain(3) = {thirdDim, 1..10, 5..10}; // using a range variable

// Domains can also be resized
var resizedDom = {1..10};
writeln("before, resizedDom = ", resizedDom);
resizedDom = {-10..#10};
writeln("after, resizedDom = ", resizedDom);

// Indices can be iterated over as tuples.
for idx in twoDimensions do
  write(idx, ", ");
writeln();

// These tuples can also be destructured.
for (x,y) in twoDimensions {
  write("(", x, ", ", y, ")", ", ");
}
writeln();

// Associative domains act like sets.
var stringSet: domain(string); // empty set of strings
stringSet += "a";
stringSet += "b";
stringSet += "c";
stringSet += "a"; // Redundant add "a"
stringSet -= "c"; // Remove "c"
writeln(stringSet.sorted());

// Associative domains can also have a literal syntax
var intSet = {1, 2, 4, 5, 100};

// Both ranges and domains can be sliced to produce a range or domain with the
// intersection of indices.
var rangeA = 1.. ; // range from 1 to infinity
var rangeB =  ..5; // range from negative infinity to 5
var rangeC = rangeA[rangeB]; // resulting range is 1..5
writeln((rangeA, rangeB, rangeC));

var domainA = {1..10, 5..20};
var domainB = {-5..5, 1..10};
var domainC = domainA[domainB];
writeln((domainA, domainB, domainC));

/*
Arrays
*/

// Arrays are similar to those of other languages.
// Their sizes are defined using domains that represent their indices.
var intArray: [1..10] int;
var intArray2: [{1..10}] int; // equivalent

// They can be accessed using either brackets or parentheses
for i in 1..10 do
  intArray[i] = -i;
writeln(intArray);

// We cannot access ``intArray[0]`` because it exists outside
// of the index set, ``{1..10}``, we defined it to have.
// ``intArray[11]`` is illegal for the same reason.
var realDomain: domain(2) = {1..5,1..7};
var realArray: [realDomain] real;
var realArray2: [1..5,1..7] real;   // equivalent
var realArray3: [{1..5,1..7}] real; // equivalent

for i in 1..5 {
  for j in realDomain.dim(2) {   // Only use the 2nd dimension of the domain
    realArray[i,j] = -1.61803 * i + 0.5 * j;  // Access using index list
    var idx: 2*int = (i,j);                   // Note: 'index' is a keyword
    realArray[idx] = - realArray[(i,j)];      // Index using tuples
  }
}

// Arrays have domains as members, and can be iterated over as normal.
for idx in realArray.domain {  // Again, idx is a 2*int tuple
  realArray[idx] = 1 / realArray[idx[1], idx[2]]; // Access by tuple and list
}

writeln(realArray);

// The values of an array can also be iterated directly.
var rSum: real = 0;
for value in realArray {
  rSum += value; // Read a value
  value = rSum;  // Write a value
}
writeln(rSum, "\n", realArray);

// Associative arrays (dictionaries) can be created using associative domains.
var dictDomain: domain(string) = { "one", "two", "three"};
var dict: [dictDomain] int = ["one" => 1, "two" => 2, "three" => 3];

for key in dictDomain.sorted() do
  writeln(dict[key]);

// Arrays can be assigned to each other in a few different ways.
// These arrays will be used in the example.
var thisArray : [0..5] int = [0,1,2,3,4,5];
var thatArray : [0..5] int;

// First, simply assign one to the other. This copies ``thisArray`` into
// ``thatArray``, instead of just creating a reference. Therefore, modifying
// ``thisArray`` does not also modify ``thatArray``.

thatArray = thisArray;
thatArray[1] = -1;
writeln((thisArray, thatArray));

// Assign a slice from one array to a slice (of the same size) in the other.
thatArray[4..5] = thisArray[1..2];
writeln((thisArray, thatArray));

// Operations can also be promoted to work on arrays. 'thisPlusThat' is also
// an array.
var thisPlusThat = thisArray + thatArray;
writeln(thisPlusThat);

// Moving on, arrays and loops can also be expressions, where the loop
// body's expression is the result of each iteration.
var arrayFromLoop = for i in 1..10 do i;
writeln(arrayFromLoop);

// An expression can result in nothing, such as when filtering with an if-expression.
var evensOrFives = for i in 1..10 do if (i % 2 == 0 || i % 5 == 0) then i;

writeln(arrayFromLoop);

