[erlang-bugs] lists:concat/1 spec inconsistent with implementation or bug in Dialyzer?
Wed Jul 28 10:06:44 CEST 2010
On 07/21/2010 07:10 PM, Rickard Olsson wrote:
> When running Dialyzer on the code below I get the following messages: "The
> call lists:concat([['a,...],...]) breaks the contract ([concat_thing()]) ->
> string()" and "Function unused_function/1 will never be called"
> Test code:
> concat() ->
> unused_function(A) ->
> The line: "lists:concat([[a]])." runs fine in the shell, but seems to cause
> the dialyzer to stop, which I assume is the reason it warns that
> unused_function/1 will never be used, even though it is used. If the arity
> is changed to 0 and the arguments removed from the call, the warning seems
> to disapear though.
> Is it a bug in the lists:concat/1's spec or in Dialyzer?
> openSUSE 11.0 (X86-64)
> Erlang R14A (erts-5.8) [source] [64-bit]
> Dialyzer v2.3.0
> Rickard Olsson
> Erlang Solutions
In my opinion, this is not a bug in the spec and certainly not in
lists:concat spec is:
-type concat_thing() :: atom() | integer() | float() | string().
-spec concat([concat_thing()]) -> string().
The line: "lists:concat([[a]])." runs (i.e. does not crash) in the shell
because the term
[a] is assumed to be a string by the lists:concat function. However,
Dialyzer is able
to distinguish a string from a list of atoms and, since the spec
declares that this
function should only by used with [concat_thing()], emits a warning. In
the lists:concat function is supposed to return a list of characters
(i.e. a string). This
is not the case with the call "lists:concat([[a]])":
So it does not crash because it can also handle lists because it has to
but it does not behave correctly when it is given lists that do not
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