[erlang-questions] Wrong advertised spec of lists:append/2

[Kostis Sagonas]

On 03/28/2013 03:36 PM, Pierre Fenoll wrote:
> Hi,
>
> As per the doc, the spec of lists:append/2 is ([term()], [term()]) ->
> [term()].
>
> However, a = lists:append([], a).
>
> Shouldn't it return [a]?

You are reading specs (and the documentation) in the wrong direction.

In library code, the spec for a function tells you two things:

  - you are supposed to call a function with terms of the given types in 
the arguments, and

  - if you issue such a call with the "expected" types, you can also 
expect that the type of the return value of the function is the one 
mentioned in the spec.

The spec says nothing about what happens when you issue a call with the 
"unexpected" arguments (as in your example). In most cases you will get 
a bad match exception, but the implementation of the function may be 
more lenient than its published specification.

Kostis