Local functions — a function declared inside a body

Summary#

A helper that belongs to one body, declared in it:

string Report(int count) {
  int margin = 2;
  var line = Label(count);                    // called before it is declared, as C# allows

  string Label(int n) => $"#{n + margin}";    // reads `margin` from the code around it
  int Fact(int n) => n <= 1 ? 1 : n * Fact(n - 1);    // calls itself

  return $"{line}|{Fact(count)}";
}

[Test]
void Local_Functions_Work_Like_Methods() {
  Assert.Equal("#6|24", Report(4));
}

Signature#

<Return> Name(<params>) { … }
<Return> Name(<params>) => <expression>;
static <Return> Name(<params>) => <expression>;     // reads nothing from the code around it

Description#

What can a local function read?#

The parameters of the function it is in, and the locals declared there with their TYPE spelled out — int margin = 2;, not var margin = 2;. A local function receives what it reads as a value, so a var local it reads is refused with a message asking for the type. Inside a class method it also reads the class's own fields, exactly as the method does:

class Invoice {
  public decimal Rate = 0.25m;

  public decimal Gross(decimal net) {
    decimal WithTax(decimal v) => v * (1m + Rate);
    return WithTax(net);
  }
}

[Test]
void A_Method_Local_Reads_The_Object() {
  Assert.Equal(125m, new Invoice().Gross(100m));
}

Can it change a variable of the code around it?#

Yes, as in C#: the local function and the body around it share the variable. What it reads is read when it is CALLED, and what it assigns is the variable itself — also when one local function calls another, or calls itself:

[Test]
void A_Local_Function_Shares_What_It_Writes() {
  int seen = 0;
  int step = 1;
  void Note() { seen += step; }
  int NoteTwice() { Note(); Note(); return seen; }
  step = 5;
  Note();
  Assert.Equal(15, NoteTwice());
  Assert.Equal(15, seen);
}

Such a local function is called by NAME in the body that declares it. Used as a value — Action a = Note;, handed on or stored — it could run where the variable is not, so that is refused by name; and it cannot wait inside itself for the other side or a durable timer. In either case, have it return the new value and assign it where the variable lives.

What does static add?#

A promise that the local function reads nothing from around it — C#'s static local function. One that does is refused, as C# refuses it.

See also#

• function — functions declared at the top level of a file
• x => … — lambdas, with or without parameter types — an inline function as a value
• class methods — functions that belong to a class