[erlang-questions] Built In Types

Kostis Sagonas <>
Mon Apr 16 23:01:59 CEST 2012


On 04/16/2012 10:16 PM, maxim minin wrote:
> Hi Erlangers,
> i try to define a type "product" like:
>
> -type product :: 'A'|'B'|'C'.
>
> But the compiler complains"type product() already defined".
> At http://www.erlang.org/doc/reference_manual/typespec.html is "product" not in the list of built-in types.
> So i have two quetions:
>
> Is "product" really a built-in type?

Hmmm... Nice discrepancy detected! This was actually news to me.

Turns out that there are four atom names, which are not built-in types, 
but are instead used internally by the type system and are in principle 
defined for all arities. These are listed in lib/stdlib/src/erl_lint.erl

is_var_arity_type(tuple) -> true;
is_var_arity_type(product) -> true;
is_var_arity_type(union) -> true;
is_var_arity_type(record) -> true;
is_var_arity_type(_) -> false.

So, for the time being, you cannot use the atoms 'tuple', 'product', 
'union' or 'record' as names of types. These should be a note of that in 
the reference_manual (or perhaps revise the representation to use less 
common atoms for some of these, e.g. '$$product').

Sorry for the inconvenience...

> Is it possible to show all built-in types in erlang shell?

Currently not.

Kostis



More information about the erlang-questions mailing list