// Array expressions can also be written with a bracket notation.
// Note: this syntax uses the ``forall`` parallel concept discussed later.
var evensOrFivesAgain = [i in 1..10] if (i % 2 == 0 || i % 5 == 0) then i;

// They can also be written over the values of the array.
arrayFromLoop = [value in arrayFromLoop] value + 1;


/*
Procedures
*/

// Chapel procedures have similar syntax functions in other languages. 
proc fibonacci(n : int) : int {
  if n <= 1 then return n;
  return fibonacci(n-1) + fibonacci(n-2);
}

// Input parameters can be untyped to create a generic procedure.
proc doublePrint(thing): void {
  write(thing, " ", thing, "\n");
}

// The return type can be inferred, as long as the compiler can figure it out.
proc addThree(n) {
  return n + 3;
}

doublePrint(addThree(fibonacci(20)));

// It is also possible to take a variable number of parameters.
proc maxOf(x ...?k) {
  // x refers to a tuple of one type, with k elements
  var maximum = x[1];
  for i in 2..k do maximum = if maximum < x[i] then x[i] else maximum;
  return maximum;
}
writeln(maxOf(1, -10, 189, -9071982, 5, 17, 20001, 42));

// Procedures can have default parameter values, and
// the parameters can be named in the call, even out of order.
proc defaultsProc(x: int, y: real = 1.2634): (int,real) {
  return (x,y);
}

writeln(defaultsProc(10));
writeln(defaultsProc(x=11));
writeln(defaultsProc(x=12, y=5.432));
writeln(defaultsProc(y=9.876, x=13));

// The ``?`` operator is called the query operator, and is used to take
// undetermined values like tuple or array sizes and generic types.
// For example, taking arrays as parameters. The query operator is used to
// determine the domain of ``A``. This is useful for defining the return type,
// though it's not required.
proc invertArray(A: [?D] int): [D] int{
  for a in A do a = -a;
  return A;
}

writeln(invertArray(intArray));

// We can query the type of arguments to generic procedures.
// Here we define a procedure that takes two arguments of
// the same type, yet we don't define what that type is.
proc genericProc(arg1 : ?valueType, arg2 : valueType): void {
  select(valueType) {
    when int do writeln(arg1, " and ", arg2, " are ints");
    when real do writeln(arg1, " and ", arg2, " are reals");
    otherwise writeln(arg1, " and ", arg2, " are somethings!");
  }
}

genericProc(1, 2);
genericProc(1.2, 2.3);
genericProc(1.0+2.0i, 3.0+4.0i);

// We can also enforce a form of polymorphism with the ``where`` clause
// This allows the compiler to decide which function to use.
// Note: That means that all information needs to be known at compile-time.
// The param modifier on the arg is used to enforce this constraint.
proc whereProc(param N : int): void
 where (N > 0) {
  writeln("N is greater than 0");
}

proc whereProc(param N : int): void
 where (N < 0) {
  writeln("N is less than 0");
}

whereProc(10);
whereProc(-1);

// ``whereProc(0)`` would result in a compiler error because there
// are no functions that satisfy the ``where`` clause's condition.
// We could have defined a ``whereProc`` without a ``where`` clause
// that would then have served as a catch all for all the other cases
// (of which there is only one).

// ``where`` clauses can also be used to constrain based on argument type.
proc whereType(x: ?t) where t == int {
  writeln("Inside 'int' version of 'whereType': ", x);
}

proc whereType(x: ?t) {
  writeln("Inside general version of 'whereType': ", x);
}

whereType(42);
whereType("hello");

/*
Intents
*/

/* Intent modifiers on the arguments convey how those arguments are passed to the procedure.

     * in: copy arg in, but not out
     * out: copy arg out, but not in
     * inout: copy arg in, copy arg out
     * ref: pass arg by reference
*/
proc intentsProc(in inarg, out outarg, inout inoutarg, ref refarg) {
  writeln("Inside Before: ", (inarg, outarg, inoutarg, refarg));
  inarg = inarg + 100;
  outarg = outarg + 100;
  inoutarg = inoutarg + 100;
  refarg = refarg + 100;
  writeln("Inside After: ", (inarg, outarg, inoutarg, refarg));
}

var inVar: int = 1;
var outVar: int = 2;
var inoutVar: int = 3;
var refVar: int = 4;
writeln("Outside Before: ", (inVar, outVar, inoutVar, refVar));
intentsProc(inVar, outVar, inoutVar, refVar);
writeln("Outside After: ", (inVar, outVar, inoutVar, refVar));

