learnxinyminutes-docs/chapel.md

---
language: Chapel
filename: learnchapel.chpl
contributors:
    - ["Ian J. Bertolacci", "https://www.cs.arizona.edu/~ianbertolacci/"]
    - ["Ben Harshbarger", "https://github.com/benharsh/"]
---

You can read all about Chapel at [Cray's official Chapel website](https://chapel-lang.org).
In short, Chapel is an open-source, high-productivity, parallel-programming
language in development at Cray Inc., and is designed to run on multi-core PCs
as well as multi-kilocore supercomputers.

More information and support can be found at the bottom of this document.

You can refer to the official site for [latest version](https://chapel-lang.org/docs/master/primers/learnChapelInYMinutes.html) of this document.

```chapel
/*
   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()
```

## Who is this tutorial for?

This tutorial is for people who want to learn the ropes of chapel without
having to hear about what fiber mixture the ropes are, or how they were
braided, or how the braid configurations differ between one another. It won't
teach you how to develop amazingly performant code, and it's not exhaustive.
Refer to the [language specification](https://chapel-lang.org/docs/latest/language/spec.html) and
the [module documentation](https://chapel-lang.org/docs/latest/) for more
details.

Occasionally check back here and on the [Chapel site](https://chapel-lang.org)
to see if more topics have been added or more tutorials created.

### What this tutorial is lacking:

* Exposition of the [standard modules](https://chapel-lang.org/docs/latest/modules/standard.html)
* Multiple Locales (distributed memory system)
* Records
* Parallel iterators

## Your input, questions, and discoveries are important to the developers!

The Chapel language is still in active development, so there are
occasional hiccups with performance and language features. The more information
you give the Chapel development team about issues you encounter or features you
would like to see, the better the language becomes.
There are several ways to interact with the developers:

* [Gitter chat](https://gitter.im/chapel-lang/chapel)
* [sourceforge email lists](https://sourceforge.net/p/chapel/mailman)

If you're really interested in the development of the compiler or contributing
to the project, [check out the master GitHub repository](https://github.com/chapel-lang/chapel).
It is under the [Apache 2.0 License](http://www.apache.org/licenses/LICENSE-2.0).

## Installing the Compiler

[The Official Chapel documentation details how to download and compile the Chapel compiler.](https://chapel-lang.org/docs/usingchapel/QUICKSTART.html)

Chapel can be built and installed on your average 'nix machine (and cygwin).
[Download the latest release version](https://github.com/chapel-lang/chapel/releases/)
and it's as easy as

1. `tar -xvf chapel-<VERSION>.tar.gz`
2. `cd chapel-<VERSION>`
3. `source util/setchplenv.bash # or .sh or .csh or .fish`
4. `make`
5. `make check # optional`

You will need to `source util/setchplenv.EXT` from within the Chapel directory
(`$CHPL_HOME`) every time your terminal starts so it's suggested that you drop
that command in a script that will get executed on startup (like .bashrc).

Chapel is easily installed on macOS with Homebrew

1. `brew update`
2. `brew install chapel`

## Compiling Code

Builds like other compilers:

`chpl myFile.chpl -o myExe`

Notable arguments:

* `--fast`: enables a number of optimizations and disables array bounds
  checks. Should only enable when application is stable.
* `--set <Symbol Name>=<Value>`: set config param `<Symbol Name>` to `<Value>`
  at compile-time.
* `--main-module <Module Name>`: use the main() procedure found in the module
  `<Module Name>` as the executable's main.
* `--module-dir <Directory>`: includes `<Directory>` in the module search path.