[erlang-questions] Built-in type var(_)

[Kostis Sagonas]

Tony Hannan wrote:
> Hello,
> 
> I was writing an MVar module (a' la Haskell) and defining the type
> 
>     -type var(A) :: {var, pid(), A}.
>     % initial value kept in reference for type only, current value held in
> process
> 
> But the compiler complained that
> 
>     "type var(_) is a builtin type; it cannot be redefined"
> 
> I don't see this built-in var mentioned in any of the documentation and
> Google didn't find it either. So where is this var type defined and can I
> use it instead of creating my own var type.

The underlying type representation needs to have a way of representing 
type variables (the A in your declaration) and it so happens that it has 
chosen something involving var/1.  I guess you are unlucky.

Simply choose some other name like mvar(A), myvar(A), my_var(A), ...

But you are right that this should probably be mentioned somewhere.

Kostis