// Similarly, we can define intents on the return type.
// ``refElement`` returns a reference to an element of array.
// This makes more practical sense for class methods where references to
// elements in a data-structure are returned via a method or iterator.
proc refElement(array : [?D] ?T, idx) ref : T {
  return array[idx];
}

var myChangingArray : [1..5] int = [1,2,3,4,5];
writeln(myChangingArray);
ref refToElem = refElement(myChangingArray, 5); // store reference to element in ref variable
writeln(refToElem);
refToElem = -2; // modify reference which modifies actual value in array
writeln(refToElem);
writeln(myChangingArray);

/*
Operator Definitions
*/

// Chapel allows for operators to be overloaded.
// We can define the unary operators:
// ``+ - ! ~``
// and the binary operators:
// ``+ - * / % ** == <= >= < > << >> & | ˆ by``
// ``+= -= *= /= %= **= &= |= ˆ= <<= >>= <=>``

// Boolean exclusive or operator.
proc ^(left : bool, right : bool): bool {
  return (left || right) && !(left && right);
}

writeln(true  ^ true);
writeln(false ^ true);
writeln(true  ^ false);
writeln(false ^ false);

// Define a ``*`` operator on any two types that returns a tuple of those types.
proc *(left : ?ltype, right : ?rtype): (ltype, rtype) {
  writeln("\tIn our '*' overload!");
  return (left, right);
}

writeln(1 * "a"); // Uses our ``*`` operator.
writeln(1 * 2);   // Uses the default ``*`` operator.

//  Note: You could break everything if you get careless with your overloads.
//  This here will break everything. Don't do it.

/* 

      proc +(left: int, right: int): int {
        return left - right;
      }
*/

/*
Iterators
*/

// Iterators are sisters to the procedure, and almost everything about
// procedures also applies to iterators. However, instead of returning a single
// value, iterators may yield multiple values to a loop.
//
// This is useful when a complicated set or order of iterations is needed, as
// it allows the code defining the iterations to be separate from the loop
// body.
iter oddsThenEvens(N: int): int {
  for i in 1..N by 2 do
    yield i; // yield values instead of returning.
  for i in 2..N by 2 do
    yield i;
}

for i in oddsThenEvens(10) do write(i, ", ");
writeln();

// Iterators can also yield conditionally, the result of which can be nothing
iter absolutelyNothing(N): int {
  for i in 1..N {
    if N < i { // Always false
      yield i;     // Yield statement never happens
    }
  }
}

for i in absolutelyNothing(10) {
  writeln("Woa there! absolutelyNothing yielded ", i);
}

// We can zipper together two or more iterators (who have the same number
// of iterations) using ``zip()`` to create a single zipped iterator, where each
// iteration of the zipped iterator yields a tuple of one value yielded
// from each iterator.
for (positive, negative) in zip(1..5, -5..-1) do
  writeln((positive, negative));

// Zipper iteration is quite important in the assignment of arrays,
// slices of arrays, and array/loop expressions.
var fromThatArray : [1..#5] int = [1,2,3,4,5];
var toThisArray : [100..#5] int;

// Some zipper operations implement other operations.
// The first statement and the loop are equivalent.
toThisArray = fromThatArray;
for (i,j) in zip(toThisArray.domain, fromThatArray.domain) {
  toThisArray[i] = fromThatArray[j];
}

// These two chunks are also equivalent.
toThisArray = [j in -100..#5] j;
writeln(toThisArray);

for (i, j) in zip(toThisArray.domain, -100..#5) {
  toThisArray[i] = j;
}
writeln(toThisArray);

// This is very important in understanding why this statement exhibits a runtime error.

/* 
      var iterArray : [1..10] int = [i in 1..10] if (i % 2 == 1) then i;
*/

// Even though the domain of the array and the loop-expression are
// the same size, the body of the expression can be thought of as an iterator.
// Because iterators can yield nothing, that iterator yields a different number
// of things than the domain of the array or loop, which is not allowed.

