Re: Why isn't -rectypes command line option the default

Re: Why isn't -rectypes command line option the default

[Jeffrey Loren Shaw]

For one reason why, see 

http://caml.inria.fr/pub/docs/manual-ocaml/libref/Lazy.html 

I remember reading other reasons, but I can't seem to find them right now.

Why isn't -rectypes command line option the default

[Erik de Castro Lopo]

Hi all,

If I try to define a recursive type like this:

    type 'a t = 'a * 'a t Lazy.t

the compiler won't accept it unless I add -rectypes to the compile
command.

I know I can avoid this by using a tag:

    type 'a t = Node of 'a * 'a t Lazy.t

but I was wondering if there was a reason why -rectypes wasn't the
default.

Cheers,
Erik
-- 
+-----------------------------------------------------------+
  Erik de Castro Lopo
+-----------------------------------------------------------+
"The very definition of hell is having to maintain someone 
else's Perl code." -- Anonymous

Re: Why isn't -rectypes command line option the default

[Zheng Li]

Erik de Castro Lopo <mle+ocaml@mega-nerd.com> writes:
> but I was wondering if there was a reason why -rectypes wasn't the
> default.
It's consciously disabled to avoid mysterious report of errors. Many careless
coding errors can be typed with recursive types, for example

# fun x-> x::x;;
- : ('a list as 'a) -> 'a list = <fun>

in this case, the programmer is probably thinking of connecting @ two
list. With recursive types, instead of a immediately type error report in
place, the function is typed without any problem, and finally some accident may
be happened far away from here, where the original function is likely to be under
several layers of composition and hard to recognized. 

So the main consideration is safety, though it's really fun to play with
recursive types sometimes.

-- 
Zheng Li
http://www.pps.jussieu.fr/~li