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How must we teach lexical scope?
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Date: 2007-03-29 (08:17)
From: Loup Vaillant <loup.vaillant@g...>
Subject: Re: [Caml-list] How must we teach lexical scope?
2007/3/28, ls-ocaml-developer-2006@m-e-leypold.de
> But perhaps I understand your problem better now: The difference
> you're wanting to make is the substitution of symbols by values at
> definition time vs. at evaluation time (I hope it is clear what I want
> to say).


> But you'll have to explain substitution at evaluation time
> anyway (when a function is called and the formal parameters are
> bound). I don't understand what your attempt to avoid to talk about an
> environment (from which a comes in the example above) will buy you.

Substitution at definition time is how I naturally thought of it. That
is, the definition:
# f x = a + x;;
was automatically replaced by:
# f x = 3 + x;;
in my head, so there were no more need for any environment.

However, I must admit such a way of thinking has its limits: as long
as the substitution is simple, that is easy. When a free variable is
some complicated piece of data (or even code), one (I) must switch to
an environment representation. In that case, the environment I think
about is only the set of free variables actually used by the function.
The environments our professors talked about included all values,
including the useless ones. I thought it was unnecessary, but I see
the trade-of, now: their process is quite long (not to mention the
syntactic burden of describing each environment) but it is systematic,
and simple. Because it is, it looks silly. I don't like environments,
but you convinced me I haven't came up with a better  solution.