/*
Classes
*/
// Classes are similar to those in C++ and Java, allocated on the heap.
class MyClass {

// Member variables
  var memberInt : int;
  var memberBool : bool = true;

// By default, any class that doesn't define an initializer gets a
// compiler-generated initializer, with one argument per field and
// the field's initial value as the argument's default value.
// Alternatively, the user can define initializers manually as shown
// in the following commented-out routine:
//
/*       // proc init(val : real) {
      //   this.memberInt = ceil(val): int;
      // }
*/

// Explicitly defined deinitializer.
// If we did not write one, we would get the compiler-generated deinitializer,
// which has an empty body.
  proc deinit() {
    writeln("MyClass deinitializer called ", (this.memberInt, this.memberBool));
  }

// Class methods.
  proc setMemberInt(val: int) {
    this.memberInt = val;
  }

  proc setMemberBool(val: bool) {
    this.memberBool = val;
  }

  proc getMemberInt(): int{
    return this.memberInt;
  }

  proc getMemberBool(): bool {
    return this.memberBool;
  }
} // end MyClass

// Call compiler-generated initializer, using default value for memberBool.
{
  var myObject = new owned MyClass(10);
      myObject = new owned MyClass(memberInt = 10); // Equivalent
  writeln(myObject.getMemberInt());

  // Same, but provide a memberBool value explicitly.
  var myDiffObject = new owned MyClass(-1, true);
      myDiffObject = new owned MyClass(memberInt = -1,
                                       memberBool = true); // Equivalent
  writeln(myDiffObject);

  // Similar, but rely on the default value of memberInt, passing in memberBool.
  var myThirdObject = new owned MyClass(memberBool = true);
  writeln(myThirdObject);

  // If the user-defined initializer above had been uncommented, we could
  // make the following calls:
  //
  /*         // var myOtherObject = new MyClass(1.95);
        //     myOtherObject = new MyClass(val = 1.95);
        // writeln(myOtherObject.getMemberInt());
  */

  // We can define an operator on our class as well, but
  // the definition has to be outside the class definition.
  proc +(A : MyClass, B : MyClass) : owned MyClass {
    return
      new owned MyClass(memberInt = A.getMemberInt() + B.getMemberInt(),
                        memberBool = A.getMemberBool() || B.getMemberBool());
  }

  var plusObject = myObject + myDiffObject;
  writeln(plusObject);

  // Destruction of an object: calls the deinit() routine and frees its memory.
  // ``unmanaged`` variables should have ``delete`` called on them.
  // ``owned`` variables are destroyed when they go out of scope.
}

// Classes can inherit from one or more parent classes
class MyChildClass : MyClass {
  var memberComplex: complex;
}

// Here's an example of generic classes.
class GenericClass {
  type classType;
  var classDomain: domain(1);
  var classArray: [classDomain] classType;

// Explicit initializer.
  proc init(type classType, elements : int) {
    this.classType = classType;
    this.classDomain = {1..elements};
    // all generic and const fields must be initialized in "phase 1" prior
    // to a call to the superclass initializer.
  }

// Copy-style initializer.
// Note: We include a type argument whose default is the type of the first
// argument.  This lets our initializer copy classes of different
// types and cast on the fly.
  proc init(other : GenericClass(?),
            type classType = other.classType) {
    this.classType = classType;
    this.classDomain = other.classDomain;
    this.classArray = for o in other do o: classType;  // copy and cast
  }

// Define bracket notation on a GenericClass
// object so it can behave like a normal array
// i.e. ``objVar[i]`` or ``objVar(i)``
  proc this(i : int) ref : classType {
    return this.classArray[i];
  }

// Define an implicit iterator for the class
// to yield values from the array to a loop
// i.e. ``for i in objVar do ...``
  iter these() ref : classType {
    for i in this.classDomain do
      yield this[i];
  }
} // end GenericClass

// Allocate an owned instance of our class
var realList = new owned GenericClass(real, 10);

// We can assign to the member array of the object using the bracket
// notation that we defined.
for i in realList.classDomain do realList[i] = i + 1.0;

// We can iterate over the values in our list with the iterator
// we defined.
for value in realList do write(value, ", ");
writeln();

// Make a copy of realList using the copy initializer.
var copyList = new owned GenericClass(realList);
for value in copyList do write(value, ", ");
writeln();

// Make a copy of realList and change the type, also using the copy initializer.
var copyNewTypeList = new owned GenericClass(realList, int);
for value in copyNewTypeList do write(value, ", ");
writeln();


/*
Modules
*/

// Modules are Chapel's way of managing name spaces.
// The files containing these modules do not need to be named after the modules
// (as in Java), but files implicitly name modules.
// For example, this file implicitly names the ``learnChapelInYMinutes`` module

module OurModule {

// We can use modules inside of other modules.
// Time is one of the standard modules.
  use Time;

