A critic of Guido's blog on Python's lambda

Marcin 'Qrczak' Kowalczyk qrczak at knm.org.pl
Sat May 13 13:44:01 CEST 2006

Alexander Schmolck <a.schmolck at gmail.com> writes:

> I'd like to see a demonstration that using the same binding syntax
> for special and lexical variables buys you something apart from bugs.

There are 3 fundamental operations related to plain mutable variables:

A1. Making a new mutable variable with an initial value.
A2. Getting the current value.
A3. Setting the new value.

and 4 operations related to dynamically scoped variables:

B1. Making a new dynamic variable with an initial value.
B2. Getting the current value.
B3. Setting the new value.
B4. Local rebinding with a new initial value.

If you don't ever use B4, dynamic variables behave exactly like plain
variables. For this reason I see no point in distinguishing A2 from B2,
or A3 from B3. Dynamic variables are a pure extension of plain variables
by providing an additional operation.

Distinguishing the syntax of A1 and B1 is natural: somehow it must be
indicated what kind of variable is created.

Mutability is orthogonal to dynamic scoping. It makes sense to have a
variable which is like a plain variable but without A3, and a variable
which is like a dynamic variable but without B3, although it doesn't
provide anything new, only allows to express more constraints with a
potential for optimization. I won't consider them here.

Common Lisp does something weird: it uses the same syntax for A1 and B4,
where the meaning is distinguished by a special declaration. Here is
its syntax:

Directly named plain variables:
A1. (let ((name value)) body) and other forms
A2. name
A3. (setq name value), (setf name value)

First-class dynamic variables:
B1. (gensym)
B2. (symbol-value variable)
B3. (set variable value), (setf (symbol-value variable) value)
B4. (progv `(variable) `(value) body)

Directly named dynamic variables:
B1. (defvar name value), (defparameter name value)
B2. name
B3. (setq name value), (setf name value)
B4. (let ((name value)) body) and other forms

Dynamic variables in Lisp come in two flavors: first-class variables
and directly named variables. Directly named variables are always
global. You can convert a direct name to a first-class variable by
(quote name).

Plain variables have only the directly named flavor and they are
always local. You can emulate the first-class flavor by wrapping a
variable in a pair of closures or a closure with dual getting/setting
interface (needs a helper macro in order to be convenient). You can
emulate a global plain variable by wrapping a dynamic variable in a
symbol macro, ignoring its potential for local rebinding. You can
emulate creation of a new first-class variable by using a dynamic
variable and ignoring its potential for local rebinding, but this
can't be used to refer to an existing directly named plain variable.

In order to create a plain variable, you must be sure that its name is
not already used by a dynamic variable in the same scope.

So any essential functionality is possible to obtain, but the syntax
is very irregular.

   __("<         Marcin Kowalczyk
   \__/       qrczak at knm.org.pl
    ^^     http://qrnik.knm.org.pl/~qrczak/

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