From: Gareth.McCaughan@... (Gareth McCaughan)
Date: 2001-12-13T10:05:18+09:00
Subject: [ruby-talk:28377] Re: The benefits of dynamic typing?

"Pixel" wrote:

[someone else:]
> > > it doesn't seem unreasonable that a language should support both
> > > static and dynamic typing.  then, if i knew at compile time what the
> > > types would be, i could tell the compiler, and i could possibly
> > > expose some problems at compile time.  but if i didn't know the
> > > types at compile time, that would be okay too.

[me:]
> > Take a look at Common Lisp, or Dylan. They have exactly
> > that feature. Good compilers for CL and Dylan do sophisticated
> > type inference, so that you don't have to declare all the
> > types you know in order to get good compile-time checking

[Pixel:]
> any links showing what is achieved in those languages? (for the checking, not
> the efficiency)

Hmm. Not sure off the top of my head. You could try
http://www.fun-o.com/ for Dylan and http://www.lisp.org/
for Common Lisp. Note that different implementations
will have different type-inferencing abilities.

Here's a brief and simplistic example in Common Lisp.
If I say

    (defun f (x)
      (let ((y (1+ x)))
        (first y)))

and then

    (compile 'f)

to the CMU Common Lisp compiler, I get this warning message:

    In: LAMBDA (X)
      (FIRST Y)
    --> CAR
    ==>
      Y
    Warning: Result is a NUMBER, not a (VALUES &OPTIONAL LIST &REST T).

which is liable to be obscure to non-Lispers, but means
"You tried to do something that expected a list, but
fed it something I know is a number.". Note that there
are *no* explicit type declarations in that code.

-- 
Gareth McCaughan  Gareth.McCaughan@pobox.com
..sig under construc