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MathGroup Archive 1992

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declaration vs. assignment and dynamic vs. static binding

  • To: mathgroup at
  • Subject: declaration vs. assignment and dynamic vs. static binding
  • From: gaylord at
  • Date: Wed, 11 Mar 1992 14:39:34 -0600

this is a question as to the difference between (functional) declaration
and (procedural) assignment.

 i am specifically trying to figure out what := and = really are. does
Mathematica (the language) use assignment or declaration or assignment with
dynamic or static binding or what and when?
i received a lot of really interesting responses, some of which i

Thomas Yan of Cornell sent ML code which I translated into M and ran

from Thomas Yan:

terminology is a problem.  i think mine is what is generally used in the cs
community.  declaration is like that for constants: you declare it, and the
value never changes.  assignment reassigns/updates an old value.  dynamic
binding is even nastier, since it is not "lexically scoped": in the
examples, the only things (e.g. functions) that see the values are those
textually nearby (after the declaration/assignment).  in dynamic binding,
values can be anywhere.

here is an example that
distinguishes all three cases.

    fun g(f,x) = let val c = 3 in f(x) end
    fun h() = let val c = 5
                   fun f(x) = x + c
                   val a = g(f,0)
                   val c = 7
                   val b = g(f,1)
               in (a,b) end

what is h()?

1) declaration: in gc, the "val=c" has no effect, since c is never used.
    essentially, g is just function application, and you get (5,6).  note
    in sml, declarations are sequential.  thus, f(x) = x + c would not be
    until c is defined, e.g. illegal before val c = 5, unless there was
    a c declared in an outer scope.

2) assignment:  in g, the "val=c" has no effect, since c is never used.
    suppose c is a local variable, and c=7 reassigns the value of c.  well,
    is in the scope of c (the scope of variables, that is, where they are
    visible for use, are just within the "let"), so it uses the local c. 
    get our friend (5,8).

3) dynamic binding: the local declarations of c in h don't matter in this
    since the c in g overrides it.  you would get (3,4).  this also shows
    why i think dynamic binding can be a problem -- suppose you defined g
in a
    separate file, before the definition of h.  there is no reason to
    the "val c = 3" to have any effect on f or x.

    of course, you might think of f as shorthand notation which just uses
    current value of c, and this can be useful.  it just requires careful

-----remainder deleted----------------


Thomas said that declaration would give (5, 6), assignment would give (5,
8) and dynamic binding would give (3, 4).

in Mathematica (if i'm translating correctly) his program reads:

Clear[f, g, h]
g[f_, x_] := Module[{c = 3}, f[x]]
h[] := Module[{c = 5},
											f[x_] := x + c;
											a = g[f, 0];
											c = 7;
											b = g[f, 1];
											{a, b}]

{5, 8}

Clear[f, g, h]
g[f_, x_] := Module[{c = 3}, f[x]]
h[] := Module[{c = 5},
											f[x_] = x + c;
											a = g[f, 0];
											c = 7;
											b = g[f, 1];
											{a, b}]
{5, 6}

It would appear that according to Thomas Yan , Mathematica uses declaration
(when f[x] = is used) and assignment (when f[x_] := is used), but not
dynamic binding.

this is confusing because of notational idiosyncracies of Mathematica which
calls = assignment and := delayed assignment.

i have codefrom other people as well that i'm presently translating but i
thought i'd send this out  to the people who sent me stuff and to groups
that might be interested.

any comments on this topic will be welcome.


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