[erlang-questions] Trouble with Erlang's lenient comparisons

[Kostis Sagonas]

JohnyTex wrote:
> Thanks for the advice guys! I managed to get Dialyzer running and it
> worked very nicely; it managed to spot some unreachable code right off
> the bat :)
> 
> However, it doesn't seem to get the more insidious errors that arise
> from comparing other types than was intended, i.e. accidentaly
> comparing a string to an integer... but since it's completely valid
> Erlang I guess it's not supposed to complain?

The answer depends on the kind of comparison that is used.

If by comparison you mean case expressions or direct matching as in

    case String of
      Integer -> ...
    end

or

    String = Integer

then dialyzer will report these to you as impossible matches.

If by comparison you mean uses of =/=, <, ... then of course these are 
allowed in Erlang and of course dialyzer will not warn you that you are 
doing something wrong there. (*)

Kostis

(*) Though it will warn you that e.g. the 'false' clause is unreachable 
in something like:

    case String =\= Integer of
      true -> ...
      false -> ...
    end