// We'll use this procedure in the parallelism section.
  proc countdown(seconds: int) {
    for i in 1..seconds by -1 {
      writeln(i);
      sleep(1);
    }
  }

// It is possible to create arbitrarily deep module nests.
// i.e. submodules of OurModule
  module ChildModule {
    proc foo() {
      writeln("ChildModule.foo()");
    }
  }

  module SiblingModule {
    proc foo() {
      writeln("SiblingModule.foo()");
    }
  }
} // end OurModule

// Using ``OurModule`` also uses all the modules it uses.
// Since ``OurModule`` uses ``Time``, we also use ``Time``.
use OurModule;

// At this point we have not used ``ChildModule`` or ``SiblingModule`` so
// their symbols (i.e. ``foo``) are not available to us. However, the module
// names are available, and we can explicitly call ``foo()`` through them.
SiblingModule.foo();
OurModule.ChildModule.foo();

// Now we use ``ChildModule``, enabling unqualified calls.
use ChildModule;
foo();

/*
Parallelism
*/

// In other languages, parallelism is typically done with
// complicated libraries and strange class structure hierarchies.
// Chapel has it baked right into the language.

// We can declare a main procedure, but all the code above main still gets
// executed.
proc main() {

// A ``begin`` statement will spin the body of that statement off
// into one new task.
// A ``sync`` statement will ensure that the progress of the main
// task will not progress until the children have synced back up.

  sync {
    begin { // Start of new task's body
      var a = 0;
      for i in 1..1000 do a += 1;
      writeln("Done: ", a);
    } // End of new tasks body
    writeln("spun off a task!");
  }
  writeln("Back together");

  proc printFibb(n: int) {
    writeln("fibonacci(",n,") = ", fibonacci(n));
  }

// A ``cobegin`` statement will spin each statement of the body into one new
// task. Notice here that the prints from each statement may happen in any
// order.
  cobegin {
    printFibb(20); // new task
    printFibb(10); // new task
    printFibb(5);  // new task
    {
      // This is a nested statement body and thus is a single statement
      // to the parent statement, executed by a single task.
      writeln("this gets");
      writeln("executed as");
      writeln("a whole");
    }
  }

// A ``coforall`` loop will create a new task for EACH iteration.
// Again we see that prints happen in any order.
// NOTE: ``coforall`` should be used only for creating tasks!
// Using it to iterating over a structure is very a bad idea!
  var num_tasks = 10; // Number of tasks we want
  coforall taskID in 1..num_tasks {
    writeln("Hello from task# ", taskID);
  }

// ``forall`` loops are another parallel loop, but only create a smaller number
// of tasks, specifically ``--dataParTasksPerLocale=`` number of tasks.
  forall i in 1..100 {
    write(i, ", ");
  }
  writeln();

// Here we see that there are sections that are in order, followed by
// a section that would not follow (e.g. 1, 2, 3, 7, 8, 9, 4, 5, 6,).
// This is because each task is taking on a chunk of the range 1..10
// (1..3, 4..6, or 7..9) doing that chunk serially, but each task happens
// in parallel. Your results may depend on your machine and configuration

// For both the ``forall`` and ``coforall`` loops, the execution of the
// parent task will not continue until all the children sync up.

// ``forall`` loops are particularly useful for parallel iteration over arrays.
// Lets run an experiment to see how much faster a parallel loop is
  use Time; // Import the Time module to use Timer objects
  var timer: Timer;
  var myBigArray: [{1..4000,1..4000}] real; // Large array we will write into

// Serial Experiment:
  timer.start(); // Start timer
  for (x,y) in myBigArray.domain { // Serial iteration
    myBigArray[x,y] = (x:real) / (y:real);
  }
  timer.stop(); // Stop timer
  writeln("Serial: ", timer.elapsed()); // Print elapsed time
  timer.clear(); // Clear timer for parallel loop

// Parallel Experiment:
  timer.start(); // start timer
  forall (x,y) in myBigArray.domain { // Parallel iteration
    myBigArray[x,y] = (x:real) / (y:real);
  }
  timer.stop(); // Stop timer
  writeln("Parallel: ", timer.elapsed()); // Print elapsed time
  timer.clear();

// You may have noticed that (depending on how many cores you have)
// the parallel loop went faster than the serial loop.

