The ‘nothing’ type

View nothingVariables.chpl on GitHub

This primer demonstrates the use of the nothing type on variables and class/record fields. A variable or field that is of type nothing is removed by the compiler and doesn’t exist during program execution.

The value none has type nothing and can be assigned to a nothing variable, or returned from a function with nothing return type.

Note

The nothing type is not to be confused with the void type, which is the type of a function that does not return any value.

Nothing variables

This variable will be removed by the compiler. It is an error to assign a non-nothing value to it, or to use it where a non-nothing value is expected.

var nothingVar: nothing;

The value none is the only value of the nothing type.

var nothingVar2 = none;

The following (commented out) statements are compile time errors:

// nothingVar = 1;
// var x = 2 * nothingVar;

A nothing variable can be passed to a generic function as long as the function doesn’t do anything that expects it to be a value. The following call to writeln() will simply print a blank line, as though the argument was never there.

writeln(nothingVar);

Conditional variables

A variable that will just be removed by the compiler doesn’t seem very useful on its own. But with the addition of compile time folding of param values, it can be used to remove variables conditionally.

config param useMultiplier = false;

If useMultiplier is false, the multiplier variable will be removed by the compiler. Since all uses of multiplier are guarded by the useMultiplier param, they will also be removed.

const multiplier = if useMultiplier then 3.5 else none;
var value = 1.0;

if useMultiplier {
  value *= multiplier;
}

writeln(value);

Conditional fields

The nothing type is extra useful for class and record fields, where a large number of instances may be created, so removal of fields could save a substantial amount of memory or cache space. An example is a record containing two different implementations of its functionality, e.g. for two different platforms.

record nothingRecord {
  param useImpl2: bool = false;
  var impl1Var1 = if useImpl2 then none else 1;
  var impl1Var2 = if useImpl2 then none else 2.0;

  var impl2Var1 = if useImpl2 then 3.0 else none;
  var impl2Var2 = if useImpl2 then "4.0" else none;

  proc myProc() {
    if useImpl2 {
      writeln((impl2Var1, impl2Var2));
    } else {
      writeln((impl1Var1, impl1Var2));
    }
  }
}

vr1 doesn’t contain the fields impl2Var1 or impl2Var2
vr2 doesn’t contain the fields impl1Var1 or impl1Var2

var vr1 = new nothingRecord(useImpl2=false),
    vr2 = new nothingRecord(useImpl2=true);

vr1.myProc();
vr2.myProc();

By leaving out the unnecessary fields, the memory and cache footprint are significantly reduced, resulting in potential performance improvements

config const n = 1000;
var A: [1..n] nothingRecord(useImpl2=false);
for vr in A {
  if vr.useImpl2 {
    vr.impl2Var1 = 1.1;
    vr.impl2Var2 = "hello world!";
  } else {
    vr.impl1Var1 = 42;
    vr.impl1Var2 = 3.14;
  }
}

Although nothing variables don’t seem useful at first glance, they can be used to conditionally remove unnecessary variables reducing memory and cache footprint. This can lead to less memory overhead and better performance.