Re: [Caml-list] Teaching bottomline, part 3: what should improve.

[Gerd Stolpmann <info@gerd-stolpmann.de>]

Am Mittwoch, den 23.05.2007, 08:49 -0400 schrieb Brian Hurt:
> Vincent Hanquez wrote:
> > On Wed, May 23, 2007 at 09:16:44AM +1000, skaller wrote:
> >   
> > > On Tue, 2007-05-22 at 18:10 -0400, David Teller wrote:
> > > 
> > >     
> > > > * Error messages of the type system are somewhat obscure. The reflex of
> > > > many students is "OCaml wants it to be of type XXX", rather than "there
> > > > is a contradiction in what I wrote". It would be nice if there was a way
> > > > to ask OCaml to display additional information on type errors.
> > > >       
> > > This is a long standing peeve of mine. Lets face it: Ocaml just lies.
> > > If it has inferred a type, then finds a contradiction, it should
> > > report both the location of the contradication AND all of the source
> > > lines that contributed to the inference.
> > >     
> > 
> > I agree, this is one of the worst thing about ocaml type inference,
> > and you sometimes end up to have to put explicit type to functions to
> > find the offending lines (usually hundreds lines before the actual line
> > that's printed), defeating the whole thing...
> > 
> > Cheers,
> >   
> 
> Hundreds of lines?  I've seen ten's of lines, but never hundreds.

I can confirm that: hundreds. This happens especially for mismatching
class types, because the compiler tries to break the mismatch down.

But beginners shouldn't use classes.

> Of course, I generally type annotate at the level of functions at
> least (using .mli files is to be encouraged, IMO).  So type errors
> generally don't escape functions.  And I keep functions reasonably
> short- tens of lines long at most...

There is certainly a point. The length of type errors depends on your
programming style. A teacher can give hints how to make everything
easier to handle. (Although it is also a valid point in education to let
the students find their limits.)

Gerd
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