Constant Declarations

In addition to variables, Chapel supports the declaration of constants which can be thought of as variables whose values cannot be changed once they've been initialized. Chapel supports two flavors of constants, each of which is introduced by a distinct keyword:

  • const introduces an execution-time constant whose value need not be known to the compiler
  • param introduces a compile-time constant whose value must be known to the compiler

Declaring immutable variables as const or param provides helpful semantic information, both to human readers of Chapel programs, and to the compiler itself.

In practice, param values are most often used in cases where compile-time operations or specialization will reduce execution-time computation, resulting in better performance. In other cases, certain language constructs are required to be param values for the sake of static typing or performance. Examples include the indices used when indexing heterogeneous tuples or the number of dimensions when declaring rectangular arrays. In situations other than the above, const declarations tend to be used since they are less restrictive as we will see below.

Declaring consts

Constant declarations are a lot like variable declarations, except for the change in keyword. As a simple example, the following statement declares an integer constant named two whose value is 2 (as you'd hope):

const two: int = 2;

Unlike variables, attempting to assign to a constant after it has been initialized will result in a compile-time error. For example, if we later attempted to perform this assignment:

two = 3;

The compiler would report the following error:

constParam.chpl:6: error: illegal lvalue in assignment

As with variable declarations, the type specification or initializer may be omitted in a const declaration, in which case the compiler will fill in any missing details:

const meaning = 42;  // inferred to be an 'int'
const zero: int;     // default initialized to '0'

Of course, since constants cannot be re-assigned, in practice they typically do include an initialization expression.

Like variables, constants support multiple comma-separated declarations per statement, where types and/or initializers may be shared:

const three = 3,
      four = 4,
      Zero, ZERO: int,         // 'Zero' and 'ZERO' are both 'int's default initialized to '0'
      language = "Chapel",     // 'language' is inferred to be a 'string'
      debug, verbose = false;  // 'debug' and 'verbose' are both 'bool's initialized to 'false'

One of the main things that distinguishes const from param declarations is that the initializing expression for a const need not be a value that is known at compile-time. For example, the following constant declarations are initialized by reading a value from the console and then using that value to do some math:

const n = read(int),     // initialize 'n' by reading an 'int' from the console
      piOverN = pi / n;  // initialize 'piOverN' by dividing 'pi' by 'n'

If we were to print their values as follows:

writeln("n = ", n);
writeln("pi/n = ", piOverN);

And then typed '4' at the console after compiling and running the program, we'd see:

n = 4
pi/n = 0.785398

As a final note, Chapel supports const declarations of any type.

Declaring params

Declaring a param value is syntactically identical to declaring a var or const, but uses the keyword param. The initializer of a param must be able to be computed at compile-time, unlike variables and execution-time constants. Thus, most of the const declarations of the previous section could trivially be converted to param declarations as follows:

param zero, Zero, ZERO: int,
      two: int = 2,
      three = 3,
      four = 4,
      meaning = 42,
      language = "Chapel",
      debug, verbose = false;

Attempting to initialize a param with a value not known to the compiler will generate an error message. For example, consider the following initializers, which were legal in const declarations:

param n = read(int),
      piOverN = pi / n;

As param initializers, they result in the following error messages since the value of n cannot be known at compile-time:

constParam.chpl:37: error: Initializing parameter 'n' to value not known at compile time
constParam.chpl:38: error: Initializing parameter 'piOverN' to value not known at compile time

Despite this restriction, Chapel knows how to perform certain compile-time computations on param values, permitting them to be initialized using expressions other than simple literal values:

param tenSquared = 10**2,
      bitMask = 1 << 8,
      bitsPerByte = 8,
      size = numBits(int),
      sizeInBytes = size / bitsPerByte,
      bigger = max(bitMask, size);

Specifically, procedures and operators that are declared to return param values are executed at compile-time and can be used to initialize param constants. A subsequent section will cover the creation of such functions.

Unlike const declarations, param values are only supported for certain types that the compiler knows how to compute with: booleans, signed and unsigned integers, real and imag floating point values, enumerated types, and strings. Literal values of these types can be considered to be anonymous param values. In the future, we expect to expand this list to include additional types, such as complex values, ranges, domains, arrays, records, classes, and unions.

When to use const vs. param

Generally speaking, programmers are encouraged to use a param whenever they are computing with a value that's known at compile-time, is one of the supported param types, and one or more of the following apply:

  • the value is used in a place where the language requires a param (e.g., indexing into a heterogeneous tuple),
  • the value is useful for enabling (or forcing) a computation to occur at compile-time,
  • the generated code should be specialized by the param value.

Programmers are encouraged to use const to declare any other immutable variable, such as:

  • constants whose values can't or won't be known at compile-time,
  • constants for which compile-time specialization is neither required nor beneficial.

The ways in which a param value can affect code specialization or optimization may seem vague now, but should become clearer as we see how param values are used to create generic types and functions, to unroll loops, etc.

While compilers can often determine that a symbol is const or param by analyzing its uses, Chapel programmers are encouraged to declare variables as constants whenever possible in order to maximize the amount of semantic information available to the compiler and to people reading the code. Such declarations can also help programmers avoid simple mistakes by having the compiler's semantic checks ensure that values which are intended to be constant are not accidentally re-assigned.