typing

[Tony Pedley]

Sean Hinde wrote:

> It may be useful to share some of our findings here at One2One on using
> Erlang for some reasonably complex jobs..
>
> Most common causes of bugs not found until runtime:
>
> 1. Referring to a non existent external function.
> 2. Mistyping (i.e. fingers not Type!) of atoms used as tags in either
> receive clauses or case clauses.
> 3. Printing Strings. The io[_lib]:format commands require you to know that a
> string is a string type before using %s to print it without double quotes,
> otherwise it crashes. In a weakly typed language with no efficient
> is_string/1 this is a royal PITA.
>
> These cause problems most in weird error capture cases which may have not
> been executed during testing (surely not). My personal favourite is
> error_report:format("Aargh").
>
> We have not tended to have problems with adding Integers to Atoms or other
> such things which type systems protect against.
>
> One could think of possible solutions to these three which do not involve a
> full type system.
>
> 1. A tool to go through a system and root out external reference problems
> (admittedly difficult to deal with apply(M,F,A)) would help. It would also
> be nice when loading a piece of code into a running system to get a result
> which included some indication of whether all the external references were
> resolved, or even an option which would only load the new code if this was
> true.
>
> 2. I guess any solution to finger trouble in atoms is getting towards a
> proper type system. Maybe something which could be declared at the start of
> a module which restricted the set of atoms which could be defined in the
> source for that file would do 80% of the job. Language Guru help needed!
>

Another solution for this one is to use macros instead of atoms

e.g.

-define(Message,message).

receive
    ?Message ->
        ok
    Any ->
        Any
end

This way any mistypes are spotted by the compiler, very usefull in genservers with a lot
of state atoms


>
> 3. Well, a string type would help!! Barring that to implement something like
> in Java (ironically strongly typed) where you can send any rubbish into a
> System.out.println and it will do its best to format it in a nice readable
> form without adding extraneous " for example. The current C derived mini
> typed language %d%s is to me something of an anachronism.
>
> Having said all that we have had two large systems in service now for
> several months and not had a single outage.
>
> Thoughts?
>
> Sean
>
> > -----Original Message-----
> > From: Thomas Arts [mailto:thomas@REDACTED]
> > Sent: 23 August 2000 07:44
> > To: James Hague
> > Cc: erlang-questions@REDACTED
> > Subject: Re: typing
> >
> >
> > James Hague wrote:
> > >
> > > >are there already some feedback on that approach ? is it
> > "academic" or
> > > frozen ?
> > > >used in projects?
> > >
> > > I asked this not too long ago.  The response was that the
> > type system was a
> > > cancelled project.
> >
> > Well, the projects that tried the typesystem were canceled,
> > that's what I
> > wrote. The typesystem self is more in a frozen phase. Some experiments
> > are going on, but on a low priority level. The problem is
> > that Erlang has
> > not been designed with a type system in mind. Adding one
> > later is possible,
> > but at a certain cost w.r.t. the freedom of programming. We
> > are investigating
> > whether we can find an acceptable compromise for things to give up.
> >
> > However, I can recommend everyone to start using teh SpecWeb
> > software in
> > order to make the datatype specifications. This is useful even without
> > a checking program.
> >
> > Regards
> > Thomas
> >
>
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--
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