Szathmáry László honlapja @ DEIK

Let's see some simple examples.

Example 1: short, implicit form

proc my_echo(x: int | float) =
echo(x)

my_echo(5) # 5

my_echo(3.14) # 3.14

#my_echo("hi") # Error

The type of x can be int or float.

This is also a generic proc, but Nim allows us to write it like this. The generic param is implicit/hidden.

Here is the proof that it's a generic proc:

my_echo[int](5) # 5

my_echo[float](3.14) # 3.14

They work.

proc my_echo[T](x: T) =
echo(x)

my_echo(5) # 5

my_echo(3.14) # 3.14

my_echo("hi") # hi

Here T is a type parameter — it works for any type.

proc my_echo[T: int | float](x: T) =
echo(x)

my_echo(5) # 5

my_echo(3.14) # 3.14

#my_echo("hi") # ERROR

This is a generic proc, but restricted to only int or float.

The proc in Example 1 is actually converted to this form by the Nim compiler!