[plt-scheme] Typed Scheme: type of filter ?

The type of `filter' in the initial environment was incorrectly given
as the simple version.  This has now been fixed in SVN.

Thanks,
sam th

On Thu, Jun 19, 2008 at 2:59 AM, Michel Salim <michel.sylvan at gmail.com> wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
> In the POPL 08 paper, the type of filter is given as
>
> (All (a b) ((a --b--> Boolean) (listof a) -> (Listof b))
>
> which would allow one to do things like
>
> (map add1 (filter number? '(1 #t 2 #f 3))
>
> in fact, Sam gave this example on Lambda the Ultimate
>
> In the released version of PLT Scheme 4.0, however, this is the type
> of filter:
> (All (a) ((<a> -> Boolean) (Listof <a>) -> (Listof <a>)))
>
> which means the above failed. As can be verified:
>
> (filter number? '(1 #t #f))
> - - : (Listof (U Integer #f #t))
> (1)
>
> The handling of booleans also strikes me as a bit odd.
>
>> > (define: bools : (Listof Boolean) '(#t #f))
>> > bools
> - - : (Listof Boolean)
> (#t #f)
>> > (define bools2 bools)
>> > bools2
> - - : (Listof Boolean)
> (#t #f)
>> > (define bools3 '(#t #f))
>> > bools3
> - - : (List #t #f)
> (#t #f)
>
> A similar behavior is exhibited for lists of numbers: '(1 1.5 2) :
> (List Integer Number Integer), so heterogenous lists are treated just
> like tuples in Haskell or ML, but is there any reason #t and #f are
> not treated as just instances of Boolean ?
>
> Thanks,
>
> - --
> Michel Salim
> -----BEGIN PGP SIGNATURE-----
> Version: GnuPG v1.4.9 (GNU/Linux)
> Comment: Using GnuPG with Fedora - http://enigmail.mozdev.org
>
> iEYEARECAAYFAkhaA9kACgkQWt0J3fd+7ZDjrwCbBdDegJPsKbpD6P6eYsgq0h81
> 1BMAn2H5+gi5a+4UVnuLEp6AHJ37tL4B
> =CTfP
> -----END PGP SIGNATURE-----
> _________________________________________________
>  For list-related administrative tasks:
>  http://list.cs.brown.edu/mailman/listinfo/plt-scheme
>



-- 
sam th
samth at ccs.neu.edu