4.1.3 Omitting the Explicit forall Clause: the Backquote Sugar

Type variables can be introduced implicitly -- without listing them in the forall clause. This provides a more concise notation for parameterized declarations by omitting the explicit forall prefix. A type variable is introduced implicitly in a declaration if:

• it is preceded by a backquote (') somewhere in its header (see more on the backquote sugar in Section 4.2), or
• it appears inside the square brackets following the name of a parameterized object or message being introduced in an object or method declaration, in which case the type variable is an explicit type parameter of that object or method.

So the declarations from the previous example can be rewritten more concisely as:

template object i_vector[T];
extend i_vector['T] isa collection[T];
method fetch(a@:i_vector['T], index:int):T  { ... }

The expressions remain the same since the backquote sugar does not affect whether or not a type parameter is explicit. Note that the names of type variables shadow names of globally visible types, whether the variables are introduced in a forall clause, using the backquote sugar, or as an explicit type parameter. For example, if a declaration introduces a type variable called int, all occurrences of int in that declaration will refer to that type variable and not the predefined integer type.