// The bracket style loop-expression described
// much earlier implicitly uses a ``forall`` loop.
  [val in myBigArray] val = 1 / val; // Parallel operation

// Atomic variables, common to many languages, are ones whose operations
// occur uninterrupted. Multiple threads can therefore modify atomic
// variables and can know that their values are safe.
// Chapel atomic variables can be of type ``bool``, ``int``,
// ``uint``, and ``real``.
  var uranium: atomic int;
  uranium.write(238);      // atomically write a variable
  writeln(uranium.read()); // atomically read a variable

// Atomic operations are described as functions, so you can define your own.
  uranium.sub(3); // atomically subtract a variable
  writeln(uranium.read());

  var replaceWith = 239;
  var was = uranium.exchange(replaceWith);
  writeln("uranium was ", was, " but is now ", replaceWith);

  var isEqualTo = 235;
  if uranium.compareAndSwap(isEqualTo, replaceWith) {
    writeln("uranium was equal to ", isEqualTo,
             " so replaced value with ", replaceWith);
  } else {
    writeln("uranium was not equal to ", isEqualTo,
             " so value stays the same...  whatever it was");
  }

  sync {
    begin { // Reader task
      writeln("Reader: waiting for uranium to be ", isEqualTo);
      uranium.waitFor(isEqualTo);
      writeln("Reader: uranium was set (by someone) to ", isEqualTo);
    }

    begin { // Writer task
      writeln("Writer: will set uranium to the value ", isEqualTo, " in...");
      countdown(3);
      uranium.write(isEqualTo);
    }
  }

// ``sync`` variables have two states: empty and full.
// If you read an empty variable or write a full variable, you are waited
// until the variable is full or empty again.
  var someSyncVar$: sync int; // varName$ is a convention not a law.
  sync {
    begin { // Reader task
      writeln("Reader: waiting to read.");
      var read_sync = someSyncVar$;
      writeln("Reader: value is ", read_sync);
    }

    begin { // Writer task
      writeln("Writer: will write in...");
      countdown(3);
      someSyncVar$ = 123;
    }
  }

// ``single`` vars can only be written once. A read on an unwritten ``single``
// results in a wait, but when the variable has a value it can be read indefinitely.
  var someSingleVar$: single int; // varName$ is a convention not a law.
  sync {
    begin { // Reader task
      writeln("Reader: waiting to read.");
      for i in 1..5 {
        var read_single = someSingleVar$;
        writeln("Reader: iteration ", i,", and the value is ", read_single);
      }
    }

    begin { // Writer task
      writeln("Writer: will write in...");
      countdown(3);
      someSingleVar$ = 5; // first and only write ever.
    }
  }

// Here's an example using atomics and a ``sync`` variable to create a
// count-down mutex (also known as a multiplexer).
  var count: atomic int; // our counter
  var lock$: sync bool;   // the mutex lock

  count.write(2);       // Only let two tasks in at a time.
  lock$.writeXF(true);  // Set lock$ to full (unlocked)
  // Note: The value doesn't actually matter, just the state
  // (full:unlocked / empty:locked)
  // Also, writeXF() fills (F) the sync var regardless of its state (X)

  coforall task in 1..5 { // Generate tasks
    // Create a barrier
    do {
      lock$;                 // Read lock$ (wait)
    } while (count.read() < 1); // Keep waiting until a spot opens up

    count.sub(1);          // decrement the counter
    lock$.writeXF(true); // Set lock$ to full (signal)

    // Actual 'work'
    writeln("Task #", task, " doing work.");
    sleep(2);

    count.add(1);        // Increment the counter
    lock$.writeXF(true); // Set lock$ to full (signal)
  }

// We can define the operations ``+ * & | ^ && || min max minloc maxloc``
// over an entire array using scans and reductions.
// Reductions apply the operation over the entire array and
// result in a scalar value.
  var listOfValues: [1..10] int = [15,57,354,36,45,15,456,8,678,2];
  var sumOfValues = + reduce listOfValues;
  var maxValue = max reduce listOfValues; // 'max' give just max value

// ``maxloc`` gives max value and index of the max value.
// Note: We have to zip the array and domain together with the zip iterator.
  var (theMaxValue, idxOfMax) = maxloc reduce zip(listOfValues,
                                                  listOfValues.domain);

  writeln((sumOfValues, maxValue, idxOfMax, listOfValues[idxOfMax]));

// Scans apply the operation incrementally and return an array with the
// values of the operation at that index as it progressed through the
// array from ``array.domain.low`` to ``array.domain.high``.
  var runningSumOfValues = + scan listOfValues;
  var maxScan = max scan listOfValues;
  writeln(runningSumOfValues);
  writeln(maxScan);
} // end main()